Chapter 25 Electromagnetic Induction

Objectives 25.1 Explain how a changing magnetic field produces an electric current 25.1 Define electromotive force and solve problems involving wires moving in a magnetic field 25.1 Describe how an electric generator works and how it differs from a motor

Objectives 25.1 Recognize the difference between peak and effective voltage and current 25.2 State Lenz's law and explain back EMF and how it affects the operation of motors and generators 25.2 Explain self-inductance and how it affects circuits 25.2 Describe a transformer and solve problems involving voltage, current, and turns ratios

Key Terms Electromagnetic Induction: Process of generating a current through a circuit by motion of a wire in a magnetic field Electromotive Force (EMF): Potential difference in volts. Not actually a force, just a voltage Electric Generator: Converts mechanical energy to electric energy (wire loops placed into a magnetic field) Motor: Electric energy to Mechanical Energy

Alternating Current Produces peak voltages, rather than a steady voltage. About 70% of the peak is the effective voltage

60 Hz

Relationship between P, V, and I in AC

Voltage leads the current – Sin graph of Voltage is slightly ahead of the current Effective power is actually ½ the power compared to a DC

Lenz’s Law The direction of the induced current is such that the magnetic field resulting from the induced current opposed the change in the field

Lenz Law Simply stated: Electrons as they move in a magnetic field create a field themselves which opposes their own movement. Sometimes called back emf Higher current = Larger opposing magnetic field Lights have most current initially before the back emf slows down the electrons

Motors and Back EMF At first, large current due to low resistance. As motor begins to turn, it begins to resist itself as the coils moving create a back emf, reducing the current. Placing a load onto a motor (a device which needs electrical energy) reduces the rate of spinning on the motor and allows more current

Back EMF When a device is unplugged, sometimes you can see a spark produced because of the back emf being large enough.

Coils proportional to voltage and current

Eddy Currents