School Science Lessons
Magnetism, magnetic fields, and forces, earth's magnetic field
2009-09-15
Please send comments to: J.Elfick@uq.edu.au
See: Interesting websites
Table of contents
29.0.0 Magnetism
29.1.0 Magnetic materials
29.1.1 Magnets, temporary and permanent magnets
29.1.2 Magnet domains and magnetization
29.1.3 Paramagnetism and diamagnetism
29.1.4 Hysteresis
29.1.5 Magnetostriction and magnetores
29.1.6 Magnetism and temperature, Curie point
6.40 Hanging magnets (Primary)
2.10 Magnetic pin chain (Primary)
29.1.1 Magnets, temporary and permanent magnets
4.67 Simple compass needle
4.68 Magnetic dip
4.69 Make a magnetizing coil
4.70 Suspended magnet
4.71 Natural magnets
4.72 Artificial magnets
4.73 Identify magnetic substances
4.74 Magnetic poles and pin chains
4.75 Cut an iron wire magnet
4.76 Magnetic fields in two dimensions
4.77 Magnetic fields in three dimensions
29.1.1.0 Storing bar magnets
29.1.1.1 Magnet assortment, natural magnets, artificial magnets
29.1.1.2 Break a magnet
29.1.1.3 Cast iron magnetic field
29.1.1.4 Identify which bar is a magnet, identify magnetic substances, rule of magnets, attraction and repulsion
29.1.1.5 Lowest energy configuration
29.1.1.6 Magnetic poles, isolated pole, freely suspended magnets
29.1.1.8 Magnetic boats
19.2.18 Extract iron, Fe, from breakfast cereal with a magnetic stirrer
29.1.2 Magnet domains and magnetization
29.1.2.1 Iron filings domains
29.1.2.2 Induced magnetic poles, magnetic induction
29.1.2.3 Hammer iron bar, magnetization in the earth's field
29.1.2.4 Magnetization by electric current
29.1.2.6 Electromagnets
29.1.2.7 Retentivity
29.1.2.8 Permalloy bar
29.1.2.9 Barkhausen effect
29.1.2.10 Magnetize magnetic material, by single touch, by double touch (divided touch), by electricity
29.1.2.11 Variation of magnetism along a bar magnet tested by spring balance
29.1.6 Magnetism and temperature
29.1.6.1 Curie point
29.1.6.2 Thermomagnetic motor
29.1.6.3 Meissner effect
29.2.0 Magnetic fields and forces
29.2.1 Magnetic fields
29.2.2 Magnetic fields and currents
29.2.3 Forces on magnets
29.2.4 Magnet / electromagnet interactions
29.2.5 Force on moving charge in a magnetic field
29.2.6 Force on current in wires
29.2.7 Torques on coils
29.3.0 Earth's magnetic field, geomagnetism, terrestrial magnetism principles, earth magnetism
29.2.0 Magnetic fields and forces
29.2.1 Magnetic Fields
29.2.1.1 Dip needle
29.2.1.2 Oersted's effect
29.2.1.3 Current through an electrolyte
29.2.1.4 Magnetic fields in two dimensions, magnet and iron filings
29.2.1.6 Magnetic fields in three dimensions, particles in oil, iron filings in glycerine, iron filings on glass plate stack
29.2.1.9 Area of contact
29.2.1.10 Gap and field strength
29.2.1.11 Shunting magnetic flux
29.2.1.12 Magnetic shielding, magnetic screening
29.2.1.13 Compare magnetic moments of two bar magnets using a deflection magnetometer (null method)
29.2.1.14 Vibrator with a magnet
29.2.1.15 Substances magnetic lines of force can pass through
29.2.2 Magnetic fields and currents
29.2.2.1 Iron filings around a wire, parallel wires, anti-parallel wires
29.2.2.2 Magnetic field around a wire
29.2.2.3 Magnetic fields around currents
29.2.2.4 Fields around currents, uniform and circular fields
29.2.2.5 Show the right-hand rule, force on charges moving through magnetic field
29.2.2.6 Biot-Savart law, Ampere's law, Ampere-Laplace law
29.2.2.7 Iron filings and a solenoid
29.2.2.8 Length of a solenoid
29.2.2.9 Small coils in a solenoid
29.2.2.10 Demountable Helmholtz coils
29.2.2.11 Field of a toroid
29.2.2.12 Iron filings on the overhead projector
29.2.2.13 Magnetic field round a bar magnet with the axis in the magnetic meridian
29.2.3 Forces on magnets
29.2.3.1 Magnets on a pivot
29.2.3.2 Levitation magnets, magnetic suspension, magnetized needle floats in air
29.2.3.3 Hanging magnets and inverse square law, pole strength of a bar magnet in the magnetic meridian using neutral points
29.2.3.4 Inverse square law, inverse fourth power, inverse seventh power
29.2.4 Magnet / electromagnet interactions
4.78 Cylindrical electromagnet
4.79 Horseshoe electromagnet
4.80 Test the strength of electromagnets
4.81 Magnetic field from electric current in a wire
4.82 Magnetic field inside an open coil, open solenoid
4.83 Electricity from a magnet and a coil
4.84 Make a simple electric motor
6.41 Make electromagnets (Primary)
29.2.4.1 Interaction of magnet and magnetizing coil
29.2.4.2 Solenoid and bar magnet
29.2.4.3 Jumping magnet
29.2.4.4 Magnetically suspended globe, unipolar motor
29.2.4.5 Floating magnetic balls, float magnetized needles
29.2.4.6 Ampere's ants
29.2.5 Force on moving charges
29.2.5.1 Cathode ray tube, CRT
29.2.5.2 e / m for electrons, measurement of e / m
29.2.5.3 Bending of an electron beam
29.2.5.4 Crookes tube
29.2.5.5 CRT and earth's magnetic field
29.2.5.6 Forces on an electron beam, magnetic deflection of cathode rays
29.2.5.7 Pinching mercury (The use of open surface mercury is illegal in some school systems!)
29.2.5.8 Magnetic pump, ion motor force on conducting field
29.2.6 Force on current in wires, parallel conductors
29.2.6.1 Parallel conductors
29.2.6.2 Interacting coils
29.2.6.3 Pinch effect simulation
29.2.6.4 Filament and magnet with a.c. / d.c., vibrating lamp filament
29.2.6.5 a.c. / d.c. magnetic contrast
29.2.6.7 Current balance
29.2.6.8 Maxwell's rule
29.2.6.9 Barlow's wheel
29.2.6.10 Electromagnetic swing
29.2.6.11 Magnetic grapevine
29.2.6.12 Electromagnetic conical pendulum
29.2.6.13 Ampere's motor, Ampere's frame
30.3.10 Dancing spring. jumping wire (LC)
29.2.7 Torques on Coils
29.2.7.1 Model galvanometers
29.2.7.2 Force on a current loop
29.2.7.3 Interacting coils
29.2.7.4 Interacting solenoids
29.3.0 Earth's magnetic field, geomagnetism
29.3.1 Simple compass needles
29.3.2 True north and magnetic north, magnetic variation (magnetic declination, magnetic deviation)
29.3.3 Magnetic dip, measure magnetic dip angles
29.3.4 North pole of magnet pointing magnetic north
4.67 Simple compass needle
See diagram 4.67.1: Simple compass needles 1. | See diagram 4.67.2: Simple compass needles 2 | See diagram 4.67.3: Plotting compass
1. Magnetize a sewing needle by stroking it with a bar magnet. Make a simple compass by the following methods:
1.1 push the magnetized needle through cardboard and suspend it on a thread,
1.2 push the needle through the projections of a cloth-covered button,
1.3 attach the needle to a strip of cardboard and balancing it over an inverted test-tube supported on a long pin.
Label the end of the magnet that tends to point north.
2. Make another simple compass needle by the following methods:
2.1. push two magnetized sewing needles through the holes of a large press stud and balancing it on the end of a needle pushed into a cork,
2.2. push a magnetized needle through thin cardboard and suspend it on a thread inside a glass jar.
3. Compare the north direction shown by a plotting compass with the directions shown by the simple compass needles. A compass needle is marked "N" at on end. This end points towards the north magnetic pole so it is the "north-seeking pole" of the magnet. The other end is the "south-seeking pole".
