17.2 Flashcards
What did JJ Thomson realise?
JJ Thomson knew cathode rays were particle beams as they carry charge. They could be affected by electric and magnetic fields.
Describe deflecting beams of charged particles in uniform electric fields…
Electrons travel from a hot cathode to a positively charged anode. Potential energy is converted to kinetic energy, and the electron then has constant horizontal velocity across the uniform electric field, as part of a beam. Weight pulls down on the electron but it has such a small effect it can be ignored. Electrons travel up, so F=-Ee.
SUVAT equations can be used to describe motion, and F=ma for vertical displacement when gravity is ignored.
What happens when current carrying wire enters a magnetic field?
In a wire carrying current, if it passes a magnetic field, there is force acting on the wire. Each electron is pushed to one side, exerting force. The sum of forces is the resultant.
What are the formulae for Ftotal and Fcharge in magnetic fields? how are these derived?
Ftotal=ILB where L is length of wire and B is magnetic field. So force of one electron is found by dividing this by N. Time taken is L/v, the time for the last electron to move distance L. As Q=Nq=It=IL/v, Ftotal=ILB=NqvB and Fcharge=qvB
What was milikan’s experiment?
Milikan oil drop experiment:
Conducted to find the mass of an electron.
1. Squirt oil through a hole in a metal plate. This falls as droplets in view of a microscope. A radioactive source gives the droplets charge.
2. Applying a PD across plates creates an electric field. The PD is made to give an electric field which balances the gravitational field, suspending the charge.
3. The supply is turned off and the oil drops, reaching terminal velocity. By timing the fall, known air viscosity is used to calculate radius of drop. From known density, the drop’s mass is found.
q=mgd/V
What is the inverse square law?
Inverse square law; doubling the length of light causes the area to increase 4 times and luminous intensity (power per unit area) falls by 4 times.
How is the electric field for a point charge derived?
Spacing of field lines indicates their strength. If there are N field lines spreading from point charge Q, density of field lines per unit area = N/4pi*r^2
As N is proportional to the charge, E is proportional to Q/r^2.
What are the formulae for field, potential, force, and energy for a point source?
Electric field=kQ/r^2 (vector). Voltage=kQ/r (scalar), Force=kqQ/r^2 (vector), Energy = kqQ/r (scalar)
