School Science Lessons
2. Scientific investigation and experiments, design of experiments
2014-03-31 sp
Please send comments to: J.Elfick@uq.edu.au
Table of contents
2.0.0 Scientific investigation
2.0.0 Scientific investigation and design of experiments
2.1.1 Carrying out scientific investigation
2.1.2 Scientific investigation and experiment design
2.1.3 Awareness and study of the world around us
2.1.4 Electrical appliances in the home
2.1.5 Heat insulation, properties of common materials

2.2.0 Design of experiments
2.2.2 Energy transfer between pendulums, by resonance
2.2.3 Electric kettle heating efficiency
2.2.4 Light bulb brightness, Joly photometer, wax block photometer
2.2.6 Human reaction time

2.0.0 Scientific investigation and design of experiments
The significance of modern science includes not only science knowledge itself but also the processes and methods of understanding science. You can obtain information through investigation then tidy it and make it more methodical to get models and rules. You can explore some ideas through experiments then draw conclusions to discover laws. Look for reasonable explanations and exchange research results with others. The findings on models and rules and laws may simplify description of observations. Investigations
and experiments may be done qualitatively and quantitatively. Physics teachers should instruct students to observe and know about related phenomena before the teacher teaches. Teachers should not explain too much but let experiment speak for itself. To apply effectively the resources of physics teaching, you must require students to do scientific investigations and experiments. Encourage students to put forward questions that need to be researched through their understanding of nature and social environment and
first hand experiences. Teachers could also bring forward questions then let students deal with them in nature and social environment. These are the prime tasks for teaching through investigations and experiments.
2.1.1 Carrying out scientific investigation
1. Put forward investigation questions and aims. Investigation questions should be significant and within students' capabilities. The aim of the investigation should be clear and concrete.
2. Choose an investigation plan. Do investigations according to certain steps. Every step must be operable and ensure the safety of students. An investigation plan should show a definite sequence.
3. Collect information.
4. Investigate, draw conclusions then put forward further questions for investigation.
Checklist of investigation skills
1. Identify variables
2. Decide procedure for investigation
3. Select and assemble apparatus and equipment
4. Check safety precautions, refer to safety guidelines from your school authority
5. Check methods to ensure accuracy of observations, system of recording and system for processing results
6. Follow instructions to do the experiment, record observations
7. Process results
8. Draw conclusions, graph results and estimate errors
9. Suggest new experiment based on results.

2.1.2 Scientific investigation and experiment design
Confirmation of the experiment aim has a wide and practical background. However, proposing an experiment aim based on completed scientific investigations is more significant to students. It is a kind of continual training of students' thinking. It not only enables students to go on exploring new knowledge but also it just reflects an attempt to affirm or correct some thing applying new conclusions. Understanding physics methods is helpful for students. For example, they may answer some questions such how to find the site of the centre of gravity of a hollow object from physics theory. You may answer the question of electric heating efficiency from investigations on electrical home appliances. You can associate the question of measuring photoelectricity efficiency with the question of electric heating efficiency. You may answer questions about refrigeration from investigations of daily materials in heat insulation.
2.1.3 Awareness and study of the world around us
Awareness of the world around us is a kind of activity every student can do and is helpful to develop students' interest and ability in physics. Methods and conclusions of investigations also have practical use in students' lives. Different students may investigate different phenomena and investigations of the same phenomena may produce different conclusions, giving students chances for exchange of information and encourage them to continue the investigations.
1. Temperature difference in a room
You need only a thermometer to measure difference in temperature. The temperature may be different during a day, a month, a season. Students should think of the method of recording data and describing the conclusion of the investigation.
2. Time of sunlight, length of shadow and estimate of time
Estimate time through the position and angle of the sun or the length of some furniture's shadow. In your own living room or yard estimate the time of sunlight the change of the angle of incidence of sunlight, the shadow of the furniture such as sun blind fixed beside a window and the change of length of the shadow.
