Electromagnetism Lenzs Law Effects of Changing Magnetic Fields

Electromagnetism Lenz’s Law

Effects of Changing Magnetic Fields l l -when a wire moves through a changing magnetic field, a current is induced in the wire -the induced current in the wire generates a magnetic field about itself which is called an induced field -the induced magnetic field will in turn interact with the external magnetic field of bar magnets through which the wire is moved - the magnetic fields oppose each other which acts to slow down or reduce the velocity of the wire

Interaction of the two Magnetic Fields l l -use the 3 rd LHR to indicate the direction of Fm on wire with the induced current - Fm is directed downwards which is opposite to the velocity of the wire (electric motor effect- Fm acting onto a current carrying wire)

Lenz’s Law l l l the direction of the induced current is such that the magnetic field resulting from the induced current opposes the action of the inducing field KEY: The velocity vector, v, will always be in the opposite direction of the magnetic force vector, Fm. http: //www. launc. tased. edu. au/ONLINE/SC IENCES/physics/Lenz's. html

Eg) dropping a magnet through an aluminum tube (conductor) http: //micro. magnet. fsu. edu/electromag/java/lenzla w/index. html

http: //www. launc. tased. edu. au/online/sciences/physics/Lenz's. html

Lab Demo: l l l Determine the magnetic force exerted onto the falling magnet mass of magnet 0. 0549 kg Weight of magnet Fg =mg Fg = 0. 5386 N Length of tube 1. 52 m Time to Fall t = 8. 00 s

Acceleration of the magnet down the tube

Examples With Lenz’s Law l l Eg) a North pole of a magnet is moved into the right side of a coil as illustrated below As the magnet is moved past the coils, an induced current in the coils (generator effect) will generate a magnetic field to oppose the motion of the magnet The right end of the coil will become a north pole to exert Fm to the right onto the magnet moved left. When the magnet is pulled out to the right, the induced current in the coils is reversed and the N pole becomes a S pole

Lenz’s Law Applied To an Electric Motor l -when a current carrying wire moves through an electric field, a back voltage is generated which opposes the direction of current flow from the voltage source

Electric Motor Effect l as the wire is forced upwards, the velocity of the wire is upwards and the induced current in the wire is generated in the opposite direction of the electron flow from the voltage supply.

Summary l the voltage induced in an electric circuit always acts in such a direction that the current it drives around a closed circuit produces a magnetic field which opposes the change in magnetic flux. "