Module 4 Practicals

Question 1

Investigating the electrical characteristics for a range of ohmic and non-ohmic components
- Method (4)
- Graphs and Calculations (1)
- Safety (1)
- Validity (5)

Answer 1

• Set up a circuit that includes a power supply, a variable resistor, an ammeter and the component of interest all in series, as well as a voltmeter across the component
• Vary the voltage across the component by changing the resistance of the variable resistor
• For each voltage, record the current 3 times and calculate the mean
• Repeat this for different components
• Plot a graph of mean current against voltage for each component
• Be careful as the components may heat up
• Switch off the circuit in between readings to prevent heating of components
• Use a wide range of voltages
• Control the equipment used and the temperature of the area
• To reduce uncertainty take more readings at more voltages
• To reduce uncertainty use ammeters and voltmeters with greater resolution

Question 2

Determine the resistivity of a wire
- Method (4)
- Graphs and Calculations (2)
- Safety (2)
- Validity (1)

Answer 2

• Measure the diameter of the wire using a micrometer
• • Measure at 3 different points and calculate a mean
• Attach the wire to a circuit with a power supply, an ammeter and a voltmeter across the wire
• Adjust the length of the wire, measuring the current and voltage each time
• Repeat the experiment twice more and calculate a mean resistance for each length
• Calculate the cross sectional area of the wire
• Plot a graph of resistance against length
• • Gradient = p/A (R=pL/A)
• Wear safety goggles in case wire snaps
• Disconnect crocodile clips between measurements to avoid them heating up
• Switch circuit off in between reading to avoid heating components

Question 3

Determining the internal resistance
- Method (3)
- Graphs and Calculations (1)
- Validity (1)

Answer 3

• Set up a circuit which includes a power supply, a variable resistor, an ammeter, a switch and a voltmeter across the power supply
• Set variable resistor to max value and measure current and voltage across power supply
• Adjust variable resistor and repeat
• Plot graph of V against I
• • V= -rI + E
• Open the switch between readings to prevent heating of the variable resistor

Question 4

Determining the maximum power of a cell
- Method (3)
- Graphs and Calculations (3)
- Safety (1)
- Validity (1)

Answer 4

• Set up a circuit which includes a power supply, a variable resistor, an ammeter, a switch and a voltmeter across the power supply
• Set variable resistor to max value and measure current and voltage across power supply
• Adjust variable resistor and repeat
• Calculate the power supplied by the battery for each reading using P=IV
• Calculate the resistance for each reading using R=V/I
• Plot a graph of power against resistance
• • It should have an arched shape in which the peak of the arch shows the maximum power of the cell
• • This peak occurs when the variable’s resistor is equal to the internal resistance of the cell
• Don’t use a high voltage as components could become too hot
• Use a range of resistances on the variable resistor to see the trend in the graph more easily

Question 5

Investigating circuits with more than one source of emf
- Method (4)
- Graphs and Calculations (2)
- Safety (3)
- Validity (1)

Answer 5

• Set up the first circuit containing two cells in series, a resistor and a voltmeter over the resistor
• Record the reading on the voltmeter
• • As the resistor is the only component this will be the potential difference supplied by the cells
• Set up the second circuit the same as the first, except place the two cells in parallel and repeat
• Repeat the entire experiment with cells of different voltages
• Calculate the expected combined potential difference for each combination
• • Series: emf = sum of emfs
• • Parallel: emf = the same across each branch
• Compare the theoretical combined potential difference and the actual p.d.
• Do not connect 2 cells of different voltages in parallel as this could lead to overheating and sparks
• Do not use high voltage cells to minimise risk of electrocution
• The resistor may get hot so be careful when touching it
• There will be some error in the value for the overall emf due to internal resistance

Question 6

Using non-ohmic devices as sensors
- Method for LDR (4)
- Method for Thermistor (6)
- Graphs and Calculations (1)
- Safety (2)
- Validity (1)

Answer 6

• Set up a circuit containing a power supply, an LDR, and a fixed resistor all in series as well as a voltmeter across the fixed resistor
• Record the voltage across the resistor for the initial light intensity
• Increase the light intensity and record the new values for light intensity and voltage
• Repeat until light intensity cannot be increased any further
• Set up a circuit containing a power supply, a waterproof thermistor, and a fixed resistor all in series as well as a voltmeter across the fixed resistor
• Place the thermistor into boiling water
• Measure the temperature of the water using a thermometer
• Record the voltage across the resistor for the initial temperature
• Place some ice in the water
• Record the voltages and temperatures for every 5 degrees decrease in temperature
• Plot a graph of voltage against light intensity/temperature
• Careful not to trip when room is dark
• Be careful when handling boiling water
• Make sure the surrounding area is as dark as possible

Question 7

Using an oscilloscope to determine the frequency and amplitude of a wave
- Method (3)
- Graphs and Calculations (1)
- Validity (3)

Answer 7

• Connect a microphone to an oscilloscope and play a note into the microphone
• Read off the time base the oscilloscope is set to (the scale of the x axis)
• Find the amplitude of the wave by reading off the distance from the middle line to the highest point on the wave
• Determine the frequency of the wave using f = 1/T where T = distance between peaks x time base setting
• Each horizontal division on the screen represents a unit of time
• The time base control varies the seconds/milliseconds per division
• You can reduce the uncertainty in the frequency measurement by altering the time base such that one full wave has the widest possible range in the x direction

Question 8

Observing polarising effects using microwaves and light
- Method for Light (3)
- Method for Microwaves (3)
- Safety (2)
- Validity (1)

