# experiments for higher physics - eyemouth high .cfe higher physics experiments 4 acceleration due

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CfE Higher Physics Experiments

1

CfE Higher Physics Experiments

Contents 1

Introduction to Uncertainties 2

g ball 3

Acceleration due to gravity using a single light gate 4

Acceleration and Angle of slope 5

F = ma 6

Explosions 7

2 coherent source loudspeaker interference 7

2 coherent source ripple tank 7

Determine using interference pattern 8

Snells Law 9

Critical Angle 10

Inverse Square Law 11

Determining Plancks Constant 12

AC peak and rms 13

Internal Resistance 14

Charge/discharge graphs for a capacitor 15

CfE Higher Physics Experiments

2

Introduction to Uncertainties

When carrying out any of these experiments it will be worth practicing dealing

with uncertainties. Even as a small exercise, becoming more familiar with this

small section will be of benefit, since many pupils find them challenging (and

tend to ignore them, in the hopes that they will go away)

Random uncertainties, which will arise in the taking of multiple readings and is

applied to an average is given by:

randomuncertainty =maximumvalue minimumvalue

numberofvalues

This value can be reduced by repeating your experiment several times.

For the readings that you take, make a note of the reading uncertainty in the

reading.

Analogue - take the reading uncertainty as a half of the smallest division, i.e. on a 30cm ruler this is usually half a millimetre, 0.0005 m.

Digital - take the reading uncertainty as one of the smallest division, i.e. on a voltmeter displaying 1.27 V this would be 0.01 V.

Systematic uncertainties, which arise from repeating the same measurement but

with a consistent error, i.e. a shrunken ruler.

You will need to convert uncertainties into percentage uncertainties in order to

carry them through to your final value.

percentageuncertainty =absoluteuncertainty

value 100

You will most likely apply the largest percentage uncertainty to your final

value, converting it back into an absolute uncertainty. At higher level this is

usually as complicated as it gets. Combining percentage uncertainties will play

a stronger role in Advanced Higher Physics.

CfE Higher Physics Experiments

3

g - ball

A g - ball is a specialised piece of Physics equipment which will time the duration of a fall. Combined with an accurate measurement of the distance travelled it can be used to determine g, the gravitational field strength. The theory here is that with initial velocity u = 0. We can use:

s = ut +

at s =

at

If we drop the ball from a range of heights we are able to measure a range of times. Repeats and averages help to increase the reliability of our results:

Initial Height (m)

Time taken to reach the ground (s)

1 2 3 average

0.2

0.4

0.6

0.8

1.0

1.2

By plotting the graph of falling distance against time taken squared, we should get a straight line through the origin and will find that the gradient can actually supply us with a, the acceleration due to gravity which is equal to g, the gravitational field strength. To do this, plot your results (s against time t2) and draw a straight line of best fit. Use this line (not just any 1 or 2 arbitrary points) to determine the gradient.

Theory

y = mx + c and s = ut +

at

now y = s x = t2 c = ut = 0 so m =

a

Doubling the gradient should result in a value for g. You should expect this to be approximately 9.8 ms-2

CfE Higher Physics Experiments

4

Acceleration due to gravity using a single light gate Similar to the g - ball experiment. A streamlined object of known length should be dropped from a known height through a light gate (held horizontally). The theory here is that with initial velocity u = 0. We can use:

v = u + 2as v = 2as

If we drop the ball from a range of heights we are able to measure a range of final velocities. Repeats and averages help to increase the reliability of our results:

Distance dropped (m)

Final velocity (ms-1)

1 2 3 average

0.2

0.4

0.6

0.8

1.0

1.2

By plotting the graph of falling distance against final velocity squared, we find that the gradient can actually supply us with a, the acceleration due to gravity which is equal to g, the gravitational field strength.

