Leaving Cert Physics: The Electron

Properties of an electron (4 pts)

• Orbits the nucleus
• Very small mass
• Negatively charged
• The charge on the electron is the smallest amount of charge found in nature*.

Who measured the charge of an electron

Robert Millikan

Who coined the term electron

George Stoney

Define thermionic emission

is the giving off of electrons from the surface of a hot metal.

Account for the operation of a cathode ray tube ( 6 pts)

1. A current is passed through the heating coil, causing it to heat the cathode which in turn causes electrons to be emitted (by thermionic emission).
2. Because of the high potential difference between the cathode and anode the electrons are accelerated across the tube towards the anode. As a result a current flows in the circuit.
3. Electrons which pass through the hole in the middle of the anode continue on until they hit the fluorescent screen.
4. The stream of electrons can be deflected by electric or magnetic fields, which are generated from the X and Y plates.
5. The voltage on the Y-plates is adjusted to make the cathode rays (or stream of electrons) move up or down.
6. The voltage on the X-plates is adjusted to make the cathode rays move left or right. The result is that the rays can move across the computer or television screen

Give 3 uses of the cathode ray tube

1. Cathode Ray Oscilloscope (CRO).
This is a variation of the cathode tube; it is used today in electronics as a diagnostic tool.
2. It also forms the main component in (old-style) televisions and computers*.
3. The ECG (electrocardiogram) is used in medicine to display electrical signals in the heart

what are cathode rays?

are streams of high-speed electrons

Give 4 Properties of cathode rays

• They travel from the cathode in straight lines
• They cause certain substances to fluoresce
• They can be deflected in electric and magnetic fields
• They can produce x-rays when they strike heavy metals

what formula gives electrons potential energy

the formula W= QV

The electronvolt (eV)

is the energy lost or gained by an electron when it accelerates through a potential difference of one volt.

How do electrons moving at right angles to a magnetic field move?

In a circular path

Define photoelectric effect

is the emission of electrons from a metal due to electromagnetic radiation of a suitable frequency falling upon it.

Demonstrate the photoelectric effecf

Procedure:​
1. Charge a gold leaf electroscope negatively.
2. Shine ultraviolet light on the zinc plate.
Result: ​​
The leaves fall together
It seems that shining UV light on the zinc plate liberates electrons from the zinc and therefore the legs become neutralised and fall back together.
What is equally significant is that shining visible light on the zinc has no effect - even if the light-bulb for visible light is of a higher power or if you leave it shining for a much longer time!

Einstein's Explanation

1. The energy coming from the UV lamp travels in packets called 'photons'.
2. If the photons contain enough energy they can get absorbed by an electron on the surface of the zinc metal.
3. Each photon gives all its energy to one electron.
4. A certain amount of this energy (known as the work function) goes to liberating (releasing) the electron.
5. The remainder appears as kinetic energy of the liberated electron*.
6. Shining visible light has no effect on the apparatus because the packets of energy associated with visible light are too small (frequency is too low) to liberate (release) electrons.

What did einstein's explanation prove

This "proved" that electromagnetic radiation (including light) is composed of particles (called photons).
Einstein's was later awarded a Nobel prize for his explanation.

define photon

is a bundle (discrete amount) of electromagnetic radiation

What does energy of incident of photon=

Work function + kinetic energy of photon-electron

Give formula for energy of incident photon

("phi") is known as the work function.
It represents the energy required to 'liberate' an electron from the surface of a metal.
Because is a discrete amount of energy, it can in turn be represented by the formula = hf0, where f0 is called the threshold frequency.
The value of the work function is different for different metals.

What happens to a photocell in practice

In practice it is connected up in a circuit as shown in the diagram underneath.

Describe the operation of a photocell

Operation
1. Set up as shown
2. Light of a suitable frequency shines on A (called the cathode or photocathode).
3. This releases electrons (by the photoelectric effect).
4. The electrons are attracted to point B (the anode), and from there they flow around the circuit, where they can be detected by a galvanometer, or alternatively they can be used to activate an electronic device.
5. Note that if you bring the light source closer to the photocell there is now a greater intensity of light (more photons) falling on the cathode, which will result in the release of more electrons, as observed in a greater deflection in the galvanometer or ammeter.

Give 4 Applications of photoelectric sensing devices

Controlling the flame in central heating boilers / automatic doors / fire alarms / photocells / photocopiers / light meters,

When are X-rays produced?

when high-energy electrons collide with a high density target.

Describe the operation of the X-ray tube

Operation of the X-ray tube
1. The low voltage supplies power to a filament which in turn heats the cathode at A.
2. Electrons are emitted from the hot cathode due to thermionic emission.
3. They get accelerated across the vacuum due to the very high voltage and smash into the high-density anode (usually tungsten) at B.
4. Most of the kinetic energy gets converted to heat, which must be removed with a coolant.
5. Some inner electrons in the tungsten get bumped up to a high orbital, then quickly fall back down to a lower lever, emitting X-rays in the process.
6. These X-rays are emitted in all directions.
7. Most of these get absorbed by the lead shielding, but some exit through a narrow window, where they are then used for the required purpose.

Give 3 properties of X-rats

• They are Electromagnetic Waves
• They cause ionisation of atoms
• They have high penetration powers

Give 2 uses of X-rays

• Medicine: To detect broken bones
• Industry: To detect breaks in industrial pipes

Why can X-ray production be considered as the inverse of the photoelectric effect?

X-ray production: electrons are used to produce electromagnetic radiation
The photoelectric effect: electromagnetic radiation is used to release electrons

Hazard of X-rays

They can ionise atoms in the body, causing them to become abnormal, which can lead to cancer