Answer 8

• Hold a polarising filter up to eye level
• Place another filter behind it and rotate it
• Observe that when the filters are perpendicular, no light gets through
• Place a vertically aligned metal grille in front of the transmitter (which transmits vertically plane polarised waves)
• Place a detector behind the grille
• Place another grille behind the first grille and rotate it, observing whether the detector records any microwaves as the position of the grille changes
• Do not look directly into a bright light
• Microwaves can cause burns if their intensity is too high so don’t stand in front of the transmitter when it is on
• Make sure the detector is working and the waves are vertically aligned by turning on the transmitter and checking the detector receives the waves with and without the detector

Question 9

Investigating refraction and total internal reflection of light
- Method for Semi Circular Block (4)
- Method for Rectangular Block (6)
- Graphs and Calculations (1)
- Safety (1)
- Validity (2)

Answer 9

• Place the block on a piece of paper and draw around it
• Make a mark at the centre of the straight edge and draw a normal from the point
• Shine a ray of light from a ray box towards the curved edge of the block in an arc until the ray emerges along the surface of the boundary between the material and air
• Measure the angle the ray makes with the normal using a protractor - this is the critical angle
• Place the block on a piece of paper and draw in pencil around it
• Lift the block and draw a normal line on the piece of paper
• Place the block back on the page and aim the beam at the point where the normal line meets the block
• Trace the beal line entering and exiting the block
• Measure the angle of incidence (angle between the incident ray and the normal) and the angle of refraction (angle between refracted ray and the normal)
• Repeat this rotating the beam by 10 degrees each time
• Calculate the refractive indexes of both blocks
• • Semi circular use: n = 1/sin C
• • Rectangular use: n = sin i/sin r
• Don’t shine the light at anyone or at reflective surfaces
• Ensure the beam is directed towards the same point each time
• Use a thinner laser for more accurate results

Question 10

Investigating refraction and reflection in a ripple tank
- Method (3)

Answer 10

• Use a motorised straight edged bar to produce plane waves while a small dipper produced circular waves
• Shine a light from above so that the wave pattern can be seen below the tank
• From the shadow, measure the wavelength of the water waves and investigate the angles of reflection and refraction

Question 11

Investigating superposition using microwaves
- Method (5)
- Graphs and Calculations (2)
- Safety (1)
- Validity (2)

Answer 11

• Remove the turntable wheels from the microwave so that the plate is stationary
• Place the chocolate on the turntable and place it back in the microwave
• Turn on the microwaves for 30 seconds
• Take the chocolate out and observe the pattern of melted and solid strips
• Using a ruler, measure the distance between 4 melted strips
• Divide the distance between the strips by 3 to find the distance between adjacent melted strips
• Multiply this distance by 2 to find the wavelength of the microwaves
• Microwaves can cause burns
• Use a longer bar of chocolate to reduce the uncertainty in the measurements
• Use a bar of chocolate with uniform thickness to make it easier to see where the antinodes are as the thinner parts will melt faster in a non uniform bar

Question 12

Determining the wavelength of light using a diffraction grating
- Method (3)
- Graphs and Calculations (2)
- Validity (3)

Answer 12

• Shine the laser through the diffraction grating onto the screen
• Measure the distance between the central fringe and the one beside it
• Measure the distance between the grating and the screen
• Calculate 𝜃 using tan 𝜃 = distance between fringes/distance between grating and screen
• Use: dsin 𝜃 = nλ to calculate the wavelength (d will say on the grating itself and n = 1)
• Calculate the wavelength using the 2nd and 3rd orders and find an average (the percentage uncertainty in 𝜃 is decreased if 𝜃 is as large as possible)
• Conduct in a dark room so the different fringes can be seen
• Use a monochromatic light source

Question 13

Determine the wavelength of a colour of light using diffraction from a CD
- Method (4)
- Graphs and Calculations (1)

Answer 13

• Measure the radius of the CD, r
• Place the CD in front of a lamp that is behind you and move the CD away from your eye until the chosen colour light is on the edge of the CD
• Measure the distance from the disk to your eye, D
• Repeat for other colour of light
• Calculate the angle of diffraction using tan 𝜃 = r/D

Question 14

Determine the speed of sound in air by the formation of stationary waves in a resonance tube
- Method (4)
- Graphs and Calculations (2)
- Safety (1)
- Validity (1)

Answer 14

• Fill the resonance tube halfway with water
• Hit a tuning fork with a hammer and hold it above the tube
• Lower an open ended measuring cylinder into the water until the intensity of the sound is amplified, this is when resonance is heard
• Measure the current height of the cylinder out of the water
• Calculate the wavelength of the sound by multiplying the current height by 4
• Find the speed of sound by multiplying the wavelength by the frequency of the tuning fork
• Don’t let the tuning fork touch the resonance tube as the vibrations can break the tube
• Repeat the experiment 3 times and calculate a mean wavelength

Question 15

Determine the Planck constant using LEDs
- Method (5)
- Graphs and Calculations (1)
- Validity (1)

Answer 15

• Set up a circuit to include a power supply and a fixed resistor in series with a diode connected to a potential divider connected to the fixed resistor
• Also include an ammeter and a voltmeter in the potential divider part of the circuit
• Find the wavelength of the LED from its packaging
• Find the threshold voltage of the LED by recording the potential difference at which the LED starts to emit light
• Repeat with different LEDs
• Plot a graph of voltage against 1/wavelength
• • eV = hc/λ → Vλ = hc/e
• Make sure LED is in a dark box in a dark room