Properties of the Electron
- It is a particle that orbits the nucleus of an atom.
- It has a very small mass.
- It is negatively charged. Charge = e.
- This is the smallest amount of charge found in nature.
- It is the indivisible quantity of charge.
- Millikan first found the value of the charge.
Thermionic Emission
Thermionic Emission is the giving off of electrons from the surface of a hot metal.
The Cathode Ray Tube
Cathode rays are streams of high speed electrons moving from the cathode.
How a Cathode Ray tube works:
- A low voltage is placed across the filament. Current flows through and it becomes white hot, this heats the cathode.
- Thermionic emission occurs at the cathode. Electrons are emitted.
- There is a large voltage between the anode and the cathode. The anode is positive with respect to the cathode. The electrons from the cathode accelerate towards the anode. They can get up to great speeds as they are not opposed by gas molecules as there is a vacuum.
- They go through the hole in the anode and go to the end of the tube.
- At the end of the tube there is a screen coated with fluorescent material, when the electron strikes the screen, its kinetic energy is converted into light energy.
Cathode Rays
- They cause certain substances to give off light when they are struck. (e.g. Zinc Sulphide)
- They have kinetic energy.
- They can be deflected in electric and magnetic fields.
- They are invisible but their presence can be detected in a tube by making them strike fluorescent material.
Energy of electrons in an electric field
Loss (gain) in potential energy = gain (loss) in kinetic energy eV = ½ mv2
The Electronvolt (eV)
The amount of energy gained or lost by an electron when it moves through a potential difference of one volt.
When charge Q moves through voltage V, the work done is given as W=QV (e)(1) = (1.6×10-19J)(1)
Applications of a Cathode Ray Tube
- Television
- Computer monitor
- Cathode Ray oscilloscope (CRS)
- ECG – electrical signals in the heart
- EEC – electrical signals in the brain
The Photoelectric Effect
The emission of electrons from the surface of a metal by electromagnetic radiation of a suitable frequency.
To Show the Photoelectric effect
- Use electroscope and UV light.
- Place zinc plate on cap and charge the electroscope negatively.
- Shine UV light onto the zinc and the gold leaf collapses.
- Put glass between the UV and the zinc and the leaf will not go down.
- Visible light frequencies will not affect the leaf.
- Ultraviolet light causes electrons to be emitted from the zinc.
- Won’t work with a positively charged electroscope as the electron get attracted back.
The Photocell
- Conducts electric current when light of a suitable frequency shines on it.
- The size of the electric current is directly proportional to the intensity of the light.
- Photocell has a cathode and an anode in a glass tube.
- Photocathode is a semi cylinder, coated with material that will undergo photoemission
- Anode is a rod in the centre of the tube.
- Tube contains a vacuum.
- Light of a suitable frequency strikes the cathode – electrons are emitted – attracted to anode, a small current flows in the circuit.
To demonstrate the action of a Photocell
- Set up circuit.
- Measure distance from photocathode to light source, and the current, change the distances and measure the same things.
- Plot graph. Y = photocurrent. X = 1/distance2
- Photocurrent proportional to 1/d2 Intensity of light proportional to 1/d2
- Photocurrent proportional to Intensity of light
Threshold of Frequency
For a given metal, the frequency below which photoelectric emission will not occur.
Light of a higher frequency will cause photoelectric emission.
Work function
The minimum energy needed to remove the loosest electron from the surface of a metal.
=hfo = planks constant x threshold of frequency
Einstein’s Explanation of the Photoelectric Effect
- Light must be considered as a stream of “packets of energy”. Each packet is called a photon or quantum of energy.
A photon is a packet of electromagnetic energy.
- The energy of a photon is proportional to the frequency.( f goes up, so does e)
- E = hf (h is planks constant)
- The brighter the source, the more photons it gives out per second. (greater intensity)
- The energy needed to remove the loosest electron is the work function
- Electrons can only get the energy from one photon.
- If the energy in a photon is less than the work function, no electrons are emitted.
- If the energy is greater, they are.
- If the energy is greater, the extra energy is converted into kinetic energy.
Einstein’s Photoelectric Law
- The velocity of an electron emitted range from 0 to a definite maximum.
- Max velocity (max kenotic energy), increases with frequency.
Explanation
The kinetic energy of the fastest electron emitted is the difference between the energy of the photon and the work function
1/2mv2max = hf – work function = hf – hfo
Electrons which are more tightly held will be emitted at a lesser speed.
As f increases, so does v.
Applications of Photoelectric sensing devices
- Burglar alarms
- automatic doors
- counting items on a conveyer belt
- soundtrack of a film
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.
- It was discovered by Rontgen.
The Hot Cathode X-Ray tube
- At the Cathode, thermionic emission occurs and a beam of electrons is produced.
- High voltage accelerates electrons to the anode to a very high speed.
- 1% of the energy is converted to x-rays.
- 99% is converted to heat. Circulating coolant removes this. Target often made of tungsten as it has a very high melting point.
- Tube surrounded by lead shield – doesn’t let x-rays through, apart from through a small window.
X-Ray production is the inverse of the photoelectric effect.
Photoelectric =) electromagnetic energy to kinetic energy
X-Ray =) kinetic to electromagnetic
Properties of X-rays
- Electromagnetic radiation, wavelength 10-9 to 10-15m.
- They ionise the material that they pass through. (knock electrons from atoms)
- They penetrate through materials – denser the material, the more it absorbs and less penetration.
- Not deflected in magnetic or electric fields.
- Make certain materials fluoresce (zinc sulphide)
- Can cause photoemission.
Uses of X-Rays
X-Ray photos bones, food with barium sulphate photo of stomach and intestines, destroying cancer cells, detect cracks/flaws in metals, find thickness of things, photograph the inside of machines without taking them apart.
Hazards
Ionising radiation, have a damaging effect on human tissue.