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Required practicals 8 & 10

AQA GCSE Physics · Topic 4.6 Waves · 4.6.1.2 and 4.6.2.2

Waves and Infrared Required Practicals: Ripple Tank, Metal Bar and Leslie Cube

In this lesson you'll learn to: measure the wavelength and frequency of ripples and work out their speed with v = f × λ, measure the speed of sound in a metal bar with an oscilloscope and v = s ÷ t, and use a Leslie cube to show which surfaces emit the most infrared, with the control variables and the conclusion examiners want.

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Start here — do the practicals

Don't read anything yet. On tab 1, take a photo to measure the wavelength, then film the ripples in slow motion and count how many pass the red tape in one second. On tab 2, hit the bar and read the oscilloscope (try every time-base). On tab 3, fill the Leslie cube, wait, and test all four faces at the same distance.

Vibration generator

Your measurements

Wavelength: not measured yet. Take a photo.

Frequency: not measured yet. Film it in slow motion.

Live, the waves are too close together and too quick to measure. A photo freezes them so you can lay a ruler along them. Slow motion lets you count them going past the red tape.

Face towards the detector

Distance

Results: rank the surfaces (1 = most infrared)

surface	distance	reading	rank

Physics with Kate · physicswithkate.com

▲ Three tabs: the ripple tank, the metal bar and the Leslie cube

You've done all three. Now learn to write them up.

The notes below give each method step by step, the variables, the safety points and the conclusions, the way a 6-mark method question wants them.

Read the notes ↓

✎

Revision notes

These are two of the AQA required practicals. Both are on the Combined Science and the Physics courses, and the exam can ask you to describe the method, spot a control variable, or do the calculation.

Required practical 8a: the speed of waves in a ripple tank

A vibration generator moves a dipper up and down in the water and makes straight ripples. A lamp above the tank shines through the water, and the ripples cast bright and dark lines on a card (screen) underneath. The bright lines are the crests.

The waves are very close together and move quickly, so you can't measure them by eye.

📷

Wavelength λ

Put a ruler beside the shadow pattern and take a photo, which freezes the waves. Measure the length of 10 waves and divide by 10.

Why 10? One wave is too short to measure accurately. Ten waves ÷ 10 is more accurate.

🎥

Frequency f

Stick a piece of red tape on the side of the tank as a marker. Film in slow motion with a stop clock in shot, play it back and count how many waves pass the tape in 1 second.

Waves passing a point in 1 s is the frequency, so you don't need the time period.

Then v = f × λ, with λ changed from cm to m. The workbook does it a second way: time how long 10 waves take to pass the point, divide by 10 to get the period T, then f = 1 ÷ T. Both give the same answer.

Required practical 8b: the speed of sound in a solid

Rest a metal bar on two supports and measure its length with a metre ruler.
Put a microphone at one end, connected to an oscilloscope.
Hit that end with a hammer. The oscilloscope shows a pulse (a blip) the moment the bar is hit.
The sound travels to the far end, reflects and comes back. A second, smaller blip appears when it reaches the microphone. It's smaller because some energy is lost on the way.
Count the divisions between the two blips and multiply by the time-base (time per division) to get the time.
The sound went there and back, so distance = 2 × length. Then v = distance ÷ time.

Measure from peak to peak. The echo is always smaller than the first blip.

⭐ Choosing the time-base

Pick a time-base that puts both blips on the screen and as far apart as possible. Too big and the blips crowd together and are hard to read. Too small and the echo runs off the screen.

Required practical 10: infrared radiated by different surfaces

A Leslie cube is a hollow metal cube whose four sides have different surfaces: matt black, shiny black, matt white and shiny silver.

Fill the cube with boiling water and put a thermometer in it.
Wait a minute or so. Metal is a good conductor, so every face reaches the same temperature as the water. That's what makes it a fair test.
Point an infrared detector at one face from a measured distance and record the reading.
Turn the cube and measure each face from the same distance. Work quickly, because the water cools.

Variable	In the Leslie cube practical
Independent (you change it)	the type of surface
Dependent (you measure it)	the infrared detector reading
Control (keep the same)	the distance from the detector to each face; the temperature of the water (every face at the same temperature)

⭐ The conclusion

Matt black surfaces are the best emitters of infrared. Shiny silver surfaces are the worst (light shiny surfaces are poor emitters).

