Download Experiments for Higher Physics

CfE Higher Physics Experiments
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 λ using interference pattern
8
Snell’s Law
9
Critical Angle
10
Inverse Square Law
11
Determining Planck’s Constant
12
AC peak and rms
13
Internal Resistance
14
Charge/discharge graphs for a capacitor
15
1
CfE Higher Physics Experiments
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
× 100
value
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.
2
CfE Higher Physics Experiments
“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)
0.2
0.4
0.6
0.8
1.0
1.2
1
Time taken to reach the ground (s)
2
3
average
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
now
y=s
x = t2 c = ut = 0
and
s = ut + at
so
Doubling the gradient should result in a value for g.
You should expect this to be approximately 9.8 ms-2
3
m= a
CfE Higher Physics Experiments
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)
0.2
0.4
0.6
0.8
1.0
1.2
1
Final velocity (ms-1)
2
3
average
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
now
y = v2 x = s
c = u2 = 0
and
v = u + 2as
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.
4
CfE Higher Physics Experiments
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!
Don’t be silly with the angles here, 45˚ is definitely too steep!
Angle of slope
(˚)
2.5
5
7.5
10
12.5
15
1
Acceleration (ms-2)
2
3
average
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?
5
CfE Higher Physics Experiments
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 I’m sure you will get there.
Carry out repeats and calculate averages. Record results as follows:
Driving force
(W = mg) (N)
1
Acceleration (ms-2)
2
3
average
1
Acceleration (ms-2)
2
3
average
↓
Mass
(M + m) (kg)
↓
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
When the Newton is defined this simplifies to F = ma!
6
a∝!
CfE Higher Physics Experiments
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?
7
CfE Higher Physics Experiments
Determine λ 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 θ 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 the distance
between the grating and the screen to find θ (see diagram below)
Record results in a table as shown:
Maximum m
-3
-2
-1
0
1
2
3
Equation:
d sin θ
mλ
∆x
θ
sin θ
0
0
0
Plot m as the x-axis, ranging from -3 to +3
Plot sin θ as the y-axis. Plot all 4 quadrants.
The resulting straight line through the origin should have a gradient =
given by:
gradient d
8
$
%
therefore the wavelength is
CfE Higher Physics Experiments
Snell’s Law
Using a ray box, rayzer kit or similar equipment, direct a single narrow beam towards the
centre of the flat edge of a D shaped block. Vary the angle of incidence from 0˚ (at the
normal) through to about 85˚, ensuring the light always enters the block at the centre of the
straight edge. Using a protractor measure the angles of incidence (θ1) and refraction (θ2).
Results recording
θ1 (˚)
0
5
10
15
↓
sin θ1
θ2 (˚)
0
sin θ2
↓
Data handling
You may try it but plotting θ1 and θ2 will not prove to be very fruitful as the result is not a
straight line. Plotting sin θ1 and sin θ2 will give you straight line. Depending on which you
choose to be the x and y axes then the gradient of the graph will either give the refractive
index, n or will give & from which you can then calculate the refractive index.
9
CfE Higher Physics Experiments
Critical Angle
Similar to the previous experiment. In this instance, direct the incident light at the curved
surface, so that it is not deflected bwhen entering the block, see diagram. Vary the angle of
incidence (this time inside the block) from 0˚ to 90˚, recording the angle of refraction as you
go.
Does anything unusual happen during your experiment? Refine your experiment and find the
angle at which this phenomenon happens. Record this angle of incidence.
From your notes you should know that this angle is the critical angle and that sin θ'
.
&
Use this equation and your value for the critical angle to determine the refractive index of the
block and compare to your result from the Snell’s Law experiment. Which method is more
rigorous?
10
CfE Higher Physics Experiments
Inverse Square Law
In this experiment the irradiance should be measured at a range of
distances from a light source.
This should be done in a dark room using as close to a point source as possible, avoid
extended sources.
A dedicated light sensor is not essential. A phototransistor connected to a voltmeter should
suffice where the reading on the voltmeter is not equal to but is proportional to the irradiance.
The irradiance can then be plotted against distance from the source. Students can attempt to
plot against % before finally plotting %( . Further analysis of the graph is then also possible.
Results
Distance from
source (m)
1
d
1
d
11
Irradiance
(or equivalent reading)
CfE Higher Physics Experiments
Determining Planck’s Constant
This involves several steps to complete and the analysis of several graphs.
Wavelength of LED light (rgb)
To do this use the interference method covered earlier in the course to λ for a red, green and
blue LED
Switch on voltage of LED’s (rgb)
Using a variable power supply, or variable resistor and fixed supply, monitor the current
through an LED as the voltage across it is increased. You should be able to plot the
following graph for a red, green and blue LED.
The switch on voltage should be taken as where the broken (traced back) line reaches the
Voltage axis. You should have 3 curves, one for each colour of LED.
Finding Planck’s Constant
Energy of photon:
Work done by electron:
E
W
hf
and
QV
+'
$
hc
λ
QV
V
E
1 hc
∙0 1
λ Q
Plotting switch on Voltage, V, as the y-axis and $ as the x-axis will give a gradient that is
equal to
23
electron.
4
, where h is Planck’s constant, c is the speed of light and Q is the charge on an
12
CfE Higher Physics Experiments
AC peak and rms
Set up a signal generator connected to an oscilloscope. Set the frequency and amplitude to
known values and try to create a trace on the screen where numerically the period (and
frequency) and amplitude of the wave are accurately displayed and measureable. This will
require you to think about using:
frequency
6789:%
and
V;7<=
√2V8!?
Use the timebase and voltage per division dials to do this (some fine tuning may be required.)
Now have someone else set the frequency and the amplitude of the signal generator to
unknown values. Then use only the oscilloscope to determine the new values of frequency
and Vpeak.
13
CfE Higher Physics Experiments
Internal Resistance
Set up the circuit as shown below.
Vary the resistance of the variable resistor and record a range of values for the voltage across
the cell (terminal potential difference) and the current through it. It is not essential to record
the value of the resistance in this case.
Plot the graph shown with your results and obtain your significant values as shown:
14
CfE Higher Physics Experiments
Charge/discharge graphs for a capacitor
In this experiment you should be able to record the behaviour of the voltage across an
inductor and the current passing through it, shortly after the circuit is switched “on” or “off”.
Set up the circuit as shown.
Monitor both the current and voltage readings on the meters every few seconds (data logging
equipment will make this easier, alternatively a large capacitance/resistance combination
should increase the duration of the experiment to allow for more readings to be taken). Plot
graphs of current and voltage against time.
15