Photoelectric Effect

Question 1

Photoelectric Effect

Answer 1

Emission of electrons from a metal plate when EM radiation is incident on the plate
New model of light

Question 2

Gold-Leaf Electroscope Experiment

Answer 2

Gold Leaf has low mass
Zinc Plate is supplied with a -ve charge
Electrons turn brass rod and gold leaf -ve (Repel)
Gold Leaf rises
When white light is shone, no effect
When ultra-violet light is shone, electrons are emitted
Their kinetic energy is measured
One to one Interaction of Photon to Electron

Question 3

Wave Theory

Answer 3

There is a threshold frequency, below which no electrons are emitted
Wave theory states an intense white light source carries more energy than a weak UV source, so it emits more electrons
Max KE is independent of intensity
Wave theory states higher intensity should provide far more energy for escaped electrons

Question 4

Einstein’s Photon Model

Answer 4

Photon: A quantum (small amount) of electromagnetic radiation
E = hf or E = hc/λ
h: Planck’s Constant in Databook
f: frequency in Hertz
E: Energy of a photon in Joules

Question 5

Properties of a Photon

Answer 5

A quantum of electromagnetic radiation
All photons have zero rest mass and zero charge
They travel at 3 x 10^8 ms^-1
Not affected by electric of magnetic fields

Question 6

Photoelectric Equation

Answer 6

One to One interaction between a photon and a free electron at the surface of a metal
A single electron absorbs a single photon and gains energy (photon)
Minimum energy to escape is the work function of the metal: ϕ
Remaining energy is KE(max)

hf = ϕ + KE(max)
Conservation of energy
ϕ: The minimum energy required to remove an electron from the surface of a metal plate, depends on the metal type

Question 7

Threshold Frequency

Answer 7

The minimum frequency of EM radiation that will remove electrons from the surface of a metal plate
At the frequency KE(max) = 0 and hf = ϕ
Threshold λ: Longest wavelength which releases electrons

Question 8

Range of KE(max) for electrons

Answer 8

Electrons inside the plate loose KE due to collisions, but may still escape, just without KE(max)
Some can have zero KE as they only just escape
Electrons on the surface escape with KE(max)
0 <= KE <= KE(max)

Question 9

Intensity and Photons

Answer 9

I = P / A
P = W / T
As Intensity increasers, power increases
Power is energy per second
energy is carried by a stream of photons
E = hf x N (1 second)
N is the number of photons emitted per second (With a source of Power P)

Question 10

Photoelectric Current

Answer 10

Emitted Electrons carry a charge e in the data book
Current = Charge Flow rate (per unit time) A = C/s
If intensity increases (Frequency is constant): More photons
Rate of electron emission increases
Current rises
Intensity ∝ Rate of Electron Emission
KE(max) does not depend on intensity (Does not follow wave theory)

Question 11

Wave Particle Duality

Answer 11

Photon models are explained by the photo-electric effect
Wave theory does not explain photoelectric effect
Wave theory explains diffraction and interference
Therefore, photons have wave particle duality

Question 12

Graph

Answer 12

Change Frequencies of EM radiation
Measure KE(max) of photons
KE(max)[y] = h[m]f[x] - Φ [+c]
The x-intercept is the fundamental frequency
gradient: Planck’s constant
If the metal changes, it will have the same gradient
Higher Φ or fundamental frequency means the graph shifts right and vice versa

Question 13

Experiment to find Planck’s constant

Answer 13

LEDs gave a threshold voltage, which they emit light from
When travelling through V - Gain W = VQ W = eV
Electrons convert this energy to photons with energy
E = hc/λ -> eV = hc/λ V = hc/e x 1/λ
Series Circuit with a variable power supply, resistor (for safety), LED
Voltmeter in parallel with LED
Voltage is increased until LED emits light, record V
Repeated for multiple wavelengths
V: Y-axis
1/λ: X-axis
m = hc/e
h = me/c