A good emitter is also a good absorber. So matt black is also the best absorber of infrared, and shiny silver is the best reflector.

Staying safe

Hazard	What to do
Water near the lamp and the vibration generator's mains supply	keep electrics away from the water and mop up spills straight away
Boiling water in the Leslie cube	pour carefully, keep the cube on a heat-proof mat and don't touch the faces
Hammer and a heavy metal bar	tap the end gently and make sure the bar can't roll off the supports

✍

How to write it in the exam

Describing a method for 6 marks

Name the equipment → say what you measure and how (repeat it, measure 10 and divide) → say what you keep the same and how you calculate the answer.

the apparatusthe measurementscontrol variables and the equation

Workbook page 7 · required practical 8a · the ripple tank

The apparatus, labelled.

Results: 10 waves took 2.1 s, 1.9 s and 2.0 s to pass a point. 10 waves measured 15.0 cm. Calculate the wave speed.

Mean time for 10 waves = (2.1 + 1.9 + 2.0) ÷ 3 = 2.0 s
Time period T = 2.0 ÷ 10 = 0.20 s
Frequency f = 1 ÷ T = 1 ÷ 0.20 = 5 Hz (5 waves pass the tape every second)
Wavelength λ = 15.0 ÷ 10 = 1.5 cm = 0.015 m
v = f × λ = 5 × 0.015 = 0.075 m/s

Why time 10 waves, and measure 10 waves? One wave is too quick and too short to measure accurately. Measuring 10 and dividing by 10 reduces the uncertainty.

Workbook page 7 · required practical 8b · the metal bar

Hammer, metal bar, microphone and oscilloscope.

The bar is 1.00 m long. The blips are 4 divisions apart and the time-base is 0.1 ms per division. Calculate the speed of sound in the metal.

Time = 4 × 0.1 ms = 0.4 ms = 0.0004 s
Distance there and back = 1.00 × 2 = 2.00 m
v = distance ÷ time = 2.00 ÷ 0.0004 = 5000 m/s

The first blip is the hammer hitting the bar. The second blip is the sound returning to the microphone after reflecting off the far end.

Workbook page 7 · required practical 10 · the Leslie cube

Leslie cube, thermometer and infrared detector.

Readings: matt black 92, shiny black 76, matt white 40, shiny silver 12. Rank them and write a conclusion.

Ranks: matt black 1, shiny black 2, matt white 3, shiny silver 4
Every face is at the same temperature because the cube is metal, a good conductor, full of hot water.
Matt black surfaces are the best emitters of infrared. Shiny silver surfaces are the worst.
A good emitter is also a good absorber of infrared.
Control variables: the distance from the detector to each face, and the temperature of the water (take each reading quickly).

Apply it · workbook page 7

A teapot keeps tea hot for longest if its outside is shiny silver, because that surface is a poor emitter of infrared.
Solar water-heating panels are painted matt black so they absorb as much infrared as possible.

Exam question · describe a method · 6 marks

Describe how a student could measure the speed of water waves in a ripple tank.

Switch on the vibration generator to make waves, and the lamp to cast their shadows on a card under the tank.
Place a ruler beside the shadows and take a photo. Measure the length of 10 waves and divide by 10 to find the wavelength.
Put a marker (red tape) on the side of the tank. Film the waves in slow motion with a stop clock in shot.
Count the number of waves that pass the marker in 1 second. This is the frequency.
Repeat and take a mean.
Convert the wavelength to metres and calculate the speed using v = f × λ.

Check your answer has all 6 parts: equipment · wavelength from 10 waves · marker · frequency from a count in 1 s · repeat and mean · v = f × λ.

📘 Now do it in your workbook

Page 7: label all three sets of apparatus, calculate the speed of the ripples and of sound in a metal bar, rank the four surfaces and write the conclusion, then apply it to a teapot and a solar panel.

AQA GCSE Physics Workbook · WAVES · page 7

⬇ Free: page 7 (PDF) 📄 Full workbook PDF · £2.99 Paperback on Amazon

The full workbook covers the whole of Topic 4.6 Waves, with exam-style questions and worked answers.