Modern PhysicsThe electron

An indivisible quantity of charge that orbits the nucleus of the atom

The Electron

The idea that there is a natural unit of electricity inside the atom was suggested in 1874 by Irishman G.J. Stoney.

He named the unit the ‘Electron’ in 1891

Subatomic Particles make up the atom.

Who discovered the electron?

The properties of the three subatomic particles

Orbits the nucleus Very small mass Negatively charged The charge on the electron is the smallest

amount of charge found in nature

Properties of the electron

You don’t have to know the actual charge (1.6 × 10-19 C), but you do need to know that the man responsible for first measuring this charge was Robert Millikan.

Discovering the charge of the electron

Electron Charge

Dr. Quantum

Video Links

Because an electron has mass it has kinetic energy when moving

The amount of energy depends on how fast it moves.

Energy of an Electron

An electronʼs potential energy can be converted to kinetic energy

E = ½ mv2 (Kinetic energy)

E=qV or for an electron E=eV (q = charge, e=charge of an electron, V= voltage)

Therefore eV= ½ mv2

The Electron Volt (eV) is the energy lost or gained by an electron when it accelerates through a potential difference of one volt.

From the formula E = eV, we can calculate that the energy lost or gained is (1.6 × 10–19)

multiplied by (1), or 1.6 x 10–19 Joules,

Therefore 1eV =1.6 x 10–19 Joules

The Electron Volt (eV)

What is 5.6 MeV in joules?

Energy (joules) = Energy (eV) x (1.6 × 10–

19) = (5.6 x 106) (1.6 × 10–19) =8.98 x10-13 J

Maths bit

Thermionic Emission The emission of electrons from the surface

of a hot metal Metals are made up of atoms, when the

metal is heated the electrons gain enough energy to leave the metal

How does an Oscilloscope work?

The minimum energy required to remove the loosest electrons from the surface of the metal is called the work function of the metal Φ

For thermionic emission the metal usually needs to be heated to a temperature in excess of 800ºC

The main component in the oscilloscope is the Cathode Ray Tube

It consists of a glass evacuated tube, a thin filament, two electrodes and a fluorescent screen

Back to our oscilloscope.

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.

How does it work?

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.

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

Properties of Cathode Rays

Old TV screens and computer monitors Cathode ray oscilloscope ECG EEG

Applications of Cathode Ray Tubes

A beam of electrons moving at right angles to a magnetic field moves in a circle

Force on a charged particle moving in a magnetic field

F = q v B (F=force, q=charge, v=velocity and B=magnetic flux density)

Also: Where m= mass, v=velocity, r=radius, F= centripetal

force

Electrons in a Magnetic Field

Do Questions 1,2, 3,4 and 7,8,9 on page 333

X-rays are electromagnetic radiation of extremely high frequencies. They are produced when high energy electrons collide with a metal target (tungsten).

The x-ray tube consists of a cathode heated by a filament, a vacuum, a tungsten target, a high anode voltage which accelerates the electrons.

How do X-Rays Work?

1. The low voltage supplies power to a filament which in turn heats the cathode.

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) .

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

Operation of the X-ray tube

5. Some inner electrons in the tungsten get bumped up to a high orbital, then quickly fall back down to a lower level, 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.

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

Properties of X-Rays:

Uses of X-rays Medicine: To detect broken bones Industry: To detect breaks in industrial

pipes

Hazards They can ionise atoms in the body, causing

them to become abnormal, which can lead to cancer.

The photoelectric effect is the emission of electrons from the surface of a metal due to electromagnetic radiation of suitable frequency hitting the metal.

This is the opposite of X-rays. In x-rays light is emitted when electrons hit the metal target. In the photoelectric effect electrons are emitted when light of a suitable frequency hits the metal target.

The Photoelectric Effect

Light must be considered as a bundle of energy called a photon. The energy of the photon is proportional to the frequency of the light (electromagnetic wave)

Einstein's Explanation of the Photoelectric Effect

E = energy of the photon, h = plankʼs constant, f = frequency of the EM radiation

The energy of the fastest electron emitted is the difference between the energy of the photon and the work function of the metal.

Work Function φ = h fo where fo is the threshold frequency.

Einstein's Photoelectric Law

Procedure: Charge the electroscope negatively. Shine ultraviolet light on the zinc plate.Result: The leaves fall togetherObservation: Shining UV light on the zinc plate liberates

electrons from the zinc and therefore the legs become neutralised and fall back together.

Demonstration of Photoelectric effect.

Operation Light of a suitable frequency shines on the

photocathode This releases electrons (by the Photoelectric

Effect). The electrons are attracted to 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.

The Photocell

Photo current is directly proportional to the intensity of the light

• Burglar alarms • Automatic doors • Control of burners in central heating • Sound track in films

Applications of photoelectric sensing devices

The Electron - Formulae

Page 339 questions 1,2,3,4,5

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Q11

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