Practicals paper 3 - A level Physics Edexcel

What is the objective of core practical 1 'determine the acceleration of a freely-falling object'?

To measure the acceleration due to gravity g of an object falling freely and consider the alternative methods: a) object falling through a trap door and b) object falling through a light gate

What is the safety for the 'determine the acceleration of a freely-falling object' practical?

Ensure that any apparatus that might topple over is secure

Be aware of falling objects

For method B, turn off electromagnet when not in use

How do you work out the percentage uncertainty?

(uncertainty/mean value) x 100%

How do you calculate the percentage difference?

((experiment result - accepted value)/ accepted value) x 100%

What equipment do you need method A in the 'determine the acceleration of a freely-falling object' practical?

Metre ruler or tape with millimetre resolution

Steel sphere

Electronic timer

Electromagnet to retain steel sphere

Trap door

What equipment do you need method B in the 'determine the acceleration of a freely-falling object' practical?

Metre ruler or tape with millimetre resolution

Falling object, such as a 2cm dowel, 10cm long

Means to guide the dowel through the light gate

Light gate and datalogger

What is method A for the 'determine the acceleration of a freely-falling object' practical?

1) Drop the object from rest and record the time taken t for the sphere to fall to the trap door

2) Repeat the measurement for the sphere to fall to the trap door twice more and work out the mean value.

3) Measure and record the height h fallen by the sphere.

4) Repeat the timing of the drop as you vary the height; you should take at least 6 readings

5) Use half the range in your readings for t as the uncertainty in t. Calculate the percentage uncertainty in t.

What is method B for the 'determine the acceleration of a freely-falling object' practical?

1) Drop the object from rest and record the time taken t for the dowel to pass through the light gate

2) Repeat the measurement for the dowel to pass through the light gate twice more and work out the mean value.

3) Measure and record the height h fallen by the sphere.

4) Repeat the timing of the drop as you vary the height; you should take at least 6 readings

5) Use half the range in your readings for t as the uncertainty in t. Calculate the percentage uncertainty in t.

6) Measure the length of the dowel.

How do you analyse the results of method A in the 'determine the acceleration of a freely-falling object' practical?

t^2 (y axis) against h (x axis) and work out the gradient m of the line of best fit. Calculate a value for g where g=2/m.

How do you analyse the results of method B in the 'determine the acceleration of a freely-falling object' practical?

Use your value for the length of the dowel to calculate the mean speed v of the dowel as it passes through the light gate. Plot a graph of v^2 against h and work out the gradient m of the line of best fit. Calculate a value of g where g=m/2.

How do you do the percentage uncertainty for method A of the 'determine the acceleration of a freely-falling object' practical?

The percentage uncertainty in t^2 is twice that in t. Use this to draw on your plot's error bars - in the y direction only. You can use a typical mid-range value for calculating uncertainties and need not work out a separate error bar for each value. Draw further lines of fit to calculate the percentage uncertainty in your value for g.

How do you do the percentage difference in the 'determine the acceleration of a freely-falling object' practical?

Calculate the percentage difference between your value and the accepted value of 9.81ms^-2 and comment on the accuracy of your method.

What is important to remember when drawing the graph for the 'determine the acceleration of a freely-falling object' practical?

Ensure the points plotted on a graph take up more than half of the available space on both scales. You do not always need an origin on a graph.

Keep scales simple, one big square as 5,10 or 20 is ideal. One big square as 3 or 7 is very difficult to plot on and often leads to errors.

Always consider whether or not the graph line should go through the origin. Straight lines should be drawn with the aid of a ruler - one long enough to cover the full length of the line.

How is SUVAT related to method A of the 'determine the acceleration of a freely-falling object' practical?

s=ut+1/2at^2 where u=0,a=g,s=h

t^2=2h/g and comparison with y=mx+c shows that plotting t^2 against h should be a straight line passing through the origin with gradient 2/g

How is SUVAT related to method B of the 'determine the acceleration of a freely-falling object' practical?

v^2=u^2+2as where u=0, a=g, s=h

v^2=2gh and comparison with y=mx+c shows that plotting v^2 against h should be a straight line passing through the origin with gradient 2g.

Describe any advantage using light gates in the 'determine the acceleration of a freely-falling object' practical

There should be less uncertainty in the measurement of time

Discuss the affect of air resistance on the value of g in the 'determine the acceleration of a freely-falling object' practical?

g will have been reduced by air resistance

Explain why graph in the 'determine the acceleration of a freely-falling object' practical should be a straight a line

A straight line has a constant gradient. The line should be straight because the gradient depends only on g, which is constant.

