Modern physics

Question 0

Who discovered the size of charge on an electron?

Answer 0

R. Millikan - oil drop experiment

Question 1

Thermionic emission

Answer 1

The emission of electrons from the surface of a hot metal

Question 2

Give 3 uses of cathode ray tube/

Answer 2

Television
Cathode ray oscilloscope
ECG - displays electrical signals in the heart
EEG - displays small varying electrical signals in the brain

Question 3

Photoelectric emission

Answer 3

Photoelectric emission (Photoelectric effect )is the emission of electrons from a metal surface using light ( electromagnetic radiation) of a suitable frequency

Question 4

Graph to demonstrate the effect of frequency on incident light for the photoelectric effect

Answer 4

Y axis -number of electrons emitted
X axis - frequency of incident light
Not directly proportional due to threshold frequency
If the frequency of the incident light is below the threshold frequency for the metal then no electrons are emitted

Question 5

Explain the relationship between light intensity and photo current. Describe the graph draw to demonstrate the effect if intensity of incident light

Answer 5

A brighter light gives a larger current. The number of electrons emitted is proportional to the intensity of the incident light.
Y axis - photo current (mA)
X axis intensity of incident Light

Question 6

Photon

Answer 6

A photon is a discrete quantity of electromagnetic radiation

Question 7

Explain Einstein’s theory

Answer 7

• When a photon collides with an electron at or just within the surface of the metal it maytransfer its energy to the electrons
• This transfer of energy is an all or none process i.e. The electron gets all the energy of the photon or no energy at all
• The energy of the photon is proportional to the frequency of the radiation
• The photon now ceases to exist
• Some of the energy required by the electron is used to escape the metal surface (overcome the work function) and the rest of the energy appears as kinetic energy of the emitted electron
• The number of electrons emitted depends on the number of photons i.e. the intensity of the incident light

Question 8

Explain an application of the photoelectric effect

Answer 8

When a photoelectric cell is positioned in a circuit, current will flow through the cell if light falls on it. If a beam of light light is turned on and off, this can be used to turn on and off an electric current. This in turn can be used to control other electrical circuits. This is the main idea behind the practical use of the photoelectric effect. Eg in 
Burglar alarms
Automatic doors 
Reading bar codes
Smoke alarms