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Effect of Changing Magnetic Fields: Induced EMF

Effect of Changing Magnetic Fields: Induced EMF. April 15 th 2011. Lenz’s Law. Lenz's law: states that an induced current has a direction such that its magnetic field opposes the change in magnetic field that induced the current.

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Effect of Changing Magnetic Fields: Induced EMF

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  1. Effect of Changing Magnetic Fields: Induced EMF April 15th 2011

  2. Lenz’s Law • Lenz's law: states that an induced current has a direction such that its magnetic field opposes the change in magnetic field that induced the current. • This means that the current induced in a conductor will oppose the change in current that is causing the flux to change. • We will be using our second RHR when dealing with Lenz’s law.

  3. Examples • The N-pole of a magnet is moved toward the right end of a coil. To oppose the approach of the N-pole, the right end of the coil must also become a N-pole.

  4. Cont. • Treat the coil as if the magnetic field lines are emerging from the right end of the coil. • Use the 2nd RHR – the induced current must flow in a counter clockwise direction. • What would happen if the S-pole approaches the coil? What direction would the induced current move?

  5. Cont.

  6. Lenz’s law: Generators & Motors • If a generator produces only a small current, then the opposing force on the armature will be small and the armature will be easy to turn – in contrast; larger current produced, force on the current will be larger, armature will be more difficult to turn. • When a motor is first turned on, a large current flows because of the low resistance of the motor.

  7. Cont. • As the motor begins to turn, the motion of the wires across the magnetic field induces the back – EMF that opposes the current flow – this reduces the net current flowing through the motor. • If a mechanical load is placed on the motor, slowing it down, the back EMF is reduced and more current flows.

  8. Cont. • The heavy current required when a motor is started causes voltage drops across the resistance of the wires that carry current to the motor. • Think of having the dome light/headlights on in your car when you go to start it – they dim as the key is turned over. • As the motor picks up speed, the voltage will rise again and the bulb will brighten.

  9. Cont. • *** When the current to a motor is interrupted by turning off a switch in the circuit or by pulling the motor’s plug from a wall outlet, the sudden change in magnetic field generates a back EMF that can be large enough to cause a spark across the switch or between the plug and the wall outlet.***

  10. Eddy – Current Damping. • A sensitive balance uses Lenz’s law to stop its oscillation when an object is placed on the pan. • A piece of metal attached to the balance arm is located between the poles of a horseshoe magnet, as the balance arm swings, the metal moves through the magnetic field. • Currents called eddy-currents are generated in the metal – these currents produce a magnetic field that acts to oppose the motion that cause the currents – slowing down the metal arm.

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