What is the objective of core practical 2 'determine the electrical resistivity of a material' practical?

To use a metal wire to determine the resistivity of a metal

What is the safety for the 'determine the electrical resistivity of a material' practical?

Voltages and currents are small and so present no specific hazard

Disconnect wire between readings as it may get hot

Normal laboratory safety procedures should be followed

What is the equipment for the 'determine the electrical resistivity of a material' practical?

1.05m of 34swg constantan wire

metre ruler

2 leads, one with a crocodile clip on one end

digital multimeter switched to the lowest ohms scale with both the leads plugged in

micrometer screw gauge

What is the method for the 'determine the electrical resistivity of a material' practical?

1) Attach the crocodile clip to the wire at the 'zero' end of the metre ruler.

2) You will use the 4mm plug at the free end of the second lead to make contact with different places o the wire. You will record the resistance R at that point and the length l from the 'zero' end. Place the 4mm plug approximately 10cm from the 'zero' end and record R and l.

3) Move the plug along the wire in steps of 10cm and record R and l at each step.

4) Measure the diameter of the wire.

5) Estimate the uncertainties in your measurements.

In the 'determine the electrical resistivity of a material' practical, why do you need to press the plug firmly onto the wire to obtain a steady reading on the multimeter?

When two metals are in contact, an emf can be established. The area of contact between the clip and the wire might be quite small. The area of contact between the plug and the wire is very small.

What graph do you draw for the the 'determine the electrical resistivity of a material' practical?

Use the equation p=RA/l.

Plot a graph of R against l and measure the gradient m.

Use your value of m and the diameter d to determine a value for the resistivity p of the metal. Use p=mA where A is the cross-sectional area of the wire.

How do you do the uncertainties in the 'determine the electrical resistivity of a material' practical?

Determine a value for the uncertainty in your values for the gradient and A. Hence calculate the uncertainty in your value for the resistivity.

How do you do analysis of the 'determine the electrical resistivity of a material' practical beyond the graph?

Look up a value for p from two different sources; one should be online and one from elsewhere. Find the percentage difference between the sources and your value. Use the uncertainties you calculated to comment on your measurements.

Explain how you might change the apparatus in the 'determine the electrical resistivity of a material' practical to calculate your value for the resistivity with greater resolution

If you originally use a multimeter that reads to two significant figures, using a voltmeter and an ammeter that reads to three significant figures would provide greater resolution.

Explain why plotting a graph in the 'determine the electrical resistivity of a material' practical improves your accuracy

It makes it easier to spot anomalies and is a good, quick way of finding a weighted average

Explain why you need to use a wire in the 'determine the electrical resistivity of a material' practical and explain what shape of sample would be suitable for plastic

You use a wire because the resistance is measurable and not very small. For an insulator such as a plastic, the sample should be quite thick and fairly short so that the resistance is again measurable. This is difficult for a good insulating material.

Identify sources of uncertainty in the 'determine the electrical resistivity of a material' practical. Consider accuracy and comment on the effect of the uncertainties.

The three sources of uncertainty are: 1) contact resistance between wire and plug, 2) resistance between crocodile clip and wire at 'zero' end of wire, 3) Crocodile clip at 'zero' mark. These all shift the line of best fit up the graph and do not affect the gradient so the accuracy should not be affected by these.

Explain why the current in the wire in the 'determine the electrical resistivity of a material' practical should be small

The wire must not heat up as resistivity is temperature dependent.

What is the objective for the core practical 3 'determine the emf and internal resistance of an electrical cell'?

To make measurements using an electrical circuit

What is the safety for the 'determine the emf and internal resistance of an electrical cell' practical?

Voltages and currents are small so present no specific hazard. Normal laboratory safety procedures should be followed.

What equipment do you need for the 'determine the emf and internal resistance of an electrical cell' practical?

Electrical cell such as 1.5V

Resistor to act as internal resistance, labelled r in diagram

100 ohm variable resistor labelled R

voltmeter on 2V range

ammeter on 200mA range

6 leads to make electrical connections

What is the method for the 'determine the emf and internal resistance of an electrical cell' practical?