Theory

y = mx + c and v = u + 2as

now y = v2 x = s c = u2 = 0 so m = 2a Halving the gradient should result in a value for g. You should expect this to be approximately 9.8 ms-2

Alternative This experiment can also be done by measuring the time taken to break the light gate beam. Using the length of the object the student can then calculate the velocity themselves.

CfE Higher Physics Experiments

5

Acceleration and Angle of slope

A straightforward investigation looking at the relationship between the angle of a slope and the acceleration of a vehicle allowed to roll down the slope. For instantaneous acceleration a single light gate and double mask will be best. The interface can be set to calculate acceleration for you as long as you provide a mask length and both masks are of that same length.

Some predictions here. If the slope is at an angle of 0 then the vehicle cannot roll down it, acceleration will be zero. If the slope is at an angle of 90 then the vehicle will fall vertically downwards due to gravity, acceleration will be 9.8ms-2. We cannot logically or practically expect values out with this range! Dont be silly with the angles here, 45 is definitely too steep!

Angle of slope ()

Acceleration (ms-2)

1 2 3 average

2.5

5

7.5

10

12.5

15

Plot the relationship between angle (x-axis) and acceleration (y-axis). There is an angle present here. Is it worthwhile trying to plot the sine or cosine of the angle?

CfE Higher Physics Experiments

6

F = ma We have been using F = ma almost as long as we have been studying Physics. Now you should be able to show that it makes sense.

An air track is used to minimise surface friction. The unbalanced driving force (F) is provided by the hanging mass and can be given by W = mg. The total mass being accelerated is in fact given by (M + m) so be careful here! There are 2 experiments here:

1. Constant mass (M + m), changing force (mg). Transfer mass from M to m and record both mg and acceleration.

2. Constant Force (mg), changing mass (M + m). Decrease mass of M but keep m constant to maintain constant mg.

Quite tricky conceptually but Im sure you will get there. Carry out repeats and calculate averages. Record results as follows:

Driving force (W = mg) (N)

Acceleration (ms-2)

1 2 3 average

Mass (M + m) (kg)

Acceleration (ms-2)

1 2 3 average

Plot the two graphs, acceleration against driving force and acceleration against total mass (M + m). If you have made a hypothesis you should confirm that

1. acceleration is proportional to the driving force (in this case mg). 2. acceleration is inversely proportional to the total mass (M + m)

This leads to the observations that:

a F and a !

When the Newton is defined this simplifies to F = ma!

CfE Higher Physics Experiments

7

Explosions This is a simple version of a collisions experiment. Begin with 2 stationary trolleys with a loaded spring between them, place these trolleys between 2 light gates. The initial velocity of both vehicles is zero and so total initial momentum is zero. When the spring is released the trolleys will move in opposite directions with speeds determined by their masses. Use the light gates to measure these speeds. Now use your knowledge of the conservation of momentum to determine whether or not the final velocities (after the explosion) are sensible.

2 coherent source loudspeaker interference Set up 2 loudspeakers connected to the same signal generator. Place them a small distance apart and walk past them both listening very carefully for any changes in the sound that you hear. Compare this to the signal from a single speaker Repeat this for different frequencies and possibly different spacing between the speakers. Comment on what you observe. Can you explain it using your knowledge and understanding of waves and interference?

2 coherent source ripple tank Set up 2 bobs connected to the same motor above and suspend them in a ripple tank. Projecting light through the tank will cast shadows showing the positions and movements of waves. Compare a single source to the twin source. Sketch what you see. Can you explain the observation using your knowledge and understanding of waves and interference?

CfE Higher Physics Experiments

8

Determine using interference pattern

In this experiment an equation is again used to determine an unknown value. To improve the quality, reliability and certainty of this we use a graphing method to analyse the result. Set up a fairly standard laser and grating experiment

Take note of the line spacing of the grating, you may need to convert from lines per millimetre. To find the angle it may be easier to determine the distance between the central maximum and the first, second, third order maximum etc. This can then be used along with th

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