3. Length, height and distance
Record the following lengths using a metre rule:
3.1 Stretch your palm as wide as possible and press your thumb and middle finger on a tabletop then record the distance between your thumb and middle finger.
3.2 Stretch your arms flatly then record the distance between your hands.
3.3 Step a pace as long as possible then record the length of the pace.
3.4 Measure your height when you stand next to a door.
These measurements may be useful for measuring some distances. Estimate the space you live by applying the above measurement. How many square metres in a room? How high are the down lights in the ceiling above the floor? How far your home is from some place where you often go? If you live at a multi storey building, measure the height from the floor of your room to the outside floor with a long rope. Can you measure the building height from the number of stairs and your hands?
2.1.4 Electrical appliances in the home
See: Electrical Hazards | See diagram 32.2.3
Before preparing to teach this topic, select, examine and report on a useful electrical appliance, e.g., air conditioner, boiler, calculator, charger, clock, dishwasher, doorbell, fan, freezer, fryer, hair dryer, heater, iron, mixer, motor, printer, radiator, refrigerator, roaster, shaver, telephone, television, toaster, torch, toy, vacuum cleaner, Xmas tree lights. Examine the nameplates and study the instruction manuals.
Report on the following:
1. Correct name and use of each electrical appliance
2. Normal or allowed working voltage and current
3. Working principles including a circuit diagram
4. Power input or useful power output, resistance and other properties
5. Operating method and points for attention
6. Safety characteristics, including the safety operating conditions so that the operator will not be hurt and apparatus not to be damaged .
7. Relevant times and terms of use, e.g. guarantee period, scrap period, date of production, continuous operating time.
8. Examine how the appliances convert electric energy to other forms of energy and think about how to design an experiment project to measure the efficiency of energy transformation.
Points for attention before preparing to teach this topic
See 19.3.5: Microwave cooking
1. Be clear on how to switch off the power in an emergency and the exact position of the appliance.
2. Any old or discarded appliance that requires mains power to operate should be inspected and repaired only by a qualified electrician. If you have any doubt about the operating status or safety of any electrical appliance, do not under any circumstance connect it to mains supply. As a rule, all appliances that require mains voltage to operate should be tested periodically by a qualified electrician. Check with your local electricity supply authority about how often these checks should be done.
Be careful! Mains electricity can kill!
Other pieces of equipment contain high vacuum tubes, such as television sets and microwave ovens.
Breaking the glass container that is evacuated can cause injury from flying glass.
Do not use exposed wires to connect a circuit.
Pay special attention to whether the leads of the electrical appliance discarded for a long time are exposed or ageing. You must wrap with electrical insulating tape or replace all exposed or ageing wires.
Check for damaged three pin plugs, exposed flex wire and exposed ends before the experiment.
3. Use only ammeters, voltmeters and power meters authorized for use in schools. Use only low voltage power packs up to 12 V. Check the circuit before connecting the last lead to the source of power, especially if an ammeter is in the circuit. Make the first connection to the source of power by switching on and off very quickly to check whether you have connected ammeters and voltmeters correctly with correct deflection and reading not off the scale.
4. Plug the three pin plug into a normal three pin socket. Do not change the pin and the socket.
5. Teachers should check all experiments involving electricity no matter the voltage before they allow students to energize circuits.
