Turning points

Question 1

Cathode rays

Answer 1

When a potential difference is applied across a discharge tube with a low pressure gas inside of it, the tube will begin to glow with glowing brightest at the cathode.

Question 2

Properties of cathode rays

Answer 2

• Have a mass
• Have a negative charge
• Have the same properties no matter what gas is used in the discharge tube
• Have a very large charge to mass ratio

Question 3

Process by which the discharge tube begins to glow

Answer 3

1) The high potential difference across the discharge tube will pull electrons off the gas atoms, forming ion and electron pairs
2) The positive gas ions are accelerated towards the cathode and when they collide with it they release even more electrons
3) The electrons are accelerated along the tube (because the gas is at low pressure, the electrons are accelerated to high speeds) and collide with gas atoms causing them to become excited. The atoms will quickly de-excite and release photons of light.

Question 4

Thermionic emission

Answer 4

When a metal is heated until the free electrons on its surface gain enough energy and are emitted.

Question 5

Electron guns

Answer 5

Electron guns use a potential difference to accelerate electrons, which are released from the cathode by heating it (thermionic emission). The electrons are accelerated towards the anode (which has a small gap) and the electrons which pass through this gap form a narrow electron beam which travels at a constant velocity beyond the anode. Work done on a charge particle is: W=QV, therefore the work done on an electron accelerated through a potential difference V is: W=eV.

As the electrons move from the cathode towards the anode, its electrical potential energy is converted into kinetic energy. At the anode, (1/2)mv^2 = eV

Question 6

How to use Thomson’s crossed fields to find the specific charge of an electron

Answer 6

This apparatus involves magnetic and electric fields which a perpendicular to each other, where the electric field and magnetic fields deflect the electrons in opposite directions.
1) Electrons are accelerated using an electron gun and enter the apparatus perpendicular to direction of both fields. The electrons will be deflected upwards by the electric field, while being deflected downwards by the magnetic field
2) The strengths of these fields are adjusted until the electron beam passes through the crossed fields undeflected, therefore the electric and magnetic forces are equal and opposite

Question 7

Millikan’s determination of the electric charge

Answer 7

An atomizer is used to spray tiny droplets of oil, which are negatively charged due to friction. These droplets fall until they reach two parallel plates which form a uniform electric field, as the droplets are charged they will experience an electric force. The strength of the field can be adjusted by changing the potential difference between the plates, until the observed oil droplet becomes stationary, meaning that its weight is equal to the electric force it experiences upwards: EQ=mg

Question 8

Newton’s corpuscular theory of light

Answer 8

Newton theorised that light was formed of tiny particles called corpuscles, with the following explanations for these properties of light:
- Reflection - the corpuscles collide with the surface and a repulsive force pushes them back, causing their component of velocity perpendicular to the surface to change direction, while their component of velocity parallel to the surface stays the same.
- Refraction - as the corpuscles approach a denser medium, short-range forces of attraction cause their component of velocity perpendicular to the surface to increase, while the parallel component of velocity stays the same, therefore the light will bend towards the normal. According to Newton’s explanation light travels faster in denser mediums.