Practicals_checked

Question 1

Practical 3.1.1d(i)
How do you determine the power and focal length of a lens?
What are the uncertainties in the power and f values?

Answer 1

You can determine the focal length of a thin converging lens by:

1. Setting up a small filament lamp as the source of rays, the object, and lens to display the image on an opaque screen,
2. Place the bulb at say, 0.200m, away from the lens i.e. u = -0.200m and turn on the power supply. Move the screen until you can see a clear picture of the filament on the screen.
3. measure the distance between the lens and the screen. This is v. Record u and v in a table.
4. Repeat the experiment at least 5 more times for a range of different u values [ensuring that you can always see an image]
5. Work out 1/u and 1/v for the data you have collected.
6. Plot 1 / v on the y-axis and 1 / u on the x-axis giving you a straight line, the y-intercept is the power 1 /f of the lens. the reciprocal of this will give you the focal length. you could also add 1/u and 1/v to get 1/f average the values for 1/f and reciprocate for the average power

The uncertainty in the image distance is much more likely to affect the overall uncertainty in power and f compared to object distance as it’s hard to tell when exactly the image is in focus

Question 2

3.1.1d(ii) Observing Polarisation using Microwaves

1. How do you this experiment?
2. Why is one grille needed?
3. Explain the results

Answer 2

1. You can investigate the polarisation of microwaves using a microwave transmitter and a microwave receiver linked to a voltmeter

• Place a metal grille between the microwave transmitter and receiver as shown in the diagram below - microwave transmitters transmit vertically polarised microwaves so you only need one metal grille.
• The intensity of microwaves passing through the grille is at a maximum when the direction of the vibration of the microwaves and the wires on the grille are at right angles to each other.
• As you rotate the grille, the intensity of polarised microwaves able to pass through the grille decreases, so the reading on the voltmeter decreases
• When the wires of the metal grille are aligned with the direction of the polarised waves, no signal will be shown on the voltmeter. The intensity drops to zero when the wires are aligned with the direction of polarisation of the microwaves, because the grille is absorbing their energy.

1. Only one grille is needed because the microwave transmitter emits microwaves that are already vertically plane-polariised
2. The grille’s electrons are excited by the microwaves vibrating electric field, the energy of the incoming microwaves is absorbed by the grille and re-emitted in all directions, but only a few of these are vibrating in the direction of the receiver.
   All energy is absorbed when they’re parallel, resulting in 0 intensity at receiver; even at 90 degrees, some electrons are still excited so there’s a small drop in intensity

Question 3

3.1.1d(ii) Observing Polarisation with Polarising Filter

Answer 3

You can observe polarisation by shining unpolarised white light through two polarising filters

1. Align the transmission axes of two polarising filters so that they are both vertical. Shine unpolarised light on the first filter. Keep the position of the first filter fixed and rotate the second one
2. Light that passes through the first filter will always be vertically polarised.
3. When the transmission axes of the two filters are aligned all of the light that passes through the first filter also passes through the second filter
4. As you rotate the second filter, the amount of light that passes through the second filter varies.
5. As the second filter is rotated, less light will get through it as all the vertical component of the second filter’s transmission axis decreases. This means the intensity of the light getting through the second filter will gradually decrease.
6. When the two transmission axes are at 45 deg to each other, the intensity will be half that getting through the first filter. When they’re at right angles to each other no light will pass through - the intensity is zero.
7. As you continue turning, the intensity should then begin to increase once again.
8. When the two axes realign [after 180deg rotation] all the light will be able to pass through the second filter again.
9. passing through varies, if you rotate through to 90 degrees, the intensity will reduce to 0

at 45 degrees to the first one, the intensity passing through the second filter will be exactly half the light getting through the first filter.

Question 4

3.1.2d(i) Investigating IV Characteristics for Ohmic and Non-Ohmic Components

1. What does circuit look like?
2. How do you carry out the investigation?
3. How do you reduce uncertainties?

Answer 4

1. Set up circuit as shown in the diagram below
2. The term IV characteristic refers to a graph which shows how the current, I, flowing through a component changes as the potential difference, V, across it increases.

• You can investigate the IV characteristic of a component using a test circuit like the one in the diagram below.. Use the variable resistor to alter the potential difference across the component and the current flowing through it, and record V and I
• Repeat your measurements and take averages to reduce the effect of random error on your results.
• Plot a graph of current against potential difference from your results. This is the IV characteristic of the component and you can use it to see how resistance changes.
• The shallower the gradient of a characteristic IV graph, the greater the resistance of the component
• A curved line shows that the resistance of the component changes with the potential difference across it.
• *Reducing Uncertainties**
• record current at each Voltages, 3 times to get a mean current, reducing effect of random errors
• make sure to turn off circuit after every reading to prevent heating of components which would affect results
• You can reduce uncertainties by using voltmeter and ammeter with higher resolution

Question 5

Finding resistivity (and conductance of a wire)

1. Describe a method to determine these.
2. How can you reduce uncertainties?

Answer 5

1. Set up circuit as shown, flying lead to be connected to a point 10cm along ruler when increasing L

• first measure diameter of wire at 3 different points to get a mean diameter, halve and find C.S.A
• The test wire should be clamped to a ruler with the circuit attached where the ruler reads zero.
• Attach flying lead to the test wire - the lead is just a wire with a crocodile clip [or pointed tip] at the end to allow connection to any point along the test wire.
• Record the length of the test wire connected in the circuit, the voltmeter reading and the ammeter reading
• Use these readings to calculate resistance of the length of the wire using R = V/I
• Repeat this measurement and calculate an average resistance for the length.
• Repeat for several different lengths.
• Plot the results on a graph of R against L and draw a line of best fit.
• The gradient of the line of best fit is equal to R/L = p/A So multiply the gradient of the line of best fit by cross-sectional area to find resistivity.
• The other components of the circuit also have a resistance but the gradient of the graph is not affected by the resistance within the rest of the circuit.
• To find conductivity, find the reciprocal of resistivity.

Reducing Uncertainties

• Must keep current constant to stop heating, can also disconnect wire to stop it from heating
• To find % uncertainty in R, add the % uncertainty’s for V and the % uncertainty for I
• %uncertainty for Area will be 2 * %uncertainty in diameter
• So overall %uncertainty in resistivity is adding %uncertainties for A and R

Question 6

Determining Internal Resistance of Cell

1. Describe the typical setup
2. Explain a method to determine the internal resistance
3. How can this method be improved

Answer 6

• Set up as shown
• Set variable resistor to max value
• Close switch and record values of current and potential difference, open switch between readings to prevent heating up of variable resistor
• Gradually decrease resistance of variable resistor and find new values of potential difference and current
• EMF= V + Ir, so V = -r(I) + EMF
  
  • (y = mx +c)
• plot graph of V against I.
  
  • gradient is -r
  • and the y-intercept is EMF

Improvements

Calibrate voltmeters and ammeters or use higher resolution ones