1) The cell and the resistor, labelled r, should be connected in series and used as a single circuit element. As a plan for this practical, draw a circuit that will connect the 100ohm variable resistor across the cell (and resistor r) and measure the pd V across R and the current I through it.

2) Have your circuit design checked by your teacher, set it up on the table and then get them to check it again before you begin.

3) Comment on how you will ensure the equipment is safely used and will continue to function properly.

4) Vary the resistance of the variable resistor and record values for V and I. Take readings for the whole range of the variable resistor.

How do you analyse the results of the 'determine the emf and internal resistance of an electrical cell' practical?

1) The mathematical model for the circuit is E-Ir=V

2) So V=-rI+E. A graph of V against I should give a straight line of gradient -r.

3) Measure the gradient of your graph and compare it with the manufacturer's value of the resistor r. Ensure you take account of the powers of 10 (prefix) in the measurement of current.

4) Justify the number of significant figures that you use in your answer.

5) Comment on the likely accuracy of your values for E and r.

Why is it unnecessary to repeat the readings from the circuit in the 'determine the emf and internal resistance of an electrical cell' practical?

There is very little random error and very little judgement is required when taking the readings. For this reason, you should take more than the usual six readings so you have more points on your graph. This will make it easier to recognise anomalies.

When the internal resistance is large in comparison to the external resistance, the terminal potential difference falls to a small value. This is used to make high voltage supplies safe for use in a laboratory. Explain how this makes the supply safe in the 'determine the emf and internal resistance of an electrical cell' practical.

When a human body is connected across the terminals the resistance is about 25kΩ. If the internal resistance of the supply is 5MΩ, the terminal pd falls to a low value with very little current flowing, making it safe.

In the 'determine the emf and internal resistance of an electrical cell' practical, it should not matter whether the voltmeter is connected across R or across the cell. This is partly because of the low resistance of the ammeter. Explain why.

There will be a very small potential difference across the ammeter. It is assumed to be so small that it can be ignored. This means that the potential difference across R and across the cell is the same.

The intercept of your graph in the 'determine the emf and internal resistance of an electrical cell' practical will be very close to the true value for the emf of the cell. Account for any difference.

The voltmeter does not have an infinite resistance. Any small current will cause a pd across the internal resistance, reducing the terminal pd below the emf.

Explain any difference between your value for r in the 'determine the emf and internal resistance of an electrical cell' practical and the manufacturer's value

The cell itself has an internal resistance but this is very small

What is the objective of core practical 4 'Use a falling-ball method to determine the viscosity of a liquid'?

To time the fall of a ball through washing-up liquid to determine the viscosity

What is the safety for the 'Use a falling-ball method to determine the viscosity of a liquid' practical?

Washing-up liquid spills are very slippery and must be cleared up at once. Have paper towels to hand. Normal laboratory safety procedures should be followed.

What equation do you use to find the viscosity in the 'Use a falling-ball method to determine the viscosity of a liquid' practical?

η= (2r^2g(p-σ))/9v where η is the viscosity of the liquid, r is the radius of the ball, p is the density of the ball, σ is the density of the liquid and v is the terminal velocity.

Which equation shows that the vector sum of the forces on the ball in the 'Use a falling-ball method to determine the viscosity of a liquid' practical is 0?

weight-drag-upthrust=0

What are the uncertainties like in the 'Use a falling-ball method to determine the viscosity of a liquid' practical?

The percentage uncertainty in r^2 is double the percentage uncertainty in r. The percentage uncertainties in the densities should be small enough to ignore.

What equipment do you need for the 'Use a falling-ball method to determine the viscosity of a liquid' practical?

long tube made of transparent material filled with washing-up liquid - supported so it stays vertical

spherical objects of various diameters

stop clock or timer

rubber bands to mark distances

metre ruler

micrometer screw gauge

What is the method for the 'Use a falling-ball method to determine the viscosity of a liquid' practical?

1) Weigh the balls, measure their radius r and hence calculate density p of the balls.

2) Place three rubber bands around the tube. The highest should be far enough below the surface of the liquid to ensure the ball is travelling at terminal velocity when it reaches this band. This is where the timer is started. The remaining two bands should be far enough apart to allow two reasonable time intervals to be measured. This will enable you to measure the terminal velocity twice for each ball.

3) Start the timer when the ball passes the highest rubber band. Use the lap timer facility to record the time taken t1 to fall to the middle rubber band and stop the timer when the ball passes the lowest rubber band, this is t2.