4.68 Magnetic dip
See diagram 4.68: Magnetic dip
Push a steel knitting needle through cylindrical cork at right angles to its long axis. Push a pin into the centre of each end of the cork to act as an axle. Balance the cork through its axle of pins on knife edges. Magnetize the steel knitting needle using a magnetizing coil. Balance the cork again. The earth's magnetic field pulls one end of the needle downwards. Fix a spirit level, or a glass tube containing a bubble in water, above the knitting needle. Use a protractor to measure the angle of dip between the horizontal spirit level and the knitting needle. At the north magnetic pole or at the south magnetic pole the needle should point straight down. At the equator the knitting needle will be about parallel to the spirit level.
4.69 Make a magnetizing coil
See diagram 4.69: Magnetizing coil
Use glass tubing wound with close turns of insulated copper wire to magnetize steel knitting needles.
4.70 Suspended magnet
See diagram 4.70: Suspended magnet
Use loops of cotton to suspend two magnets freely. Bring each pole of the two magnets close to, but not touching, each other. Show that like poles repel and unlike poles attract.
4.71 Natural magnets
A form of magnetite, iron (II, III) oxide, called lodestone, acts as a magnet when freely suspended. It was probably first discovered in China where they used it for the first magnetic compasses.
4.72 Artificial magnets
Look for low-cost artificial magnets in discarded loudspeakers, telephone receivers and other equipment. Artificial magnets have different shapes, e.g. "Alnico", horseshoe magnet, pairs of bar magnets with a soft iron keeper, cylindrical magnets. Store artificial magnets in pairs in a box, north to south, and south to north. Be careful! Keep magnets away from computer diskettes (floppy discs) and colour television screens.
4.73 Identify magnetic substances
Collect objects made of different substances, e.g. paper, wax, brass, zinc, iron, steel, glass, cork, rubber, aluminium, copper, gold, silver, wood, tin. Test each object with a magnet to see which objects a magnet attracts or does not attract. Bring a soft iron wire and hard steel or piano wire near a compass needle to see if a magnetic field affects it.
4.74 Magnetic poles and pin chains
See diagram 29.2.3.2: Pin chain
1. Use a 6 cm length of iron wire. Draw one end of a magnet along it once only and in one direction from end to end. Lay the wire on a piece of paper then test for magnetism by sprinkling iron filings over it. The iron filings are not attracted equally along its whole length. The areas of strongest attraction are the magnetic poles of the piece of wire. Use adhesive tape to removes iron filings from a strong magnet.
2. Pick up a pile of pins with the magnet. Leave one pin attached to the magnet. Take off another pin and bring it close the end of the first pin. They will stick together by magnetic force. Connect all the pins to make a magnetic pin chain.
3. Pick up a pile of pins with the magnet. Leave one pin attached to the magnet. Take off another pin and bring it close the end of the first pin. They will stick together by magnetic force. Connect all the pins to make a magnetic pin chain.
4. Estimate the strength of bar magnets by using a magnetized object to attract pins or paper clips and estimate this object's magnetization effect by the number of attracted pins or paper clips.
5. Use light thread to attach a paperclip to the desk with adhesive tape. Hold a strong magnet above the paperclip and see it rise.
4.75 Cut an iron wire magnet
Cut in half the magnetized steel wire from 4.171. Test both ends of each broken portion. The magnetism found on each side of the break has opposite polarity. Cut off a very small piece of the wire magnet and test it with iron filings. The smallest piece of the wire is a magnet with opposite poles.
4.76 Magnetic fields in two dimensions
See diagram 4.76.1: Iron filings over bar magnet | See diagram 4.76.2: Iron filings over combinations of magnets
1. Sprinkle iron filings evenly on a thin card. Hold the card high over a bar magnet then carefully lower it until it almost touches the magnet. Tap the card gently with the end of a pencil. The iron filings move into a pattern showing the magnetic field.
2. Repeat the experiment with two bar magnets in different positions. The iron filings tend to line up in "lines of force", "field lines". Hold a plotting compass above the lines of force and compare their direction with the direction of the compass needle. Put an unmagnetized piece of soft iron near two bar magnets on the desk and observe the interesting magnetic fields formed.
3. Make permanent records of the magnetic field by the following methods
3.1. Spray over the iron filings with a paint sprayer.
3.2 Replace the card with photographic paper in a dark room. Shine a bright light on it and develop the print.
3.3 Dip a white sheet of paper in melted wax. Let it cool then sprinkle iron filings on the solid wax. Hold the paper over a strong magnet to allow the iron filings to move into lines of force patterns. Hold a hot iron over the iron filings to let them sink into the wax.
3.4 Photocopy the iron filings on transparent paper, but do not use a strong magnet near a photocopy machine.
4.77 Magnetic fields in three dimensions
Add oil to iron filings in a container. Shake to see if the filings will go into suspension in the oil. Use a concentration of oil that allows the iron filings to remain suspended then bring a magnet to the container to develop a pattern of iron filings in three dimensions. Make a permanent record using water glass or liquid plastic.
4.78 Cylindrical electromagnets
See diagram 4.78: Cylindrical electromagnet
1. Use an iron bolt 5 cm long with a nut and two washers. Put a washer at each end and screw the nut on to the bolt. Leave 30 cm of wire then wind three layers of bell wire on the bolt between the washers. Leave another 30 cm of wire then cut the wire. Twist together the two ends of the wire. Wind insulating tape around the ends of the bolt to prevent the wire unwinding. Remove insulation from the two ends of the wire to link the electromagnet in a circuit with two dry cells or lead cell accumulators in series. Use a headlight bulb in series with the electromagnet.
2. Connect the circuit and then pick up pins and nails. Disconnect the circuit and see the iron objects fall. The magnetic force exists only when you turn on the current. Use a plotting compass to test the poles at each end of the electromagnet. Reverse the connections to the source of electricity and test the poles again.
4.79 Horseshoe electromagnet
See diagram 4.79: Horseshoe electromagnet
Do NOT use a lead cell accumulator, car battery, for this experiment because the resistance of these coils is low and the current will be too large with a significant fire risk. If you use horseshoe magnets or C-shape magnets, wind the coil in opposite directions on each arm of the magnet. Use an U-shape piece of iron. Wind a coil of three layers of bell wire on each straight arm of the iron, but not on the curving part. Leave 30 cm of wire before you start winding the coil from the end of one arm. Cross to the other arm. Wind a coil of three layers and leave 30 cm of wire at the end. Wind three layers of wire on this pole then wind insulating tape around the wires so they cannot unwind. Remove the insulation from the ends of the coil, connect the horseshoe magnet in series with a car headlight bulb, connect to two dry cells or lead cell accumulators, and test the poles of the electromagnet. One pole should be a north pole and the other pole should be a south pole. If each pole has the same polarity, you have wound the second coil in the wrong direction so you must unwind the coil and rewind it in the opposite direction. Use the magnet to attract different things.
4.80 Test the strength of electromagnets
Do not use lead cell accumulators for this experiment because the resistance of these coils is low and the current will be large with a significant fire risk.
1. Wind 25 turns of bell wire on a straight iron bolt and connect one dry cell or lead cell accumulator to the ends of the wire. Record the number of pins or paper-clips you can pick up with the electromagnet.
2. Repeat the experiment with two dry cells or lead cell accumulators connected in series.
3. Wind on 25 more turns of wire in the same direction. Join them to the first 25 turns. Repeat the experiment.
4. Repeat the experiment with two dry cells or lead cell accumulators connected in series.
5. Wind on another 50 turns. Join them to the first 50 turns. Repeat the experiment.
6. Repeat the experiment with two dry cells or lead cell accumulators connected in series. Remove 50 turns and rewind them on the bolt in the opposite direction.
7. With 100 turns so wound, repeat the experiment with two dry cells or lead cell accumulators connected in series.
4.81 Magnetic field from electric current in a wire
Pull 25 cm of insulated copper wire through a hole in the centre of a small white piece card. Connect the ends of the wire to a battery through a car headlight bulb. Fix the card in a horizontal position. Fix the wire in a vertical position. Sprinkle iron filings evenly on the card. Switch on the current. Tap the card gently with the end of a pencil. The iron filings move into a pattern showing the magnetic field. Switch off the current. Repeat the experiment using a small plotting compass instead of iron filings. Compare the directions of the compass needle to the patterns of iron filings on the card. Repeat the experiment with the direction of current reversed.
