Electromagnetic Waves

# Electromagnetic Waves - Magnetic forces on electric charges...

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Magnetic forces on electric charges Like electric forces 1. The force on q is proportional to the magnitude of q 1. The force on q is proportional to the magnitude of B 2. The force on a negative charge is opposite the force on a positive charge Not like electric forces 4. The force is proportional to the velocity v 5. The magnitude and direction of F depends on the angle between v and B F=qvBsin θ

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Magnetic fields Unit for magnetic fields is T(Tesla) A 1 T field is a large magnetic field The Earth’s magnetic field is only about 0.0005 T A superconducting magnet can be on the order of 10 T
Clicker question a) b) c) d)

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Clicker question a) b) c) d)
Direction of force |F| = qvB sin θ gives the magnitude What about the direction? F is perpendicular to v and to B What if I have a magnetic force acting on a charge over a distance d? How much work does it perform? W=Fd(cos θ ) where θ is the angle between F and d Zero The force is always perpendicular to the displacement The force changes the direction of the particle, it causes an acceleration, but no change of kinetic energy.

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Motion in a magnetic field We’ve seen that the magnetic force acting on a moving electric charge is always perpendicular to the direction of motion A force that is always perpendicular to the motion changes the direction of motion but not the speed If I have a particle moving perpendicular to a uniform magnetic field, that particle will undergo uniform circular motion F c = qvB = ma r = mv 2 r r = mv qB This is what happens in a cyclotron, like the NSCL next door.
Forces on currents An electric current is just a bunch of moving electric charges So there should be a force acting on an electric current moving in a magnetic field in the same direction as the force would be on the individual moving charges I

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Force on a current Same direction as force on a positive charge moving in the direction of the current where we define the direction of the current as the direction that positive charges would move even though a current is the flow of electrons
Electric motors How does an electric motor work?

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Electromagnetic Waves - Magnetic forces on electric charges...

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