Adjust the position of the rubber bands if your first test is not suitable.

4) Once you are happy with the position of the rubber bands, measure the distance d1 between the highest and middle rubber band. Then, measure the distance d2 between the highest and lowest bands.

5) Repeat at least three times for balls of this diameter and three times for each different diameter.

How do you analyse the results of the 'Use a falling-ball method to determine the viscosity of a liquid' practical?

1) For each diameter, calculate the mean values for t1 and t2.

2) Use these mean values and the distances to calculate mean values for the terminal velocity of each ball. Repeat for all balls to obtain a mean value.

3) By considering the spread in your repeated readings, estimate the uncertainty of your mean values. This is usually half the range.

How can you reduce error in the 'Use a falling-ball method to determine the viscosity of a liquid' practical?

Position your eyes at the horizontal level of the rubber bands when starting and stopping the timer. You need to develop a good technique for measuring the time so that you are consistent. For example, if you read the time as the bottom of the ball crosses the top of the band, you should always do so. Make sure you also measure your distances from this point.

Why would it improve the 'Use a falling-ball method to determine the viscosity of a liquid' practical if you used a simple electromagnet to release the ball bearing?

If the ball can be released from rest in the liquid, the tube need not be so long. The highest rubber band can then almost be at the circuit.

How would you change the method for the 'Use a falling-ball method to determine the viscosity of a liquid' practical if the only tubes available are very short?

You can use two rubber bands and measure only one time interval.

Why should the tube used in the 'Use a falling-ball method to determine the viscosity of a liquid' practical have a large enough diameter that the ball does not move towards the wall?

Bernoulli's principle means that it would change the flow regime rendering Stoke's law an inappropriate model.

Explain why you would not use light gates in the 'Use a falling-ball method to determine the viscosity of a liquid' practical to measure time

The liquid is coloured and the low intensity of the light transmitted will make the timing unreliable. It is likely that the ball will not fall through the relatively narrow beam of a light gate.

In the 'Use a falling-ball method to determine the viscosity of a liquid' practical, sometimes the ball falls close to the wall. Comment on the effect that this will have on the measurements.

Bernoulli's principle means that, as the ball approaches the wall, the liquid is accelerated. Because of this, the pressure reduces and the ball moves closer to the wall making any effect more pronounced. The streamlines for the flow will no longer be symmetrical and Stoke's law is unlikely to be an appropriate mathematical model.

How would you use your uncertainty values in the 'Use a falling-ball method to determine the viscosity of a liquid' to estimate the uncertainty in your value for the viscosity of the washing up liquid?

Time is being measured at two intervals for each ball, it is possible to estimate an uncertainty for the terminal velocity of each ball. Using balls of the same diameter will allow a mean value to be found for the terminal velocity of balls of that diameter. Thus a viscosity can be calculated for each diameter of balls and the overall mean found using the value for each diameter. The terminal velocity is likely to be the major source of uncertainty since we are ignoring the uncertainty in the densities. You can compare the percentage differences between your values for viscosity for different diameters and the calculated uncertainty from your measurements.

What is the objective of core practical 5 'Determine the young modulus of a material'?

To take measurements of a long wire to determine the Young modulus for copper

What is the safety for the 'Determine the young modulus of a material' practical?

The wire will be under tension so safely spectacles should be worn.

Although the masses used are not too heavy, you should still take care when adding and removing them.

What are the three equations used in the 'Determine the young modulus of a material' practical?

E=σ/ε

σ=mg/A (m is the mass added)

ε=x/L

What equipment do you need for the 'Determine the young modulus of a material' practical?

3.1m length of 36swg copper wire

two wooden blocks and clamp to secure one end of the wire

bench pulley

slotted masses up to 600g and hanger

metre ruler

micrometer screw guage

small piece of sticky label or similar to mark position on wire

90 degrees set square

What is the method for the 'Determine the young modulus of a material' practical?

1) Fix the bench pulley at the end of the bench. Trap one end of the wire between the two wood blocks and secure these to the bench approximately 3m from the pulley. Lay out the wire so that it passes over the pulley and attach the slotted mass hanger to the end. Measure the diameter d of the wire.

2) Lay the metre ruler under the wire near the pulley and attach the sticky label to act as a length marker. You judge the length by looking vertically down over the edge of the paper onto the scale of the metre ruler. The set square will help you to do this.

3) Measure the length of wire L from the wood blocks to the edge of the paper.