4.82 Magnetic field inside an open coil, open solenoid
See diagram 4.82: Open solenoid
Wind five evenly spaced turns of bell wire around a wooden cylinder. Slide the coil off the cylinder. Fit the cylinder into slots in a piece of cardboard so that the cardboard appears to cut the coil in half lengthways. Connect the coil to the terminals of a dry cell or lead cell accumulator or low voltage power supply using a car headlight bulb in series. Sprinkle iron filings evenly on the card. Switch on the current. Tap the card gently with the end of a pencil. The iron filings move into a pattern showing the magnetic field. Note the pattern inside the coil and outside the coil. Switch off the current. Repeat the experiment using a plotting compass instead of iron filings.
4.83 Electricity from a magnet and a coil
See diagram 4.83: Produce electricity with a magnet and a coil
Connect a coil of fifty turns of bell wire to a current detector. Use long connecting wires so that the coil, and the magnet are away from the compass in the current detector. Hold the horseshoe magnet or bar magnet in your left hand and the coil of bell wire in your right hand. Hold the coil vertically. Pass one pole of the magnet through the soil while observing the compass needle in the current detector. When the coil moves through the magnetic lines of force, an electric current moves through the circuit.
4.84 Make a simple electric motor
See diagram 4.84: Simple electric motor | See diagram 4.84.1: Simple electric motor
1. horseshoe magnet, 2. axle, 3. commutator, 4. coil, 5. brass strip, 6. electric motor with 3 coils, A Contact (brush), Aw Wire from contact to coil, B Contact (brush), Bw wire from contact to coil
Fix a simple coil, mounted on an axle, between the poles of a horseshoe magnet. Two wires from the coil connect to the commutator. The commutator is a cylindrical insulator revolving on the axle with two strips of brass attached. The commutator rotates with the coil. Each brass strip is joined to one wire from the coil. Two carbon contacts, brushes, touch the side of the commutator and allow electric current to pass from the battery to the commutator. Electric current goes from the battery to brass strip A then along wire Aw, through the coil then back through wire Bw and brass strip B then back to the battery to complete the circuit. When the commutator and coil make one half turn, the current enters through brass strip B and returns through brass strip A, reversing the current in the coil. The electric motor runs more smoothly if more than one coil is used. This electric motor uses a permanent magnet but most electric motors use a field coil that forms a more powerful electromagnet.
Using Fleming's left hand rule, direction of thumb is thrust, first finger is magnetic field and second finger is current. In the diagram, side 7 to 8 of the coil has upward force on it and side 9 to 10 has downward force on it. So the coil turns until it it vertical and the brushes no longer touch the brass strips because of the gaps between them, and no current flows. However, due to inertia of the commutator, the coil keeps turning so side 7 to 8 is now on the right side and side 9 to 10 is on the left side. The brushes touch the brass strips again and the coil keeps turning clockwise.
29.1.1 Magnets, temporary and permanent magnets
Magnetic materials, alloy magnets, ceramic magnets, temporary and permanent magnets
See diagram 29.4.0: Permanent bar magnet
Magnets are masses of a substance that can repel or attract the same substance. Magnets may be temporary magnets or permanent magnets, e.g. bar magnets in school laboratories or compass needles. Permanent magnets are made of steel. The magnetic length, 2L, is the distance between the two poles and is always less than the physical length. Permanent magnets can be made from alloys, e.g. "Alnico" and "Alcomax" (nickel, iron, aluminium, cobalt, copper), or ceramic iron oxide compounds called ferrites, e.g. barium ferrite, nickel ferrite. Permanent magnets are used in telephones, electric motors and bicycle dynamos. Ferrites as powders are used as a magnetic coating in audi tapes and computer floppy disks. Do not carry out magnetism experiments near large masses of magnetic material or near apparatus or wires through which an electric current is passing. Moving electric charges cause magnetism. Diamagnetism occurs when a substance is weakly affected by a strong magnet, e.g. bismuth, mercury. Paramagnetism occurs when substances can produce a weak magnetic field in the same direction as that of a strong magnet, e.g. tungsten, aluminium. Ferromagnetism occurs in the ferromagnetic materials iron, cobalt, nickel, and ferrite metallic oxides when groups of atoms, called domains, have the same directions of spin. External magnetic forces can induce magnetism, i.e. magnetize ferromagnetic materials. Induced magnetism can be temporary magnetism as in the soft iron used in electromagnets or permanent magnetism as in hard steel. Temporary magnetism lasts only if the external source of magnetism lasts. However, even permanent magnetism can be lost by hammering or heating. Some solutions of salts, e.g. MnCl2, FeCl3, CoSO4 show some magnetic susceptibility using a Quincke type glass tube.
29.1.1.0 Storing bar magnets
See diagram 29.1.1.0: Stored bar magnets
Magnets can lose their magnetism if you treat them roughly or do not store them in pairs with soft iron keepers, N to S and S to N. A magnetized ring of iron keep its magnetism better than a bar of iron with two magnetic poles. So the "keepers" keep the magnetic flux in a magnetic circuit with no free magnetic poles. Store artificial magnets in pairs in a box, north to south and south to north. Keep magnets away from computer diskettes and colour television screens!
29.1.1.1 Magnet assortment, natural magnets, artificial magnets
The most common natural magnets are a form of magnetite, iron (II, III) oxide, called lodestone that acts as a magnet when freely suspended. Lodestone was common in Magnesia in the Kingdom of Lydia, an ancient kingdom now in western Turkey. Previously, a lodestone was supposed to have magical properties! Lodestone attracts small nails. Two pieces of magnetite in paper stirrups come to rest on the magnetic meridian. Magnetite was probably first discovered in China and was used for the first compasses. Look for low cost artificial magnets in discarded loudspeakers, telephone receivers and other equipment. Artificial magnets have different shapes, e.g. "Alnico", horseshoe magnet, pairs of bar magnets with a soft iron keeper, cylindrical magnets, C-magnets, U-magnets, "Alcomax" magnets, and powerful magnets. Store artificial magnets in pairs in a box, north to south and south to north. Keep magnets wall away from computer diskettes and colour television screens!
1. List all the different kinds of magnets:
1.1 in the laboratory,
1.2 in the home,
1.3 in a motor car.
2. Suspend a large lodestone in a cradle with the south pole painted white. Use a bar magnet is used to show attraction and repulsion.
29.1.1.2 Break a magnet.
1. A magnet attracts nails. Break it and note that the broken pieces have formed new magnetic poles.
2. Break a magnetized steel wire in half. Test both ends of each broken portion. The magnetism found on each side of the break has opposite polarity. Break off a very small piece of the wire magnet and test it with iron filings. The smallest piece of the wire is a magnet with opposite poles.
29.1.1.3 Cast iron magnetic field. Cast iron filings in gelatine. Cast iron filings in acrylic over one pole of a magnet.
29.1.1.4 Identify which bar is a magnet, identify magnetic substances, rule of magnets, attraction and repulsion
See diagram 29.1.1.4: Like and unlike poles
1. Two bars look alike one is a magnet and the other is not a magnet. With two similar bars of iron one magnetized use the end of one to lift the middle of the other. 2. Many iron and steel objects are magnetized without you knowing it. You can detect this magnetism with a compass. If a rod is magnetized, it must, like the compass needle, have a north pole and a south pole. The rule of magnets is that two unlike poles attract and two like poles repel. So one pole of the needle will be attracted to the end of the rod and the other repelled. If the rod is not magnetized, both poles of the needle are weakly attracted to the end. Collect objects made of paper, wax, brass, zinc, iron, steel, glass, cork, rubber, aluminium, copper, gold, silver, wood, tin. Test each object with a magnet to see which ones are attracted that are not. Bring a soft iron wire and hard steel or piano wire near a compass needle to see if it is affected by a magnetic field.
29.1.1.5 Lowest energy configuration
Magnets held vertically in corks are placed in a dish of water. When a coil around the dish is energized the magnets move to the lowest energy configuration.
29.1.1.6 Magnetic poles, isolated pole, freely suspended magnets
See diagram 4.76.1: Magnetic field of a bar magnet
1. Magnetism is the strongest at the poles of a magnet. Use a bar magnet, a horseshoe-shaped magnet, a magnetized needle and other magnetized objects. Immerse them in fine iron filings then take them out. Note that most filings are at the poles. Scatter iron filings or iron powder over every part of a magnet and note that some filings slip off the magnet and some filings are attracted at its poles.
