Electrons Flashcards
How are cathode rays produced in a discharge tube?
The gas must be at sufficiently low pressure.
If voltage applied to the tube is high enough, some of the gas atoms are ionised.
Positive ions created near cathode surface are attracted to cathode. Photons are emitted when some of the ions and electrons produced by ionisation recombine.
Some electrons do not recombine and move towards anode. This causes excitation by collision of gas atoms. Photons are emitted when the atoms de-excite.
What were J.J. Thomson’s conclusions about cathode rays?
They have energy, momentum and mass.
They are negatively charged.
They have the same properties, regardless of what gas is used.
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What is thermionic emission?
Thermionic emission can produce electrons without a gas.
When a metal is heated, some of the free electrons gain enough kinetic energy to leave the metal surface. The electrons can be accelerated by an electric field.
What affects the speed of the electrons produced in thermionic emission?
Increasing the p.d. increases the force attracting the electrons towards the anode.
This results in the electrons gaining higher speed.
How can you calculate the specific charge of an electron?
Using a magnetic field
r = mv/Be
e/m = v/Br
What was the significance of the specific charge of an electron?
Thomson showed that electron specific charge is 1.76 x 10^11 C/kg.
This was 1860 times larger than that of a hydrogen ion (largest specific charge known at the time).
Describe Millikan’s oil drop experiment.
Controlled the motion of charged oil droplets using the electric field between oppositely charged parallel plates.
Made any charged droplet stay stationary by adjusting the p.d. between the plates until the electric force was equal and opposite to the weight of the droplet.
With no electric field present, an oil droplet falls at terminal velocity due to the drag force (viscous force) equaling the weight of the droplet. From this he was able to work out the radius of the droplet and therefore the mass.
What was the significance of Millikan’s results?
He found that the charge Q was always an integer multiplied by 1.6x10^-19 C. He concluded that the charge of the electrons is 1.6x10^-19 C.
