Photoelectric Effect and X-Rays

Question 1

Define the photoelectric effect.

Answer 1

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

Question 2

On what does the number of electrons emitted depend?

Answer 2

The number of electrons emitted depends on the intensity of the incident light.

Question 3

On what does the energy of the electrons emitted depend?

Answer 3

The energy of the electrons depends on the frequency of the incident light.

Question 4

Justify how x rays production is the opposite of the photoelectric effect.

Answer 4

In x ray production electrons hit a metal target and photons are emitted.
In the photoelectric effect photons hit a metal target and electrons are emitted.

Question 5

Give three applications of a photocell/the photoelectric effect

Answer 5

Control of central heating burners
Light meters for cameras
Reading bar codes

Question 6

Define thermionic emission.

Answer 6

Thermionic emission is the emission of electrons from the surface of a hot metal

Question 7

Explain why the leaves of a gold leaf electroscope collapse when UV light is shined on them.

Answer 7

Ultraviolet radiation causes photoelectric emission. Electrons leave the surface of the zinc which reduces the size of the negative charge on the gold leaves. The reduces the potential difference between the gold leaves and the outer body of the electroscope causing the leaves to collapse.

Question 8

Explain why the leaves don’t collapse when the zinc is covered with an ordinary piece of glass?

Answer 8

Ultraviolet radiation cannot pass through ordinary glass so photoemission doesn’t occur.

Question 9

Explain why the leaves don’t collapse when the zinc is illuminated with green light?

Answer 9

The frequency of green light is lower than ultraviolet radiation and below the threshold frequency of zinc so photoemission cannot occur.

Question 10

Explain why the leaves don’t collapse when the electroscope is charged positively.

Answer 10

Because any electrons emitted will be attracted back to the positively charged electroscope. The charge on the electroscope remains constant and the leaves do not collapse.

Question 11

Define X-rays.

Answer 11

X-rays are electromagnetic radiation of very short wavelength and very high frequency.

Question 12

Who discovered X-Rays?

Answer 12

W. Rontgen

Question 13

In an x-ray tube, how are the electrons emitted from the cathode?

Answer 13

Electrons are emitted from the cathode by thermionic emission.

Question 14

In an x-ray tube, how are electrons accelerated?

Answer 14

A very high voltage between the cathode and the anode is used to accelerate the electrons.

Question 15

Write down a formula for the kinetic energy gained by an electron?

Answer 15

E = q x V; q=charge V=Voltage/Potential difference
Or
qV=hf

Question 16

Why was the quantum theory of light revolutionary?

Answer 16

The theory suggests that light consists of photons. I.e. discrete bundles of energy rather than being a continuous wave.

Question 17

Define photon.

Answer 17

A photon is a discrete quantity of electromagnetic radiation.

Question 18

For the photoelectric effect how is the intensity of the light varied?

Answer 18

Vary distance from light source to photocell
Vary the current/voltage to bulb in light source

Question 19

How do x-rays differ from light rays.

Answer 19

Unlike light rays, x-rays can penetrate matter and can cause ionisation.

Question 20

Give three uses of x-rays.

Answer 20

Medical analysis of bones in the human body.
Luggage scanners at airports
Astronomy
Irradiation of foods

Question 21

What happens to the majority of the energy when the electrons hit the target metal? As a result what kind of metal must be used?

Answer 21

The majority of the energy is converted to heat energy.
So the target metal must have a very high melting point. E.g. tungsten

Question 22

Give two properties of radio waves.

Answer 22

They travel at the speed of light.
They can travel in a vacuum.

Question 23

Why are radio waves not very penetrating?

Answer 23

They have low frequencies and therefore low energies and low penetration.

Question 24

Name an electromagnetic wave that may induce cancer.

Answer 24

X-rays; they ionise cells in the human body.

Question 25

What is a p-n junction?

Answer 25

A p-n junction is where a p-type semiconductor is in contact with an n-type semiconductor

Question 26

On the frequency to energy of an electron graph what is the 1. slope of the graph 2. The point where it crosses the x-axis.

Answer 26

1. Plank’s constant 2. The threshold frequency

Question 27

Formula for energy of a photon.

Answer 27

E=hf

Question 28

State two design features of an x-ray tube that account for an increase in heat energy.

Answer 28

The tungsten metal target has a high melting point.
The cooling fins remove heat from the inside of the tube.

Question 29

Give two uses of a cathode ray tube.

Answer 29

-Television
-Cathode ray oscilloscope

Question 30

Define work function.

Answer 30

The minimum amount of energy required to remove the most loosely bound electron from the surface of a metal.

Question 31

Explain how electromagnetic radiation causes the temperature of the earth to increase according to the greenhouse effect.

Answer 31

When high frequency radiation is absorbed by the surface of the Earth, the planet’s internal energy increases and the surface gets hotter. This energy cannot escape the atmosphere because of the greenhouse effect and the build up of greenhouse gases.

Question 32

Explain why not all electrons emitted will have the same energy.

Answer 32

Not all the radiation will have the same frequency which will emit electrons of lower energies.
Not all electrons have the same binding energy.

Question 33

Explain how electromagnetic radiation heats up the Earth according to the greenhouse effect.

Answer 33

High frequency infrared radiation is absorbed by the surface of the Earth causing it to heat up and gain energy. However the earth cannot release this energy because of the greenhouse effect. CO2 in the atmosphere traps the heat and reflects the infrared radiation back onto the Earth heating it once more.

Question 34

What are cathode rays?

Answer 34

Cathode Rays are electrons emitted from the cathode.

Question 35

Distinguish between the photoelectric effect and thermionic emission.

Answer 35

Photoelectric: Light. Thermionic: Heat

Question 36

On what does the intensity of an x-ray depend? On what does the wavelength, penetrating power and energy of x-rays depend?

Answer 36

Number of electrons striking the target per second.

Voltage across the tube.