2. Magnetize a piece of iron wire or a needle by rubbing with a bar magnet. Find its poles with iron filings. Cut into two the magnetized iron wire or needle with pliers then test it with filings again. Each piece still has two poles. Cut each piece into two parts again then test them with filings. Each small piece has two poles. No matter how short the remaining wire is, it has two poles.
3. Use a 6 cm length of steel wire or piano wire. Draw one end of a steel magnet along it once only and in one direction from end to end. Lay the wire on a piece of paper then test for magnetism by sprinkling iron filings over it. The iron filings are not attracted equally along its whole length. They call the areas of strongest attraction the "magnetic poles". Pick up a pile of pins with the magnet. Leave one pin attached to the magnet. Take off another pin and bring it close the end of the first pin. They will stick together by magnetic force. Connect all the pins to make a magnetic pin chain.
4. To isolate a magnetic pole pass a long magnetized knitting needle through a cork and float it on water.
5. Make a freely suspended magnet. Use loops of cotton to suspend two magnets freely. Bring each pole of the two magnets close to, but not touching, each other. Show that like poles repel and unlike poles attract.
29.1.1.8 Magnetic boats
See diagram 29.1.1.8: Floating magnetic pins
1. Stroke pins with the north pole of a bar magnet. Very carefully lower the pins into water so that they float. Note how they line up end to end. Move the pins to make circles, north to south poles.
2. Stroke three pins many times with the north pole end of a magnet in the same direction so that their points attract each other. Put each pin in a little paper boat made of greaseproof paper. Put the boats in a dish of water. The boats will line up end to end in a north south direction.
29.1.2 Magnet domains and magnetization
Making a magnet, stroking, electrical solenoid, electrically demagnetizing, induced magnetism and paper clip chain
29.1.2.1 Iron filings domains, magnetization
Outside magnetic effects can cause the domains in a ferromagnetic material to act strongly in one direction to magnetize the substance temporarily in soft iron and permanently in hard steel. Soft iron is used in electromagnets so the magnetism can be "turned on" and "turned off". Permanent magnetism is necessary for compasses and permanent magnets but heating or hammering can destroy it. The measure of magnetic strength is magnetic moment, m. It is the torque on a magnet placed at right angles to a magnetic field. The magnetic moment is a vector. As the magnetic moments of molecular current inside matter are in the same direction, the matter shows a character that can attract the things made by iron, cobalt, nickel, and metallic oxides (ferrites) called magnetism. The process of obtaining the magnetism acted on by a magnetic field is called magnetization.
A tube of compressed iron filings is magnetized and then the iron filings are agitated. A set of magnetic needles on pivots orients randomly until a magnet is brought close.
29.1.2.2 Induced magnetic poles, magnetic induction
See diagram 29.1.2.2: Induced magnetism
1. A chain of nails is supported by a magnet each becoming a magnet by induction. A soft iron bar held collinear with a permanent magnet will become magnetized by induction. Use a compass needle to show the far pole of the bar is the same as the near pole of the magnet.
2. Put an iron bar on a block of wood. Hold an iron nail near one of its ends vertically. The nail can drop down when you release it showing that the iron bar has not magnetized. Hold a strong magnet near the other end of the iron bar. The nail does not drop when you release it, showing that the magnetic induction from the magnet has magnetized the iron bar. Remove the magnet and check if the iron bar is still magnetized by dropping the nail again.
3. Put a compass on the table. Hold an iron nail 15 cm in length with its sharp end near the north pole of the compass. Bring the north pole of a bar magnet near the other end of the iron nail but do not let them touch each other. Observe if the north pole of the compass moves. Remove the magnet after the compass points to the direction of north again. Bring the south pole of the magnet near the other end of the nail. Observe how the compass moves.
29.1.2.3 Hammer iron bar, magnetization in the earth's field
See diagram 4.67.3: Plotting compass
1. Hammer the end of a soft iron bar in the earth's magnetic field. Pound a soft iron bar held in the earth's field. A permalloy bar does not need to be pounded. Hammer a soft iron bar held parallel to the field of the earth. A bar of permalloy is magnetized by simply holding it in the earth's field.
2. Temporary magnetism lasts only if the external source of magnetism lasts. However, even permanent magnetism can be lost by hammering or heating.
3. Hold a soft iron bar pointing to the north and sloping downwards with the lower end against a thick piece of plastic. Hammer it down into the plastic. Lay the iron bar on the plastic, put a piece of paper over it and sprinkle iron filings on the paper. The iron filings move into a pattern showing that the iron bar has become slightly magnetic.
4. Hold a plotting compass near the iron bar and notice any movement of the compass needle. When you hammer the iron, some of its particles line up with the earth's magnetic field lines so that they point to the north.
29.1.2.4 Magnetization by electric current
Place an iron core in a solenoid. Magnetize with direct current and demagnetize by reducing alternating current to zero.
29.1.2.6 Electromagnets
A magnet powered by a 1.5 V battery lifts a large weight. An electromagnet with 25 turns of wire and one dry cell can lift over 100 kg
29.1.2.7 Retentivity
Retentivity means ability to respond after stimulus is removed, e.g. in ferromagnetic substances measured as residual flux density. In a magnetic hysteresis loop, retentivity is the value of B at zero magnetic field and is called remanence. A soft iron bar will cling to a U shaped electromagnet when the current is turned off but no longer attract after it is pulled away.
29.1.2.8 Permalloy bar
Permalloy is an alloy with high permeability and low hysteresis loss. It contains 78.5% Ni and 21.5% Fe + possibly other elements, e.g. Cu, Cr, Co. Permalloys are used in magnetic shields and computer memory chips. Iron filings stick to a permalloy bar held parallel to the earth's magnetic field but fall off when it is held perpendicular. A small strip of iron sticks to a permalloy rod when it is held in the direction of the Earth's field.
29.1.2.9 Barkhausen effect
The Barkhausen effect is observed when the steady increase in a magnetizing flux produces jumps in the magnetization of ferromagnetic materials. Magnetic domains in the core of a small coil can be heard flipping as a magnet is moved by using and an audio amplifier. Insert various cores into a coil connected to an audio amplifier and spin a magnet around it. Stretch an iron nickel alloy wire through a coil and bring a magnet close to show sudden simultaneous magnetization. Soft iron and hard steel cores are placed in a small coil attached to an audio amplifier and the assembly is inserted into a magnetic field. A soft iron core inserted in a small coil connected to the input of an audio amplifier.
29.1.2.10 Magnetize magnetic material, by single touch, by double touch (divided touch), by electricity, magnetizing iron by contact
See diagram: 29.4.1: Magnetizing by touch
1. Demagnetize the specimen, e.g. steel knitting needle, using a solenoid carrying an alternating current or by heating the specimen to dull red heat along its whole length and plunging it into cold water. Fix the specimen under a brass drawing pin stuck into the bench. Stroke the specimen ten times in the same direction with one pole of a permanent bar magnet with marked poles. Note the pole used and mark the end of the specimen where the pole first meets the specimen. Tests for magnetism in the specimen using iron filings. Find the polarity of the specimen with a plotting compass.
2. Demagnetize the specimen. Fix the specimen under a brass drawing pin stuck into the bench. Stroke the specimen by using opposite poles of two permanent bar magnets with marked poles. Stroke the specimen ten times. Note the poles used and mark the end where the pole first meets the specimen. Tests for magnetism in the specimen using iron filings. Find the polarity of the specimen with a plotting compass.
3. Demagnetize the specimen. Adjust the sliding contact of the rheostat to half the resistance. Put the specimen in the solenoid. Close the switch. Note the direction of flow of the current, from positive through the circuit to negative. Note the direction of winding of the solenoid. Open the key. Remove the specimen from in the solenoid. Mark it to show its position in the solenoid. Test for magnetism in the specimen using iron filings. Find the polarity of the specimen with a plotting compass.
4. Magnetize iron by contact and demagnetization. Stroke a nail on a permanent magnet and it will pick up iron filings. Magnetize an iron bar in a solenoid then pound it to demagnetize. Stroke a steel needle with a permanent magnet to magnetize and pass it through an a.c. solenoid to demagnetize.