4) Add masses to the hanger and record the position of the marker against the metre ruler. Calculate the extension x for each mass added.

5) You might notice significant creep occurring at higher loads. This indicates the elastic limit has been exceeded.

What is the analysis of the results for the 'Determine the young modulus of a material' practical?

1) Plot a graph of mass added against extension.

2) Measure the gradient of the straight portion of the graph and use this to calculate the Young modulus for the copper.

3) Research a value for the Young modulus of copper and comment on your result.

What measure of weights would allow more readings to be taken in the 'Determine the young modulus of a material' practical?

0.5N

Why is it important that the length of the wire in the 'Determine the young modulus of a material' practical is long?

The extension will only be small before creep sets in. A long wire makes the extension large enough to read. Since x=FL/AE, a large value for L and a small value for A make x sufficiently large to measure with a metre ruler.

Describe a good technique for measuring the diameter in the 'Determine the young modulus of a material' practical

Take pairs of readings at right angles to each other. This ensures the wire is circular. Repeat these readings down the length of the wire: five times should be sufficient. Calculate the mean of all 10 values.

Explain why a value with two significant figures is suitable for the answer for the 'Determine the young modulus of a material' practical

The extension is very small and, using a metre ruler, we get only one significant figure. Plotting the mass against extension for the gradient means that we might trust two significant figures although one significant figure is strictly correct.

What is the objective of core practical 6 'Determine the speed of sound in air using a 2-beam oscilloscope, signal generator, speaker and microphone' ?

To use appropriate instrumentation to measure sound signal

What is the safety for the 'Determine the speed of sound in air using a 2-beam oscilloscope, signal generator, speaker and microphone' practical?

The electromotive forces are small and electric currents negligible

The sound should not need to be so loud that any discomfort is felt

Follow the usual electrical precautions for mains apparatus including a visual inspection of the supply lead

What are the equations used for the 'Determine the speed of sound in air using a 2-beam oscilloscope, signal generator, speaker and microphone' practical?

v=fλ and f=1/T where T is the period of one oscillation. You can measure this from the oscilloscope screen.

What equipment do you need for the 'Determine the speed of sound in air using a 2-beam oscilloscope, signal generator, speaker and microphone' practical?

signal generator with loudspeaker

oscilloscope with 2-beam facility and microphone connected to one input

2 metre rulers

leads

What does the display look like on the oscilloscope in the 'Determine the speed of sound in air using a 2-beam oscilloscope, signal generator, speaker and microphone' practical?

The oscilloscope will display two traces the signal fed to the loudspeaker and the signal is received by the microphone. As the distance between the microphone and the speaker is increased, the phase of the signals varies and the traces on the screen move past each other.

What is the method for the 'Determine the speed of sound in air using a 2-beam oscilloscope, signal generator, speaker and microphone' practical?

1) Place the microphone next to the oscilloscope and place the speaker about 50 cm away, facing the microphone. Turn on the signal generator and set it to about 4 kHz. Adjust the oscilloscope to show the microphone signal with about three cycles on the screen.

2) Connect the signal generator output to the second oscilloscope input (as well as the speaker) and adjust the controls to display three cycles of this signal.

3) Adjust the spacing on the screen and the distance between the speaker and microphone so that the bottom of one trace is just level with the top of the other.

4) Adjust the separation so that a trough on the top trace exactly coincides with a peak on the lower trace. Place the metre ruler alongside the microphone and speaker and record the

distance between the microphone and speaker.

5) Move the speaker away from the microphone and observe one trace sliding over the other.

Move the speaker so that the trace has moved exactly one cycle. The troughs and peaks

should just touch again. Record the new distance between the microphone and speaker. The

difference between the two distances is one wavelength.

6) Continue to move the speaker away from the microphone and record each successive distance

where the peaks of one trace coincide with the troughs of the other.

7) Calculate a mean value for the wavelength of the sound and estimate the uncertainty in this measurement.

8) Use one of the traces to determine the frequency of the sound. You will achieve a greater resolution this way than using the scale on the signal generator. Estimate the uncertainty in this measurement. You should be able to measure the frequency to three significant figures and the

wavelength to at least two.

How do you take repeats for the 'Determine the speed of sound in air using a 2-beam oscilloscope, signal generator, speaker and microphone' practical?

Using the scale on the signal generator, halve the frequency and repeat the measurements for frequency and wavelength. You might need to increase the separation beyond 1 m. You might try this experiment at much higher and lower frequencies to observe the effect.

