Board Work What are the directions of the forces on the opposite charges moving with velocity v through magnetic field B? B v +−

Moving conductors through magnetic fields

Poll Question B I What is the direction of the Lorentz force on the charges moving along this wire? A. B. C. D. E. F.

Current in a Field Feels a Force F B I

Loop in a Field Has Torque B I I

Torque Turns a Motor I I current I force F B

– Motion Creates a Potential v B induced potential +

Rotating Loop Creates Circuit B V V

B V V

Rotation Powers a Generator v v Motion V V induces a potential which generates a current that charges the battery B through the field

Magnetic Induction Transforming energy with magnetism

Objective Explain the electric potential created by a changing magnetic field.

What’s the Point? This is how electric current is harnessed to do mechanical work. It’s another way to think about the Lorentz force. ☺

Lorentz Force Reminder F = qv  B Source: Griffith, The Physics of Everyday Phenomena F is directed out of the screen.

Magnetic Flux Field through a surface  = BA Effectively the number of field lines enclosed by a loop

Flux Change Creates Potential Faraday’s Law The rate of magnetic flux change through a loop equals the electric potential induced around the loop. V =V =  tt

Magnetic Flux Flux changes as loop turns abcde At a, c, e: Greatest flux , least change At b, d: Zero flux, fastest flux change  /  t

Induced current direction Lenz’s Law When a changing magnetic flux induces a current, the induced current’s magnetic field opposes the flux change.

Lenz’s Law Example From a to b: –flux decreases;  into face of loop ← –Loop current increases –Induced field increases out of face of loop abcde Induced current

Lenz’s Law Explains Generator   B changing field opposing field needed I

Board Work If the loop inside this magnetic field expands, in what direction will the induced current around it be? B What is the Lorentz force on each side? Which direction is the flux change? What current would oppose it?

How a Microphone Works microphone geophone Flux inside loop becomes greater

Wave-Powered Generator Originally appeared in Smithsonian magazine.

How a Loudspeaker Works loudspeaker induced motion of magnet current current- generated field attracts or repels magnet

Reading for Next Time Electromagnetic Waves