29.1.2.11 Variation of magnetism along a bar magnet tested by spring balance
See diagram 29.1.2.11: Variation of magnetism
Place a bar magnet on a piece of squared paper. Tie a soft iron nail to the hook of a spring balance. Let the bar magnet attract the nail then try to pull the nail off the magnet. Record the needed pulling force. Start the experiment from one pole of the bar magnet and test every 2.5 cm. Show the readings of the spring balance on a graph. Let the distance at the first end be zero, graph the distance on the horizontal axis, the needed pulling force on the vertical axis. Draw a graph to show the distribution of the magnetism in a bar magnet. Magnetism is strongest at the magnetic poles
29.1.3 Paramagnetism and diamagnetism
Place paramagnetic and diamagnetic crystals between the poles of a large electromagnet. Place small samples of bismuth, aluminium, glass between the poles of a strong electromagnet. Suspend samples of bismuth and copper (II) sulfate by threads. A large horseshoe magnet attracts the copper (II) sulfate and repels the bismuth. A dollar bill is attracted by a strong magnet. Pull the bubble in a carpenter's level with a strong magnet. Pull liquid air drops around on a sheet of paper. Liquid oxygen sticks to the pole pieces of a strong electromagnet until it evaporates. Fill a test-tube with liquid oxygen. Suspend the test-tube by a long string attached to the ceiling. Bring a powerful magnet to the side of the test-tube. The position of the test-tube changes due to paramagnetsim.
29.1.4 Hysteresis
Hysteresis is the lag of an effect behind the cause of the effect. A hysteresis loop is a closed figure obtained by plotting magnetic flux density, B, against magnetizing field, H, when H increases and decreases. The area of the loop measures the energy lost during magnetization. Hysteresis loops for laminated steel and ferrite cores as saturation is reached can be displayed on an oscilloscope. The hysteresis loop for the iron core of a transformer is shown on an oscilloscope. Parallel iron bars suspended in a coil show hysteresis when slowly magnetized. Water is boiled by magnetic hysteresis waste heat.
29.1.5 Magnetostriction and magnetores
Nickel constricts and cobalt steel lengthens when magnetized. Place sample rods in a solenoid and show the effect by optical lever.
29.1.6.1 Curie point
The Curie point is the temperature above which a ferromagnetic material becomes paramagnetic. Iron under magnetic attraction is heated until it falls away. Upon cooling it is again attracted. A counterweighted iron wire is attracted to a magnet until heated red with a flame. A long soft iron wire held up by a magnet falls off when the wire is heated past the Curie point. A length of soft iron wire heated with 110 V d.c. through a rheostat shows loss of magnetic properties when it passes through recalescence (loss of heat in ferromagnetic material as crystal structure and magnetic properties change). Monel metals (nickel based alloys) have curie points between 25 degrees C and 100 degrees C depending on the alloy. A rod of nickel is attracted to a magnet when cool but swings away when heated.
29.1.6.2 Thermomagnetic motor
Local heating of permalloy tape or nickel rings in a magnetic field will cause rotation. The rim of a wheel of Monel tape is placed in the gap of a magnet and heat is applied to one side to make the wheel turn. A thin strip of magnetic alloy around the rim of a well balanced wheel is placed in the gap of a magnet with a light focussed on a point just above the magnet. Heating changes the magnetic properties and the wheel rotates.
29.1.6.3 Meissner effect
The Meissner effect occurs in a diamagnetic material which expels all its magnetic flux when cooled below its critical temperature and a magnetic field is applied. Such a material can show superconducting levitation. Cool a superconductor and a magnet floats over it owing to magnetic repulsion. Place a small powerful magnet over a disc of superconducting material cooled to liquid nitrogen temperature. A magnet / cork in a vial filled with salt water so the float just sinks is placed over the superconductor
29.2.0 Magnetic fields and forces, magnetic lines of force (field lines), magnetic flux, plot magnetic field lines, field patterns of permanent magnets and current carrying wires, loops and solenoid, force field comparisons to gravitational or electric fields, measurement of magnetic field strength by the simple current balance technique, magnetic field, plot magnetic field lines
See diagram 29.2.1: Lines of force | See diagram 29.2.3.3: Deflection magnetometer
The "amount" of magnetism is called magnetic flux. The magnetic field at a place, also called the magnetic induction or magnetic flux density, is represented by the vector B, unit tesla, T = 1 weber per square metre, Wb / m2. At any place in the magnetic field B is tangential to the magnetic field line drawn through that place. F (newtons) = q (coulombs) x v (volts) x B (tesla) sin a (where a = the angle between the magnetic field lines and the direction of a moving charge). The cgs unit for B is the gauss (G). I G = 10-4 T. Magnetic flux through an area is the product of the component of B perpendicular to the surface area, A, and 1. Magnetic flux = B x A Cos a (the angle between the direction of the magnetic field and the area at right angles), in weber, Wb. The deflection magnetometer is designed to compare the strengths of two magnetic fields acting at right angles to one another. It consists of a small magnet ns pivoted at the centre of a large circular scale graduated in degrees and a light aluminium pointer. Any deflection of the magnet causes the pointer to move through the same angle. The field to be tested is always placed perpendicular to the earth's horizontal component, i.e. in a magnetic east-west direction. If the strength of the field = H oersted and the earth's horizontal component = H0 oersted, by the parallelogram of forces law, the resultant R makes an angle a with Ho. The magnet aligns itself with this resultant field, i.e. is deflected through an angle a measured on the circular scale. H / H0 = tan a, so H = H0 tan a.
Magnetic fields of force can be shown as lines of magnetic force, magnetic flux. You can draw lines of magnetic flux around a source of magnetism to show the magnetic field so that the tangent at any point gives the direction of the magnetic field. The lines of magnetic flux are drawn as going from the north pole to the south pole. A magnetic compass aligns itself in the direction of the magnetic field, i.e. a tangent to the line of magnetic force at that point. Magnetic field is sometimes called the "flux density". When two or more magnetic fields interact, the result is equal to the vector sum of the separate fields.
29.2.1 Magnetic fields, magnetic fields and forces, lines of force
See diagram 29.2.1: Lines of force
A magnetic field is a field of force that appears on magnetic poles or magnets, i.e. around a magnetic body. Also a magnetic field appears around a current-carrying conductor and is associated with the motion of electrons in atoms. The strength and direction of a magnetic field is measured by magnetic flux density, B, SI unit tesla, or magnetic field strength, magnetizing force, H, SI unit ampere per metre.
Magnetic flux is related to the product of the magnetic permeability of the medium and the magnetic field intensity normal to the surface, SI unit weber, CGS unit maxwells.
1. Cover a bar magnet with a piece of stiff white paper. Sprinkle iron firings on the paper and tap it lightly. The iron filings line up along the lines of force from north pole to south pole. Hold a small magnetic compass, plotting compass, above the paper. It aligns itself to the direction of the magnetic field. Move the compass around to see the directions of magnetic field at different places.
2. Put paper over a magnet. Scatter iron filings on it. Tap the paper lightly, and a pattern forms. The curved lines of the iron filings show the direction of the magnetic force. Make the pattern permanent by dipping paper into melted candle wax and let it cool. Scatter iron filings on it. Hold a hot iron over the wax after the formation of the magnetic lines. The pattern will be fixed.
3. Place a piece of heavy paper over a bar magnet. Sprinkle iron filings on the paper and tap gently. The pattern shows the direction of the field of the magnet. You can use a sheet of glass instead of the paper. You may also plot the field with the aid of a small compass, placing it in various positions near the bar magnet and noting the direction in which the needle points. The iron filings form themselves into lines because each filing, being in a magnetic field, becomes itself a tiny magnet. The north pole of each tiny magnet is attracted by a south pole of a magnet near by and the filings arrange themselves into lines.
4. The magnetic curves by sprinkling iron filings over a glass plate may be preserved indefinitely a glass is warmed on the smooth surface of a hot plate. Put a piece of paraffin and let it spread evenly in a thin layer over the surface. Remove the glass plate and let the surplus paraffin running off. Form the image with iron filings that do not stick to the iron, so if the image is unsatisfactory the filings may be removed and a new figure taken. To fix the curves, the plate of glass is again placed on the warming stove. Cover the surface of the paraffin with white paint so the curves appear on a white background. For a simpler process, cover one surface of stiff white paper with a layer of paraffin by warming over an iron plate, spread the filings over the cooled surface and fix them with a hot iron or gas flam
29.2.1.1 Dip needle, magnetic dip
See diagram 4.68: Magnetic dip indicator
Use a large compass needle or dip needle as an indicator of magnetic field. Construct a magnetoscope by hanging needles from the edge of a small brass disc. Use a dip needle show the inclination and local direction of the earth's magnetic field. Explore the magnetic field around a long wire with a compass needle or dip needle.