How do you analyse the results of the 'Determine the speed of sound in air using a 2-beam oscilloscope, signal generator, speaker and microphone' practical?

1) Multiply your values for wavelength and frequency to obtain a value for the speed of sound for each of the frequencies used. Hence, find a mean and percentage difference.

2) Use the uncertainties from your measurements to calculate a percentage uncertainty in your

individual values for the speed of sound.

Comment of the sources of uncertainty in the 'Determine the speed of sound in air using a 2-beam oscilloscope, signal generator, speaker and microphone' practical

The trace on the screen can be quite thick and so there is some uncertainty about exactly

where to place the speaker each time. Similarly, the resolution of the screen measurement for the frequency is about 2 mm.

What will the percentage difference and uncertainties be like for the 'Determine the speed of sound in air using a 2-beam oscilloscope, signal generator, speaker and microphone' practical?

The percentage difference might well be very small if the measurements are accurate. The

percentage uncertainties are likely to be about 2% each. So this looks like an accurate result

and is likely to be close to the true value on the day.

In the 'Determine the speed of sound in air using a 2-beam oscilloscope, signal generator, speaker and microphone' practical, . When the traces slide one cycle past each other, the speaker has moved one wavelength. Explain this.

Since the x-axis displays time, we can tell that the sound is taking one extra period to travel

from the speaker to the microphone. The distance travelled in one period is the wavelength.

Why is 4kHz a suitable frequency for the 'Determine the speed of sound in air using a 2-beam oscilloscope, signal generator, speaker and microphone' practical?

We need a wavelength that can be measured to a good resolution using a metre ruler.

What are the objectives for core practical 7 'Investigate the effects of length, tension and mass per unit length on the frequency of a vibrating string or wire'?

To carry out an investigation into standing waves

To develop the skills to carry out further investigations

What is the safety for the 'Investigate the effects of length, tension and mass per unit length on the frequency of a vibrating string or wire' practical?

There are no hazards associated with this experiment if rubber is used as the medium. If using metal wire safety spectacles should be worn. Follow the usual electrical precautions for mains apparatus including a visual inspection of the

supply lead.

What are the equations for the 'Investigate the effects of length, tension and mass per unit length on the frequency of a vibrating string or wire' practical?

v=fλ , v^2=T/μ , f^2λ^2=T/μ

What is the equipment for the 'Investigate the effects of length, tension and mass per unit length on the frequency of a vibrating string or wire' practical?

2 m length of rubber 'string'

vibration generator connected to a signal generator

bench pulley

slotted masses and hanger

metre ruler

What is the method to find the effect of tension on frequency of a wave on a string for the 'Investigate the effects of length, tension and mass per unit length on the frequency of a vibrating string or wire' practical?

1) Attach 10g of mass to the end of the pulley and calculated tension as followed: Tension= mass added x g.

2) Switch on transducer and increase frequency until first harmonic is formed.

3) First harmonic is observed when there is a standing wave on the string with 2 nodes and 1 antinode using wavelength = twice the length of the string.

4) Using timebase on oscilloscope, find frequency of first harmonic (1/T) - record the frequency

5) Increase the amount of mass up to 100g in 10g increments, finding the first harmonic, frequency and tension each time

6) Plot frequency against tension and frequency against square root tension to investigate the relationship between the two variables

What is the method to find the effect of μ on frequency of a wave on a string for the 'Investigate the effects of length, tension and mass per unit length on the frequency of a vibrating string or wire' practical?

1) Measure the mass and length of the strings between transducer and pulley using a metre ruler and a mass balance

2) Calculate the mass per unit length, μ; μ=mass of string/ length of string

3) Keep 10g of mass on the end of the pulley (keep tension constant) and the length of the string constant

4) Change the string's mass per unit length by using a thicker string or different material

5) Plot frequency against μ and draw line of best fit to determine the relationship between the two variables.

What is the method to find the effect of the length on frequency of a wave on a string for the 'Investigate the effects of length, tension and mass per unit length on the frequency of a vibrating string or wire' practical?

1) Measure the length of the string between transducer and pulley using a metre ruler

2) Keep 10g of mass on the end of the pulley (keep tension constant)

3) Change length of string between pulley and transducer (use same string to keep μ constant)

4) Record and plot fundamental frequency against length to find the effect of length on fundamental frequency

What would make your measurement in the 'Investigate the effects of length, tension and mass per unit length on the frequency of a vibrating string or wire' practical have a greater resolution?

