Lecture 9 - Electromagnetism

Electromagnetic induction - Faraday

• changing magnetic flux through a loop induces an electromotive force (emf) or voltage = current

• such a current is called an inducted current

• faster the magnet movement → larger the current

• direction of magnetic motion determines the direction of the induced current

• an induced emf is always in a direction that opposes the original change in flux that it caused

applications of induction

• microphone

• credit card reader

• seismograph

Faraday’s Law of Induction

if flux in a coil changes over a change in time, the average emf is calculated by: -(change in flux/change in time)

emf = - N (change in (BA costheta)/time)

Current and emf formula

I = emf/R

transformer and transmission of power

• transformer is a device for increasing or decreasing an ac voltage.

• consists of two coils linked by an iron. core

• a changing emf in one coil induces the emf in the other coil

• the ratio of emfs is equal to the ratio of the number of turns in each coil

Step up or Step down

step up = primary has less coils than secondary, less volts too

step down = primary has more coils than secondary, more volts too

when a transformer increases the output voltage, does it mean that the output power is also increased

no, conservation of energy says that input power = output power

why the voltage is stepped down when transmitting power from the power plants to homes?

when secondary voltage is increased, secondary current is decreased, power loss in line is P = I²R, therefore voltage is stepped down to reduce power loss in the line