6. Never allow students to work unsupervised on electrical experiments.
7. Ensure that no other appliances are working before starting the test.
2.1.5 Heat insulation, properties of common materials
See diagram 23.1.5
Organization and guidance: Learn the function of the heat insulation of common materials. Find out which of the materials is the best insulator and probably help you when you are in state of emergency such as pouring a spoon of boiling porridge into your dinner pail made from stainless steel while you have dinner in your school dining room. The simplest method of doing so is to feel a heated thing insulated by these materials. However, distinguishing the degree of their heat insulation in detail is difficult. The following way could distinguish their heat insulation in detail and tell you the difference between their insulation. To do this, set up four big beakers and four small ones, as shown. Pour the same amount of hot water into each small beaker, then put each small beaker containing hot water into each big one. Select three kinds of heat insulators such as pieces of polyester plastics, pieces of papers and pieces of wood. Fill the space between a big beaker and small beaker with these materials. Compare the degree of this heat insulation by measuring the drop in temperature of the water in small beakers at the same time. The fourth large beaker contains only air, and it is a control, against which you can compare the other beakers. Controlling other variables to make a reliable comparison between them is necessary. For example, the water must be the same temperature in each beaker, the quantities of the materials filled in each beaker must be identical, the original temperature of the large beaker should be the same. Let the students think if there are other control factors in this experiment. Put a thermometer in each beaker and cover with a piece of paper. Record the temperature in each small beaker at one minute intervals. Keep doing this at least 10 minutes. You can judge which is the best heat insulating material according to these 10 data in each group. Plot a graph of temperature against time. Draw all three graphs on the one sheet of graph paper to see the conclusion clearly. Continue to repeat this procedure if you have more materials to distinguish.
2.2.2 Energy transfer between pendulums, by resonance
See diagram 15.4.12
Study how the time taken for energy transfer between pendulums depends on 1. the distance between hanging points of the pendulums and 2. the length of the pendulums.
Suspend a 100 cm strong string between two stands. Attach two threads 2.5 cm each side of the centre of the strong string. Attach 100 g weights to the end of each thread so that the length of the thread is 50 cm. Pull one weight to the side through a 60o angle to the vertical. While noting the time in seconds, release the weight so that it swings freely back and forth as a pendulum but does not touch the stationary second pendulum. The energy of the first pendulum transfers to the second pendulum. The first pendulum swings less until it stops swinging and the second pendulum swings more until it has the original swing of the first pendulum. Note the time when the first pendulum stops. The energy of the first pendulum transfers to the second pendulum. Note the time when the second pendulum stops. Note the times for five transformations of energy. Calculate the average time needed for one transformation of energy. Repeat the experiment by increasing the distance between the hanging points of the pendulums.
Repeat the experiment by shortening the length of the thread. Repeat the experiment by changing the initial angle of swing.
How does time of transfer depend on the following:
1. distance between pendulums,
2. length of pendulums,
3. original angle of swing of pendulums?
Note that the distance between pendulums affects the tension in the strong string.
Table 2.2.2
Experiment distance
d
length
l
angle
a
Transfer 1
(seconds)
Transfer 2
(seconds)
Transfer 3
(seconds)
Transfer 4
(seconds)
Transfer 5
(seconds)
Total
(seconds)
Average
(seconds)
1 (control) 4.5 50 60o . . . . . - -
4 (distance) 10 50 60o . . . . . . .
3 (length) 4.5 100 60o . . . . . . .
4 (angle) 4.5 50 30o . . . . . . .

2.2.3 Electric kettle heating efficiency
See diagram 32.2.3
Any kettle used to heat water can lose heat to its surroundings and to the materials from which it is constructed. The heat produced by the heat source does not only heat the water. You can measure the heat efficiency of an electric kettle by doing a simple experiment.
BE CAREFUL! Be sure that water cannot come into contact with the power supply. Some simple heating elements are bare wire and should not be used for this experiment! Do not operate an electric kettle with wet hands! Be sure that students and teachers cannot be scalded by steam.
1. Record the power rating of the heater element.
2. Measure and record the temperature of 500 mL of water and pour it into a kettle.
3. Switch on the power supply to the kettle and start timing how long it takes the kettle to bring the water to boil 4. Switch off the power supply when the water boils, and record the time it took for the water to come to boil.
5. Empty the kettle and allow the element to cool to room temperature then repeat steps 2, 3 and 4 and  find the average time to bring the water to boil.
To calculate the efficiency of the kettle you need to find how much energy the water absorbed to bring it to boiling point. Use the formula Q = mc (Tf - Ti), where m = mass of water, c = specific heat of water, Tf = final temperature and Ti = initial temperature. Then divide this value by the time it took to bring the water to boiling and you get the power consumed in boiling the water. Finally you divide this value by the power rating of the element to give the efficiency of the kettle.