29.2.1.2 Oersted's effect
Show Oersted's effect with a compass needle and a long wire carrying a heavy current to explore the magnetic field around a long wire. A compass deflects above and below a current carrying wire. Hold a current carrying wire over a bar magnet on a pivot and the magnet moves perpendicular to the wire. Arrange four compass needles around a vertical wire running through plexiglass. Pass a current of 50 amps through a heavy vertical wire and investigate the magnetic field with a compass needle. Pass a heavy current from a storage cell through a long wire and use a compass needle to investigate the nearby magnetic field. When demonstrating Oersted's effect using large currents, use flat braided brass cable instead of copper wire.
29.2.1.3 Current through an electrolyte
Use a compass needle detects the magnetic field from 2 amps flowing in an electrolyte. Detect a magnetic field produced current in copper electrolyte and a gas discharge tube with a large compass needle.
29.2.1.4 Magnet and iron filings, magnetic fields in two dimensions, field of a magnet
See diagram 29.2.1: Lines of force
1. Sprinkle iron filings on a glass sheet placed on top of a bar magnet.
2. Sprinkle iron filings evenly on a thin card. Hold the card high over a bar magnet then carefully lower it until it almost touches the magnet. Tap the card gently with the end of a pencil. The iron filings move into a pattern showing the magnetic field. Repeat the experiment with different types of magnets or with two bar magnets in different positions. The iron filings tend to line up in "lines of force" ("field lines"). Hold a plotting compass above the lines of force and compare their direction with the direction of the compass needle. Make permanent records of the magnetic field.
2.1 Spray over the iron filings with a paint sprayer
2.2. Replace the card with photographic paper in a dark room. Shine a bright light on it and develop the print. Dip a white sheet of paper in melted wax. Let it cool then sprinkle iron filings on the solid wax. Hold the paper over a strong magnet to allow the iron filings to move into lines of force pattern. Hold a hot iron over the iron filings to let them sink into the wax.
3. Sprinkle iron filings on a sheet of plexiglass over a magnet. Sprinkle iron filings on a magnet between two glass plates. Sprinkle iron filings on a glass sheet covering a bar magnet.
4. Put two bar magnets, a piece of soft iron, and wood blocks thicker than the magnets and iron on the table and cover with a piece of white cardboard. Scatter fine iron filings on the cardboard. Tap the cardboard from the side and note how the fine iron filings settle into a pattern.
5. To make magnet circles, push a wire up through a piece of thin cardboard. Drop iron filings evenly over the cardboard. Connect the wire to a dry cell battery. Tap the cardboard and the iron filings form circles around the wire because of the magnetic field from the flow of electric current in the wire.
29.2.1.6 Magnetic fields in three dimensions, particles in oil, iron filings in glycerine, iron filings on glass plate stack
1. Pour a spoon of fine iron filings into a bottle then 3 / 4 fill with sticky liquid, e.g. water glass or oil. Close the bottle then forcibly shake it to make the iron filings suspend evenly in the liquid. Place a strong magnet near the bottle. Note the distribution of the iron filings. You may save the distribution of the filings after it cools and solidifies if the liquid is transparent plastic in liquid state.
2. Use a suspension of carbonyl nickel powder in silicon oil as an indicator of magnetic field.
3. Make a sandwich of iron filings in glycerine between two glass plates. Soft iron bars extend the poles of a permanent magnet into a projection cell with iron filings in an equal mixture of glycerine and alcohol.
4. Make a 3-D view of magnetic fields by sprinkling iron filings on a series of stacked glass plates.
29.2.1.9 Area of contact
If one end of a magnet 1 cm in diameter is reduced to 0.5 cm in diameter, the small end lifts a much larger piece of iron than the large end. An electromagnet supports less weight when the face of the ring is against the pole than when the curved edge is against the pole. A soft iron truncated cone will support less weight when the large end is in contact with the face of an electromagnet.
29.2.1.10 Gap and field strength
Vary the gap of a magnet and measure the field with a gaussmeter (Name in USA term for instrument that measures magnetic flux density.)
29.2.1.11 Shunting magnetic flux
Pick up a steel ball with a bar magnet then slide a soft iron bar along the magnet towards the ball until it drops off.
29.2.1.12 Magnetic shielding, magnetic screening
Slide sheets of copper aluminium and iron between an electromagnet and an acrylic sheet separating nails from the magnet. Displace a hanging soft iron bar by attraction to a magnet then interpose a sheet of iron. A test magnet is used to show the shielding properties of a soft iron tube with various magnetic field generators. Hold a magnet above a nail attached to the table by a string then interpose a sheet of iron. Two horizontal sheets of glass separated by and air space intervene between an electromagnet and collection of nails being held up. Insert a sheet of iron into the space and the nails drop. The following demonstration could be in the Capacitors and Dielectrics section: Place a compass in the gap of an electromagnet and reverse the field at various rates, then use a sensitive magnetometer.
29.2.1.13 Compare magnetic moments of two bar magnets using a deflection magnetometer (null method)
See diagram 29.4.5
Find the magnetic lengths of two bar magnets 2L1 and 2L1, using a plotting compass. Place the magnetometer so that the pointer indicates 0o at one end and the arms lie magnetic east-west. Place magnet 1 at a distance d, to give a deflection of 30o. Adjust the position of magnet 2 in the other arm of the magnetometer so that there is no deflection of the pivoted magnet P. Record d1 and d2. Repeat the experiment twice, with d1 so that magnet 1 alone gives initial deflections of 45o and 60o and record d1 and d2. When there is no deflection of the pivoted magnet P, the magnetic fields at P from the two magnets, must be equal and opposite. Taking each pair of readings of d1, and d2, calculate M1 / M2 each of the three cases from the formula above. Calculate the mean value M1 / M2. If the magnets are short and powerful so that L is small compared with d, then L2 is negligible compared with d2.
29.2.1.14 Vibrator with a magnet
See diagram: 29.3.7
Place an U-shaped magnet with one pole up, the other pole down, at the edge of the table. Put a needle or a razor blade on the pole that is down. The needle or razor blade will stand vertically between the two poles. Beat the needle in the centre of the magnetic field slightly by a pencil at right angles to the magnetic lines of force. Note how the needle moves. Move the needle up or down, i.e. change the length of the needle in the magnetic field. Repeat the experiment, observe the variations of the vibrating frequency of the needle.
29.2.1.15 Substances magnetic lines of force can pass through
Put a bar magnet on the table and cover with a piece of paper. Put different substances, e.g. wood, glass, copper, zinc, cardboard, paper, plastic, iron, aluminium, on the paper over the bar magnet. Put iron filings on a piece of stiff white paper. Hold the paper over the substances and tap the paper from the side until some pattern forms. You can distinguish which substances can allow magnetic lines of force to pass through them by observing the pattern of iron filings on the paper. A magnetic field acts though all these materials except iron.
29.2.2.1 Iron filings around a wire, parallel wires, anti-parallel wires
Sprinkle iron filings around a vertical wire running through the centre of a plexiglass sheet.
29.2.2.2 Magnetic field around a wire
Iron filings show the field of a wire passing through a sheet of plexiglass. Sprinkle iron filings around a vertical wire running through plexiglass.
29.2.2.3 Magnetic fields around currents
Sprinkle iron filings around a current carrying wire loop, coil and solenoid.
29.2.2.4 Fields around currents, uniform and circular fields
Use iron filings to show the resultant of a vertical wire passing through a uniform field.
29.2.2.5 Show the right-hand rule, force on charges moving through magnetic field
See diagram 29.03
A positive charge crossing a magnetic field line experience a force in the direction found by using the open right-hand rule: fingers point in the direction of the magnetic field (north to south), thumb points in the direction of movement of the positive charge, palm of the hand pushes in direction of force on the positive charge.
The size of the force on the positive charge depends on the product of four factors:
1. the size of the charge, q,
2. the velocity of the charge, v, in m / s,
3. the strength of the magnetic field, B,
4. the angle between the direction of movement of the positive charge and the direction of the field lines, a.
A positive charge moving parallel to field lines experiences no force. Move a compass around a vertical wire carrying a current, then reverse the current.
29.2.2.6 Biot-Savart law, Ampere's law, Ampere-Laplace law
The Biot-Savart law expresses the intensity of magnetic flux density produced at a point at a distance from a current-carrying conductor. It gives Ampere's law, Ampere-Laplace law, that expresses the force between parallel current-carrying conductors in free space.
