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F16 PH333 7.1.1-7.1.3

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Richard Datwyler

on 7 December 2018

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Transcript of F16 PH333 7.1.1-7.1.3

7.1.1 Ohms Law
7.1.2 Electromotive force
7.1.3 Motional Emf
Electromotive force
Ohms law
There must be a force that drives current
Here this lowercase f is force per unit charge
In electrodynamics, the force is from E and B fields using Lorentz force.
Generally it is only from the E field because v x B is small (it is often used in plasma physics however)
If the E is constant you can rewrite and eventually get
Velocity and heating
This idea of a force causing current leads to a misleading idea of acceleration
but the current isn't. it is moving at a slow drift speed
yet the electrons are energetic and move with a thermal energy.
and there is a great equations of how conductivity is related to charge density and temperature
These thermal collision heat the wire, and that power is in the familiar Joule heating
not a force.
Motional Emf
Pulling a wire through a magnetic field will push charges around creating a potential, an emf.
Also useful to define Emf from magnetic flux
there are 3 main ways to change this flux. And there is an issue with the directions of things, but we'll save that for next lesson.
Magnetic flux
The magnetic flux is a counting game. How much magnetic field goes through cross-sectional region.
As this flux changes, this will induce an emf and a possible current in a wire that has its flux changing in time.
two cylinders are separated by material with conductivity, what is the current through it for some length L,if they have a potential difference of V.
"I'm not sure I understand the emf integral."
add it up all the way around
in a perfect conductor the net force is zero
only in the battery
"I heard emfs are really dangerous. What makes an emf bad for health? "
"Figure 7.8 was confusing reading."
"How does drift velocity play a part in current and whats the difference between it and thermal velocity?"
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