If you measure as large a length as possible, or as many half-wavelengths as possible

Identify the main sources of uncertainty in the 'Investigate the effects of length, tension and mass per unit length on the frequency of a vibrating string or wire' practical

The measurement of a resonant frequency unless the calibration of the signal generator is accurate. Measuring the wavelength is also uncertain as a thick blur is observed at the nodes.

Describe what is difficult to get right in the 'Investigate the effects of length, tension and mass per unit length on the frequency of a vibrating string or wire' practical and how to get it right

It will probably be the sharpness of resonance that causes the biggest problem. Adjusting the

frequency while looking closely at a node is a technique to gain the largest response. Looking

at the amplitude is probably less helpful.

How can a standing wave be set up and used to determine a value for the speed of electromagnetic radiation

Using resonant cavities for lasers and radio frequency waves

What is the objective for core practical 8 'Determine the wavelength of light from a laser or other light source using a diffraction grating'?

To make measurements of laser light passing through a diffraction grating to determine the wavelength of the light.

What is the safety for the 'Determine the wavelength of light from a laser or other light source using a diffraction grating' practical?

Lasers should be Class 2 and have a maximum output of 1 mW. They present little risk in the laboratory,

providing sensible precautions are taken to reduce the risk of the accidental passage of light into eyes. Follow the usual electrical precautions for mains apparatus, including a visual inspection of the supply lead.

What equations do you need for the 'Determine the wavelength of light from a laser or other light source using a diffraction grating' practical?

nλ=dsinθ, where sinθ can be derived from tanθ=s/D and s is the distance between maxima, d is the distance between adjacent slits so d=1/N where N is the number of slits per metre

What equipment do you need for the 'Determine the wavelength of light from a laser or other light source using a diffraction grating' practical?

Source of laser light

Diffraction grating, supported at the same height as the laser beam

Metre ruler

What is the method for the 'Determine the wavelength of light from a laser or other light source using a diffraction grating' practical?

1) Place the laser approximately 4 m away from a large wall and place the diffraction grating in front of it. Arrange for the beam to pass through the grating at normal incidence and meet the wall perpendicularly.

2) Measure the distance D between the grating and the wall.

3) Turn on the laser and identify the zero order maximum (straight through). Measure the distance

s to the nearest two first order maxima. Calculate the mean of these two values. The first order is the maximum produced according to n = 1 in the equation nλ = d sin θ

4) Measure s for increasing orders.

5) Repeat for a diffraction grating with a different number of slits/mm.

How do you analyse the results of the 'Determine the wavelength of light from a laser or other light source using a diffraction grating' practical?

1) Since the angle is not small, you will have to calculate θ from your measurements for s and D. Calculate a mean θ value for each order.

2) Calculate a mean value for the wavelength of the laser light and compare your value with the accepted wavelength (of 635 nm for the standard school red laser).

How dangerous are the lasers in the 'Determine the wavelength of light from a laser or other light source using a diffraction grating' practical?

Care should be taken that

reflected laser light is unlikely to

enter any eyes although lasers

are now thought less harmful

than was once the case. Normal

reflexes, such as blinking and

turning away, are most helpful if

laser light of this strength does

enter the eye.

State the advantages of using laser light in the 'Determine the wavelength of light from a laser or other light source using a diffraction grating' practical

Laser light is monochromatic so the maximum is small. It is also coherent, so the maximum is produced without further focussing. The light is bright and a large value for D can be used without the maxima becoming too dim to see properly.

Explain why a metre ruler is suitable for measuring the distance in the 'Determine the wavelength of light from a laser or other light source using a diffraction grating' practical

A resolution of 1 mm is suitable when measuring distances of around 70 cm for the 300 slits/mm grating and around 25 cm for the 100 slits/mm grating. This gives percentage uncertainties of 0.1% and 0.4% respectively.

Suppose that a white light laser were possible. Describe what the diffraction maxima would look

like.

Since white light is a mixture of frequencies, the diffraction would be different for each frequency. The maxima are small so, in practice, the inner edge would be blue-ish and the

outer edge red-ish.

What is the objective for core practical 9 'Investigate the relationship between the force exerted on an object and its change of momentum'?

To determine the momentum change of a trolley when a force acts on it as a function of time