29.2.2.7 Iron filings and a solenoid
A solenoid is wound through a piece of plexiglass for use with iron filings on the overhead projector. Iron filings show the field of a solenoid wound through a sheet of plexiglass. Wind a solenoid through a piece of plexiglass for use with iron filings on the overhead projector. Insert into a solenoid a glass cylinder filled with iron filings in a solution of glycerine and alcohol.
29.2.2.8 Length of a solenoid
Construct a large solenoid to make it easy to change the spacing of turns and therefore the length. Use a magnetometer or coil to show field strength.
29.2.2.9 Small coils in a solenoid
Mount an array of small coils inside a large solenoid. Small springs keep the small coils aligned randomly when no current is applied.
29.2.2.10 Demountable Helmholtz coils
Helmholtz coils are a pair of compact identical coaxial coils separated by a distance equal to their radius. Use Helmholtz coils to generate a large uniform magnetic field at a midway position.
29.2.2.11 Field of a toroid
A toroid is a coil in the shape of a ring, in geometry a torus. Iron filings show the field of a toroid which is wound through a sheet of plexiglass.
29.2.2.12 Iron filings on the overhead projector
Use iron filings in a viscous liquid, e.g. castor oil, to show magnetic field configurations. Sprinkle iron filings on plastic sheets that have a single wire, parallel wires and a solenoid passing through holes.
29.2.2.13 Magnetic field round a bar magnet with the axis in the magnetic meridian
See diagram 29.01
A magnetic field, B, exists where a charge experiences a force because of its motion. You can detect a magnetic field by a compass needle that aligns itself in the direction of the magnetic field at that place. A magnetic field refers to where a magnetic force is found, i.e. magnetic flux is present. The force found in a magnetic field has a direction at any point in the magnetic field found by putting small pieces of iron in the magnetic field. The direction is called a line of force. In a strong magnetic field many lines of force are found in a very small space, the flux density is high, magnetic field lines, lines of force, can be drawn to show the direction a compass needle would have at any place in the magnetic field. Assume that the direction of the magnetic field is the direction of a compass needle, so magnetic field lines leave north poles and enter south poles. Like magnetic poles repel, i.e. NN or SS. Unlike poles attract, i.e. NS or SN. Magnetic field lines, lines of force, can be drawn to show the direction a compass needle would have at any place in the magnetic field. The pattern of a magnetic field about a bar magnet produced by sprinkling iron filings on a piece of paper over it. Lines of magnetic force are unbroken, pass through the magnet, never cross and have the same strength A strong magnetic field has more lines of force in an area than a weak field. If you bring the north pole of one magnet close to the south pole of another magnet, the two magnets will attract each other. If you bring the north pole of one magnet near to the north pole of another magnet, these poles will repel each other. The lines of force repel each other, and the two magnets push each other away. If you turn around the magnets so that the two south poles brought together, the poles will repel each other. Like poles repel and unlike poles attract.
29.2.3.1 Magnets on a pivot
Place one magnet on a pivot and use the other to attract or repel the first magnet. Place a magnet in a cradle then use a second magnet to attract and repel the first. Show interaction between bar magnets. Show magnetic attraction / repulsion. Snap the lines of force.
29.2.3.2 Levitation magnets, magnetic suspension
Linear motors use a force from a moving linear magnetic field that react with a conducting rail. Electromagnets in the train lift it and act as the rotor of an electric motor. Eddy currents induced in the rail create an opposing magnetic field. The two opposing magnetic fields repel each other and force the conductor away from the stator in the direction of the moving magnetic field. This principle is used in the Shanghai Maglev Train and other magnetic levitation trains.
See diagram: 29.3.6
1. Tie one end of a light thread through the eye of a needle. Hold on to the other end of the thread and pull up to lift and suspend the needle. Fix an U-shaped magnet vertically on the table. Lower the needle over the north pole of the magnet and pull the "eye" end of the needle over that end of the magnet.
2. Hold the thread steady and move the magnet horizontally so the needle drags across the pole until the sharp end of the needle separates from the pole.
3. Move the magnet away then bring the magnet back with the south pole end below the sharp end of the needle. The magnetized needle floats in the air above the south pole of the magnet.
29.2.3.3 Hanging magnets and inverse square law, pole strength of a bar magnet in the magnetic meridian using neutral points
See diagram 29.2.3.3: Magnetometer
1. The inverse square law of magnetism states that the force F between two magnetic poles varies inversely as the square of the distance d between them, i.e. F is proportional to 1 / distance2. Rotate a magnetometer until the pointer indicates 0o at one end, and the arms lie magnetic east-west. Clamp the ball-ended magnet at its centre so that ball A lies vertically above, and ball B magnetic east of the pivoted magnet P to give a 35o deflection. Read both ends of the pointer to eliminate the error if the pivot is not at the centre of the circular scale. Record the distance d cm of the centre of B from the pivoted magnet P. Repeat with ball B at the same distance from, and magnetic West of, the pivoted magnet to eliminate the error if the pivot is not at the zero marks of the linear scales. Record the readings of both ends of the pointer. Repeat the above procedure for values of d to give deflections between 30o and 60o.
d cm B east of P, a1 B east of P, a2 B west of P, a3 B east of P, a4 Mean a tan a 1 / d2
. . . . . . . .
Draw a graph of tan a (y axis) against 1 / d2 (x axis). The poles of a ball-ended magnet are at the centre of each ball. Ball A has no influence on the needle since at P its field is vertical. Assuming the inverse square law, magnetic intensity H in a horizontal direction at P caused by ball B = m / d2, m is the pole strength of B. However, H = Ho tan a, Ho is the horizontal component of the earth's field at P and a is the angle of deflection of P. m / d2= (H0 tan a), so 1 / d2= [(H0 / m) tan a]. However, Ho / m is a constant, so 1 / d2 is proportional to tan a, assuming the inverse square law. If the graph of 1 / d2 against tan a is a straight line passing through the origin, the inverse square law is verified.
Hang two magnets horizontally and parallel. Use the inverse square law to compute the pole strength from the length of the suspension the saturation and mass of the magnets.
2. Find the magnetic length 2L of a weak bar magnet with known polarity. Draw the outline of the bar magnet. Put a plotting compass in several positions near one end and mark with a pencil dot the position of each end of the compass needle. Repeat the procedure at the other end of the magnet. Remove the magnet. Draw a straight line through each pair of dots, producing the lines to intersect over two small areas that are the poles of the magnet. The distance between these poles is the magnetic length 2L of the magnet.
3. Put the magnet in the centre of the paper on the board with its south pole pointing magnetic north and its axis in the magnetic meridian. Plot lines of force in the region of the neutral points P and Q. When the compass is placed on these points the needle does not set in any particular direction. Measure the distances d1 and d2 from the centre of the magnet. If the pole strength of the bar magnet is m and its magnetic length is 2L, then the field strength H at a point distance d from its centre and on its magnetic axis produced = 4mLd / (d2- L2)2. At the neutral points the field H caused by the magnet is equal and opposite to H0, the earth's horizontal component. So H0 = 4mLd / (d2- L2)2,d = average distance of neutral points P and Q from centre of magnet.
29.2.3.4 Inverse square law, Inverse fourth power, Inverse seventh power
Use a balance to measure the repulsion of two bar magnets. Make a balance out of a meter stick with a magnet on one end facing the pole of another similar magnet. Adjust the distance between the magnets and slide the counterbalance along the meter stick until equilibrium is reached. Use a bar magnet brought near a second bar magnet counterweighted and on a knife edge to roughly verify the inverse square law. Use three simple variations of magnets levitating in a glass tube to show a force varying with the inverse of the distance squared. Apparatus shows the force between two dipoles varies as the inverse fourth power of the separation. Apparatus shows the force between a magnet and a piece of soft iron varies with the inverse seventh of the separation.
29.2.4 Magnet / electromagnet Interaction
29.2.4.1 Interaction of magnet and magnetizing coil
Make a magnetizing coil by using a glass tube wound with close turns of insulated copper wire to magnetize steel knitting needles. A solenoid on a pivot and a magnet on a pivot interact. A bar magnet is mounted in a large flat coil. The deflection of a compass needle in the centre of a large coil placed in the plane of the magnetic meridian is proportional to the tangent of the current.
