EXAM 2

Question 1

Explain the IV relationship for a filament bulb

Answer 1

1. Initially, current is directly proportional to potential difference (the filament obeys
   Ohm’s law)
2. As current increases, the electrons collide more frequently with lattice resistor ions,
   transferring more energy to them per second OR as pd increases, electrons gain
   more energy and transfer more energy to lattice resistor ions
3. This causes the resistor ions to oscillate with greater amplitudes so the temperature
   of the resistor increases
4. This causes more frequent collisions between the electrons and the lattice ions
5. This results in a reduced drift velocity
6. This causes the current, I=nqvA to increase by a smaller factor than the potential
   difference (as n, q, A all fixed)
7. So, as resistance is the ratio of pd to current, if pd increases more than current does,
   the ratio R=V/I increases

Question 2

Explain the resistance behaviour of a thermistor as temperature increases (due to external
factors)

Answer 2

1. As temperature increases, the electrons in the thermistor are provided with more
   energy
2. This means they can be promoted from the valence band of the semiconductor to
   the conduction band
3. As they are free and able to conduct, the number density of free charge carriers, n
   increases
4. This results in a higher current, as I=nqvA and q, v, A are all fixed.
5. As resistance is the ratio of pd to current, R=V/I, as current can be higher for the
   same potential difference at this higher temperature, the resistance decreases

Question 3

Explain the IV relationship for a thermistor (here, the temperature increases due to the
current flowing through the thermistor so we must explain this step first)

Answer 3

1. As current increases, the electrons collide more frequently with lattice resistor ions,
   transferring more energy to them per second OR as pd increases, electrons gain
   more energy and transfer more energy to lattice resistor ions
2. This causes the resistor ions oscillate with greater amplitudes so the temperature of
   the resistor increases
3. But, as temperature increases, electrons can gain energy and so more electrons are
   promoted into the conduction band of the thermistor, so n, the number density of free
   charge carriers increases.
4. Thus, current, I= nqvA will increase by a greater factor than the potential difference.
   e. So, as resistance is the ratio of pd to current, if current increases by a greater factor
   than pd does, the ratio R=V/I decreases

Question 4

Explain the resistance, light intensity behaviour for an LDR

Answer 4

1. As light intensity increases, electrons can gain energy from the incident photons
2. electrons are promoted into the conduction band of the LDR, so n, the number
   density of free charge carriers increases.

3.. Current increases

4.. So, as resistance is the ratio of pd to current, if pd remains constant as current
increases, resistance = V/I decreases

Question 5

Explain how a standing wave is set up:

Answer 5

1. Two waves travelling in opposite directions
2. Of equal frequency and similar amplitude
3. Undergo superposition when they meet
4. Constructive interference produces antinodes – positions of maximum amplitude
5. Where waves meet in phase
6. Destructive interference produces nodes – positions of zero amplitude
7. . Where waves meet in antiphase

Question 6

Explain how to determine the wavelength of laser light experimentally and graphically

Answer 6

1. Set up equipment as shown with the diffraction grating at right angles to the light from the laser, parallel to the
   screen
2. Calculate d, the slit width which is 1/ slits per metre value
3. Measure the distance, D, between grating and the screen with a metre rule
4. Measure the distance, x, by measuring the distance between the first orders using a metre ruler and diving by 2 (to get the mean x between the 1st order and 0th order)
5. Calculate the angle θ using tanθ= x/D so θ=tan -1 (x/D)
6. Change the diffraction grating to one with a different value for the number of slits per mm, N
7. Repeat for 6 different diffraction grating slit sizes
8. Plot a graph of sinθ against 1/d
9. Sinθ = nλ x 1/d
10. y = m x
11. determine the gradient of the line of best fit

as n=1 the gradient of the line of best fit is the wavelength
12.

Question 7

Explain how the photoelectric effect indicates that light is a particle

Answer 7

1. It is observed in the photoelectric effect that only EM radiation above a certain threshold
   frequency will release electrons (no matter what the intensity)
2. So the energy of the photon, E=hf is proportional to frequency
3. The kinetic energy of the electron is only dependent on the frequency of the photon
4. In wave model energy is proportional to intensity, so in the wave model high intensity
   light of any frequency should emit electrons, and the energy of the electrons should
   depend on intensity – this is not observed
5. Instead, electrons are always released from surface of metal when energy of photon is
   larger than work function of metal
6. It is also observed in the photoelectric effect that electrons are emitted instantaneously
   from the metal
7. This indicates that energy is absorbed instantaneously –one photon is absorbed by one
   electron
8. However, in the wave model energy would be absorbed over time and not release
   electrons instantaneously
9. In the particle model, the higher the intensity, more photons would be absorbed each
   section and so there would be more electrons emitted per second – this is what is
   observed

Question 8

Explain origin of line spectra (spectral lines/ emission lines) specific to certain elements

Answer 8

1. Electrons exist in discrete energy levels
2. Electron within atom excited to higher energy level when: fast moving electron collides
   with atom, transferring its kinetic energy or current is passed through OR gas is heated
3. The electron then falls back down to lower energy level
4. Emitting a photon with an energy E equal to the energy difference between the two
   electron levels
5. E=hf is the energy of the photon
6. The photon is emitted with a specific frequency f = E/h where E is the energy difference
   between the levels
7. There are only a limited number of energy differences between levels and there only a
   corresponding limited number of frequencies emitted
8. Different elements have different energy differences between levels so produce different
   spectral lines

Question 9

Explain origin of absorption spectra

Answer 9

1. Electrons exist in discrete energy levels
2. Electron within atom (in atmosphere of sun) excited to higher energy level when it
   absorbs a photon
3. E=hf is the energy of the photon
4. The photon absorbed must have a specific frequency f = E/h where E is the energy
   difference between the levels
5. There are only a limited number of energy differences between levels and only a
   corresponding limited number of frequencies of photons absorbed
6. Different elements have different energy differences between levels so produce different
   absorption lines at specific frequencies

Question 10

Equation for power=

Answer 10

Power = 1/ focal length . units = D (dioptres)

Question 11

What do to for total power fir diverging

Answer 11

When multiple lenses are lined up on the principal axis, their powers add up (remember
to subtract any diverging lenses)

Question 12

Explain how to estimate the focal length of a converging lens

Answer 12

Pass parallel rays of light from a distant object through the lens and move a screen
behind the lens until the image is focussed. The distance between the lens and the
focussed image is the focal length