The following example is based on a kitchen kettle with an element rating of 2,200 watts:
m = 0.5 kg, c = 4186 J / kgoC, Tf = 100oC, Ti = 22oC.
So Q = 0.5 × 4186 × (100 - 22) = 163,254 Joules
The time taken to bring the water to boil was 94 seconds.
Therefore the power consumed to boil the water = 163,254 / 94 = 1,737 Watts
To find the efficiency of the kettle divide the power used to boil the water by the power output of the element and multiply by 100 to give a percentage value, i.e. (1,737 / 2,200) × 100 = 79%
The efficiency of the kitchen kettle is 79%, or 21% of the power output is wasted.

2.2.4 Light bulb brightness, Joly photometer, wax block photometer
See 6.3.1.7: Luminous intensity, candela, cp | See diagram 28.2.4: Photometer
Electric energy can be transferred not only into light energy but also heat when light bulb works. So its efficiency can be expressed as the ratio of luminous intensity to consumed electric power. Light intensity at distance s from a light source varies inversely with the distance squared. It can be measured with a light meter or a photometer. If the light meter is calibrated to the size of camera length apertures, it is called an exposure meter.
1. Using a Joly photometer (wax block photometer)
It consists of two equal paraffin wax blocks separated by a thin opaque sheet. You can adjust the positions of two light sources to be compared until the two wax blocks appear equally bright. Also known as The Joly photometer is made from two identical blocks of paraffin wax, B1 and B4, about 5 mm thick, separated by a sheet of aluminium foil. Luminous sources of light, intensity I1 and I4, are placed each side of the blocks at distance S1 and S4 from the aluminium sheet, so that B1 receives illumination only from S1 and B4 receives illumination only from S4. By viewing from the side, i.e. in the plane of the aluminium sheet, the intensity of the diffused light from the paraffin blocks can be compared. If the photometer is moved between two light sources so that the light intensity seen in each block is the same, then. I1 / S14 = I4 / S44.

2.4.1 Make a photometer
See diagram 28.2.4
Use a rectangular cardboard box, e.g. a school chalk box. Cut four identical rectangular windows in the sides of the box. Make two paraffin blocks each 5 mm thick and half the area of the window so that the two blocks can just fit side by side in the window. Make sure that the upper and lower surfaces of the paraffin blocks are smooth. Cut a piece of flat aluminium foil the same size and shape as the paraffin blocks.
Fit it between the blocks and fit the blocks and foil into the window. Fix two globes in lamp holders each side of the box. One globe of known light intensity, e.g. 40 watt frosted bulb, luminous intensity about 32 candelas. The luminous intensity of the other globe is unknown. Darken the room and turn on the power for the two globes. Slide the photometer to a position where the two sides of the paraffin blocks are equally bright. Record the distances from the aluminium foil sheet to each globe.
If I1 = known intensity, e.g. 32 candelas and I4= unknown intensity then as I1 / S14 = I4 / S44, I4 =
(32 / S14) / S44.
2.2.6 Human reaction time
See diagram 9.249: Reaction time
If a body falls from a height s, the distance it falls after t seconds = gt2 / 2. So if you measure s, you can obtain t, t = √2S / g. Hold metre ruler vertically with the zero on the scale down and the 100 on the zero on the scale up. With your arm stretched horizontally, hold the ruler vertically between the thumb and first finger with the lower edge of the first finger at the zero on the scale. Open your fingers then close them again as quickly as possible to catch the ruler again. Record the distance to the downward edge of your first finger. Repeat the experiment and calculate the average distance down the metre stick. Use t =
√2S / g to calculate the time of the falling ruler, i.e. your reaction time. Repeat the experiment under the following pairs of conditions:
1. Hold the ruler first with your left hand
2. Hold the ruler first with your left hand
3. Talk to others while doing the experiment
4. Do not talk to others while doing the experiment
5. Do not allow loud background music
6. Do not allow loud background music.
Try other contrasting conditions to see whether your reaction time is affected. Compare results with the results of other students.