Module 4 standard answers Flashcards
Describe an experiment to obtain the I-V characteristic of a resistor/filament lamp/LED/Thermistor at a constant temperature
Set up the circuit as shown (For the LED, add a resistor in seres);
Vary the potential difference across the resistor by adjusting the slider on the rheostat;
Move the slider from 1 end of the rheostat to the other to ensure the full range of potential differences from 0.0 to 6.0V;
Measure and record the current through the resistor and the potential difference across the resistor for at least six difference pairs of I and V (For the LED, move the slider by small amounts so that the potential difference at which the LED begins to light can be determined accurately);
Plot a graph of I against V;
As the gradient is constant, resistance = 1/gradient.
For the LED add a resistor in series and move the slider by small amounts
Describe an experiment to determine how the resistance of a wire varies with its temperature.
Place the wire in an oven to raise its temperature
Remove the wire from the oven and connect it in series with a cell and an ammeter.
Connect a voltmeter in parallel with the wire
Use a temperature probe to measure the temperature of the wire
Make multiple recordings of the ammeter and voltmeter readings for each temperature reading recorded.
Calculate R=V/I for each pair of readings then plot a graph of R vs T.
Describe and explain the shape of the IV characteristic of a filament lamp
The current in the lamp is not directly proportional to the potential difference across its ends as the graph is not a straight line through the origin
Hence, the filament lamp does not obey Ohm’s Law
As the current through the lamp increases, the temperature of the lamp’s filament increases;
As the temperature increases there are increased ion vibrations and more frequent electron-ion collisions.
As a result, when the temperature of the filament increases, the resistance of the filament lamp increases.
State and explain how the resistance of a filament lamp changes as the potential difference across it increases.
As the potential difference across the filament lamp increases, the resistance of the lamp increases
This is because there is more current flowing through the lamp, which increases the number of electron collisions with ions in the metal, which increases the temperature of the lamp
The higher the temperature of the filament, the higher its resistance.
Describe the features of an IV characteristic of a LED
No current flows through the LED until the potential difference is large than (1.5).
Above 1.8V the current rises almost linearly with increases in potential difference.
The LED does not obey Ohm’s law as I is not proportional to V
Below 1.5V, the LED has an infinite resistance.
Above 1.5V the LED’s resistance decreases.
Describe an experiment to determine the resistivity of the material of the resistance wire.
Measure the diameter of the wire using a vernier caliper;
Measure multiple lengths of the wire using a ruler;
For each length, find the potential difference across the wire and current flowing through the wire using the voltmeter and ammeter in the circuit;
For each pair of current and potential difference readings, calculate R=V/I
Plot R against L (the length) and the gradient of the graph is p/A
Calculate A = pid^2/4 and then p = gradient x A
List the factors which the resistance of the wire depends on
The temperature of the wire
The resistivity of the wire
The cross-sectional area of the wire
The length of the wire.
State and explain the effect on the ammeter and voltmeter readings when the temperature of the cooper wire bundle is increased
Resistance of the wire increases as its temperature increases
Voltmeter reading remains constant
The current in the circuit decreases as I=V/R and so the ammeter readings falls.
Describe how resistance/resistivity of a semiconductor is affected by temperature
The resistivity of the semiconductor decreases as its temperature is increased
The decrease in resistivity is most rapid at lower temperatures
As the temperature increases, more electrons can break free of their atoms to become conduction electrons, increasing the number density, n
As the temperature increases there are increased ion vibrations and more frequent electron-ion collisions but this effect is small compared to the increase in the number density, n.
Describe how the resistance of a NTC thermistor varies with temperature
Resistance decreases with increasing temperature
The gradient is steepest at low temperature
Larger charges in R at low temperatures gives larger changes in I and V
Therefore, the thermistor is best used in a circuit which requires switching at low temperature, e.g. fridge or frost protection heater.
Explain how you would determine experimentally the EMF and internal resistance of a charged cell. Include a circuit diagram with meters and a variable load resistor.
Set up the standard circuit diagram with the variable load resistor included.
Take a set of readings of V (Terminal Potential Difference) and I (Current) for at least 6 different positions of the variable resistor
Plot a graph of V against I
Determine the y-intercept of the line of best fit, which is the EMF of the cell
Determine the gradient of the line of best fit which is equal to -r
State wave phenomena that apply to both transverse and longitudinal waves
Reflection, refraction diffraction, interference (superposition).
There is lamp which 2 polarisers in front of it that have their transmission axes crossed. Describe and explain what the observer would see.
The observer would see no transmission
The lamp produces unpolarised light, so the first polariser would only let the vertical component of the lights oscillations pass through.
However, the second polariser will only let horizontal oscillations pass through, but the light reaching it is plane polarised in the vertical plane, and so no light is transmitted through the second polariser.
