Chapter 29 The Electron

What are the properties of the electron

• Orbits the nucleus

• Very small mass

• Negatively charged

• Charge on electron is the smallest amount of charge found in nature

Who first measured the charge on the electron?

Robert Millikan

Who coined the term ‘electron’?

George Stoney (GJ Stoney)

What is Thermionic Emission?

The giving off of electrons from the surface of a hot metal

Draw a labelled diagram of the Cathode Ray Tube and describe how it works

• Current is passed through the heating coil which heats the cathode and causes electrons to be emitted by Thermionic Emission

• High voltage difference between the cathode and anode causes the electrons to accelerate across the tube. As a result current flows

• Electrons pass through the hole in the anode and hit the fluorescent screen

• Kinetic Energy from the electron is converted to Light Energy

What is the function of the filament?

It heats the Cathode

What happens at the Cathode?

Thermionic Emission occurs and electrons are emitted

What happens when the electrons hit the screen?

Their kinetic energy is converted to light energy. Light can be seen on the screen

What causes the electrons to be accelerated across the tube?

High voltage between Anode and Cathode

Why is there a vacuum in the tube?

So that the motion of the electrons is not opposed as they move across the tube

What are Cathode Rays?

The beams of electrons (streams of high-speed electrons) produced in a cathode ray tube are called Cathode Rays

Properties of Cathode Rays

• They travel from the cathode in straight lines

• They cause certain subtances to fluoresce

• They can be deflected in electric and magnetic fields (x and y plates)

• They can produce X-rays when they strike heavy metals

What is the Photoelectric Effect

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

To demonstrate the Photoelectric Effect

Procedure

1. Charge a GLE negatively

2. Shine UltraViolet light on the zinc plate

Result

The leaf falls altogether

Observation

Shining UV light on the zinc plate liberates electrons from the zinc and therefore the leaves become neutralised and fall back together

Outline Einsteins explanation of the Photoelectric Effect

Light travels in packets of energy called Photons

Each Photon gives all of its energy to one electron

The energy needed to remove the loosest electron from the surface of a metal is the Work Function W of that metal

If the energy of each photon is less than the work function no electrons are emitted

If the energy of the photon is greater than the work function electrons are emitted

What is the Work Function?

The work function of a metal is the minimum energy needed to remove the loosest electron from the surface of that metal

What is the Threshold Frequency?

For a given metal the frequency below which photoemission will not occur is called the Threshold Frequency. Light of frequency above the threshold will cause photoemission

What is a Photon?

A photon is a packet of electromagnetic energy

The energy E of a photon is given by E=hf

Define the ElectronVolt

The electronvolt (eV) is the amount of energy gained or lost by an electron when it moves through a potential difference of 1V

Define Intensity

Intensity is defined as the incident light energy per unit area per unit time

As Frequency increases = electrons speed up

As Intensity increases = no. of electrons increases

What is a photocell?

A photocell is a device that conducts electric current when light of a suitable frequency shines on it.

The size of electric current it conducts is directly proportional to the intensity of light falling on it.

Draw a labelled diagram of a photocell

To demonstrate the action of a Photocell

1. Set up as shown

2. Light of a suitable frequency shines on A (photocathode)

3. This releases electrons by the photoelectric effect

4. Electrons are attracted to B (anode) and flow around the circuit

5. Note that if you bring the light source closer, there is a greater intensity of light (more photons) which will result in the release of more electrons and as observed, there is a greater deflection in the galvanometer

Applications of Photoelectric sensing devices

• Automatic Doors

• Fire Alarms

• Photocopiers

• Photocells

What are X-Rays?

X-rays are high frequency electromagnetic radiation produced when high speed electrons in a cathode ray tube strike a metal target that has a high melting point

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

In X-Ray production, electrons hit a target and electromagnetic radiation is given out

In the Photoelectric Effect, electromagnetic radiation strikes a target and electrons are given out

Draw a labelled diagram of a Cathode X-Ray Tube

Decribe the Operation of an X-Ray

1. Low voltage supply heats the cathode

2. Electrons are emitted by thermionic emission

3. They get accelerated across the vacuum due to the very high voltage and smash into a high density anode (tungsten)

4. Most of kinetic energy gets converted to heat, which is removed with a coolant

5. X-rays are emitted all around and some exit through a small narrow window

Properties of X-Rays

They are electromagnetic waves

they cause ionisation of atoms

they have high penetration powers

What is ionisation?

Ionisation is the process where an atom or molecule loses or gains electrons to become an ion

Uses of X-rays in Real World

Medicine: To detect broken bones

Industry: To detect pipe breaks

What is the Hazard of X-Rays

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