29.2.4.2 Solenoid and bar magnet
See diagram 4.82: Suspended solenoid
A suspended solenoid reacts with a bar magnet only when the current is on. A magnet oscillates in a coil proportional to the square of the current in the coil. When a solenoid is energized an iron core is violently drawn into the coil.
29.2.4.3 Jumping magnet
Place a bar magnet in a vertical transformer and apply d.c. with a tap switch.
29.2.4.4 Magnetically suspended globe, unipolar motor
Two magnetized knitting needles mounted as the legs of an H suspended by a string rotate when a current flows upward through a rod.
29.2.4.5 Float magnetized needles, float magnetic balls
See diagram 29.1.1.8: Floating magnetic needles
Rub eight needles on one pole of a magnet to magnetize them and make the sharp end of them being the same pole. Push each needle through a cork leaving only one cm length in the cork. Float the magnetized needles on the surface of water in a plastic bowl. Put one pole of a strong magnet above the floating magnet needles and the floating needles will change their positions to form a certain picture. Increase or decrease the numbers of the magnet needles, change the poles of the magnet needles, change the distance from the pole to magnet needles, observe if the shape of the picture changes.
2. Thousands of small magnetic balls floating freely on the surface of water form hills and hollows when excited by an a.c. magnetic field.
29.2.4.6 Ampere's ants
An amusement park display where a pushbutton controlled magnetic stirrer is under a dish of iron filings.
29.2.5.1 Cathode ray tube, CRT
Deflect the beam in an open CRT with a magnet. A magnet or battery connected to the plates
is used to deflect the beam of an open CRT.
29.2.5.2 e / m for electrons, measurement of e / m
Deflect the beam in an open CRT with a magnet. Use the earth's field to deflect the beam in an oscilloscope. Deflect the beam of an oscilloscope with large solenoids. Deflect the beam of an oscilloscope by current in wires parallel to the axis of the tube.
29.2.5.3 Bending of an electron beam
An electron beam hitting a fluorescent screen in a tube is bent by a magnet. A thin beam along a fluorescent screen is bent by a magnet or charged rod. A thin electron beam made visible by a fluorescent screen is bent when a magnet is brought near.
29.2.5.4 Crookes tube
The Crookes tube was an improved gas discharge tube, vacuum tube, that showed a striped positive column, Faraday dark space, Crookes dark space, negative glow, cathode glow. Unwanted deflections of the beam in the Crookes tube are caused by induced charge.
29.2.5.5 CRT and earth's magnetic field
A CRT is mounted so it can be oriented in any direction and rotated about its axis. Find the position that results in no deflection from the earth's field turn 90 degrees.
29.2.5.6 Forces on an electron beam, Magnetic deflection of cathode rays
A beam of free electrons is bent in a circle by large Helmholtz coils. A beam from a lime spot cathode in a large bulb is made circular by Helmholtz coils.
29.2.5.7 Pinching mercury (The use of open surface mercury is illegal in some school systems!)
A thread of mercury in a glass tube is pinched in two by the interaction of the current and the conductor.
29.2.5.8 Magnetic pump, ion motor force on conducting field
copper (II) sulfate solution flows in a circle when placed between the poles of a magnet with a current from the centre to edge. An ion motor for the overhead projector with cork dust in a copper (II) sulfate solution. Cork dust floating on a solution of zinc chloride in a circular container rotates when current is passed through the solution in the presence of a magnetic field. Cork dust shows the motion of copper (II) sulfate an ion motor. Salt solution rotates when placed in a circular dish over a magnet with electrodes at the centre and edge.
29.2.6.1 Parallel conductors (The use of open surface mercury is illegal in some school systems!)
Long vertical parallel wires attract or repel depending on the current direction. Use two heavy vertical wires 1 cm apart and pass 20 amps in the same or opposite direction. Use rectangular loops of solid wire hanging on pivots from two stands. Use parallel wires with one being a loop free to turn in a pool of mercury. Radial wires (like clock hands) spring apart when current is passed through them.
29.2.6.2 Interacting coils
Two hanging loops attract or repel depending on current direction. A narrow loop formed by hanging a flexible wire opens when current is passed. Two loops in proximity attract or repel depending on current direction.
29.2.6.3 Pinch effect
Six number 8 wires are connected loosely between two terminals. Pass 20 amps and the bundle is attracted. Six vertical parallel wires are loosely hung in a circular arrangement. Six wires in parallel attract when current passes through each in the same direction. Then sets of three wires each have current flowing in opposite directions. A high voltage capacitor is discharged through a cylinder of aluminium foil strips.
29.2.6.4 Filament and magnet with a.c. / d.c., vibrating lamp filament
A tube lamp with a straight filament on a.c. will vibrate when placed between the poles of a magnet. A magnet is brought near carbon filament lamps one powered by a.c. the other by D3. The images are projected.
29.2.6.5 a.c. / d.c. magnetic contrast
A magnet is brought near a carbon lamp filament powered by d.c. then A3.
29.2.6.7 Current balance
In a current balance a balancing mass measures the force required to prevent the movement of of one current-carrying coil in the magnetic field of a second coil carrying the same current. Current balance has a rectangular coil on knife edges and stationary windings with parallel conductors. An open rectangle of aluminium wire is balanced between the poles of a U magnet until current is passed through the part perpendicular to the field. Hang a triangular loop of wire from a spring scale in the mouth of an electromagnet and the current in the loop is varied.
29.2.6.8 Maxwell's rule (The use of open surface mercury is illegal in some school systems!)
Maxwell's rule states each part of an electric circuit the circuit experiences a force causing it to tend to move in such a direction as to enclose the maximum possible magnetic flux. Show an electric circuit that can change shape to include the maximum possible magnetic flux. A heavy wire connects two metal boats floating in mercury troughs with electrodes at one end.
29.2.6.9 Barlow's wheel (The use of open surface mercury is illegal in some school systems!)
A copper disc with current flowing from the centre to a pool of mercury at the edge rotates when placed between the poles of a horseshoe magnet. A potential is applied from the axle of a wheel to a pool of mercury at the rim while the wheel is between the poles of a magnet. Current passes from the bearings of a copper wheel mounted vertically to a pool of mercury at the base. A U shaped magnet is mounted so the current is perpendicular to the magnetic field. The copper disc in Barlow's wheel is replaced by a cylindrical Alnico magnet with the field parallel to its axis. For a variation of Barlow's wheel, an Alnico disc magnetized in the direction of the axis rotates around the axis when a current is made to flow from the axis to the rim.
29.2.6.10 Electromagnetic swing
Switch the current direction in a wire loop swing mounted above one pole of a vertical bar magnet to build up a pendulum motion.
29.2.6.11 Magnetic grapevine
A very flexible wire suspended alongside a vertical bar magnet will wrap itself around the magnet when there is a current in the wire.
29.2.6.12 Electromagnetic conical pendulum (The use of open surface mercury is illegal in some school systems!)
A vertical wire is suspended loosely from above a vertical solenoid into a circular trough of mercury. As current is passed through the wire it rotates in the trough.
29.2.6.13 Ampere's motor, Ampere's frame
A coil on a reversing switch is placed between the poles of strong magnets. A magnet is brought near and rotates a large current carrying loop. A copper rod rolls along two electrified rails over ring magnets sandwiched between steel plates. A wheel on electrified rails over a large vertical field produced by electromagnets rolls back and forth depending on the current direction. As the current is reversed in a rod rolling horizontally on a track between the poles of a strong magnet the direction of motion reverses.
29.2.7.1 Model galvanometers
Use a large working model of a galvanometer with a large coil and magnet to show the essentials. Construct a large model d'Arsonval galvanometer from a coil and a large U-shaped magnet.
29.2.7.2 Force on a current loop
A rectangular loop on wire aligns perpendicular to a magnetic field.
29.2.7.3 Interacting coils
A small free turning coil is mounted in a larger coil. Two horizontal coaxial coils the inner stationary and the outer larger coil suspended freely interact when currents are passed through in like or opposite directions. A solenoid attached to a battery is mounted in a large open Helmholtz coils assembly.
29.2.7.4 Interacting solenoids (The use of open surface mercury is illegal in some school systems!)
Two heavy copper horizontal solenoids pivot in mercury cups about a vertical axis. Suspend a solenoid and show the effects of a bar magnet on it. A vertical coil energized by a flashlight cell floats in a large pan. Use a bar magnet to move the coil.