Describe and explain what the observer would see when a third polariser is inserted between the previous two, with its axis of transmission aligned somewhere between the previous 2.
The observer would see some transmission
The lamp produces unpolarised light, so the first polariser would only let the vertical component of the lights oscillations pass through
Polariser 3 would let a component of the vertically polarised light through as its axis of transmission is not perpendicular to that of polariser 1.
The plane of oscillations of the light reaching polariser 2 is not perpendicular to the axis of transmission of polariser 2 and so a component of this light will pass through polariser 2 to the observer.
An experiment was carried out where a student observes a beam of plane-polarised light passing through a polaroid filter. The following graph shows how the intensity of the light reaching the student varied as the polaroid filter is rotated through 360 about an axis passing through its centre.
Suggest why there is a series of maxima and minima
When the axis of transmission of the polaroid is parallel to the light’s plane of polarisation there is a maximum transmission of light (At 0, 180, 360)
However, there is zero transmission of light when the Polaroid’s axis of transmission is perpendicular to the plane of polarisation of the light.
State 1 major difference between Gamma rays and X-rays
X-rays are emitted from the collision of high speed electrons with a metal surface
Gamma rays are emitted from the radioactive decay of atomic nuclei.
A ray of light enters the block and leaves from the other side. Describe how you would use the apparatus shown in the diagram to determine the refractive index of the glass.
Mark the incident and emergent rays using a pencil and ruler onto the paper
Mark on the normal line to the glass block
Connect the incident ray and emergent ray with a straight line using a ruler and pencil
use a protractor to measure the angle of the incident ray and refracted ray to the normal
Substitute into n=sin(theta1)/sin(theta2)
Describe a ripple tank experiment to demonstrate the interference between water waves which have circular wave fronts.
Two dippers are vibrated up and down from the same vibration generator.
This ensures the wave sources are coherent
To reduce the speed of waves in a ripple tank, reduce the depth
Describe how an arrangement ensures that the slits S1 and S2 act as coherent light sources.
The 2 slits are illuminated using the same light source
So the light leaving the two slits has the same wavelength and frequency, so the light leaving the 2 slits will have a constant phase difference.
Explain in terms of path difference why bright and dark lines are formed on the screen
At bright lines/fringes the path difference between 2 waves is equal to a whole number of wavelengths which results in constructive interference
At dark line/fringes the path difference between the 2 waves is equal to an odd number of half wavelengths which results in destructive interference.
Describe how the appearance of the interference pattern would change if a white light source was used instead of the monochromatic source
There would be a central white fringe
The other fringes would be coloured.
State the features of the stationary wave
Energy is trapped in pockets
There are positions where the amplitude is maximum
Adjacent points have different amplitudes
Points between the same node are all in phase
Points eithers side of a node are in antiphase.
State a similarity between a progressive wave and a stationary wave
The particles vibrate in a direction parallel to the direction of travel of the wave.
Describe the differences between a progressive wave and a stationary wave
All particles in a progressive wave have the same amplitude but particles in a stationary wave have different amplitudes
I a stationary wave there are particles which do not oscillate (at the nodes) but all particles oscillation in a progressive wave.
There is a phase difference between the oscillations of neighbouring particles in a progressive wave, whereas there is no phase difference between neighbouring particles (between the same node) in a stationary wave.
Explain how a stationary wave is formed
A progressive wave is reflected (typically by some barrier like a pulley or metal plate)
The reflected wave interferes with an incident wave
Forming points of constructive and destructive interference, known as antinodes and nodes respectively.
Describe an experiment to demonstrate how a standing wave can be produced in an air column
A tuning fork (or loud speaker) is held above a tube which is placed in a measuring cylinder containing water
The tuning fork is sounded and placed at the top of the tub
The length of the tube is changed (by moving the tube up and down in the water) until a loud sound (a resonance( is heard at the top of the tube.
The diagram shows an arrangement where the microwaves leave a transmitter T and move in a direction TP, which is perpendicular to the metal plate P.
When a microwave detector D is slowly moved from T towards P the pattern of the signal strength received by D is high, low, high, low, …
Explain:
1) Why these maxima and minima of intensity occur
2) How you would measure the wavelength of the microwaves
3) How you would determine their frequency
The microwaves transmitted by T are reflected by the metal plate and interfere with other incident microwaves, forming a stationary wave with nodes and antinodes
The locations of high intensity are places where the waves are interfering constructively
Whereas the locations of low intensity are places where the waves are interfering destructively.
The wavelength of the microwaves would be calculated by measuring the distance between 2 adjacent antinodes, which is equal to half of a wavelength
The frequency of the microwaves is then determined by using c=wavelength x frequency
