• What is an alternating current (AC)?

• What is an AC generator?

• What is an AC transformer?

• How to calculate a transformer’s expected step-up or step-down AC voltage?

ES Lesson 7: AC electricity

© 2012 C. Rightmyer, Licensed under The MIT OSI License, 20 July 2012

The magnetic field generated by a coil of wire that has a current flowing through it.

Electricity and Magnetism, Gareth Stevens Publishing, Steve Parker, 2007

A compressed coil of wire with a current passing through it acts just like a button magnet.

[Intro to College Physics, Appleton-Century-Crofts, R.D. Rusk, 1960]

ES demo 7-1. A moving magnetic field inside a coil of wire induces current in that coil.

Make: Electronics, O’reilly [Publishing], Charles Platt, 2009

BE demo 1-2

How does an AC generator work?

Continuous slip rings required to connect current to the load.

[Intro to College Physics, Appleton-Century-Crofts, R.D. Rusk, 1960]

Direction and amplitude of generated voltage

7IQT Corporation, Electric Pizza, An in house course, 2010

Electrical power delivery system

Electricity and Magnetism, Gareth Stevens Publishing, Steve Parker, 2007

Direction and amplitude of the generated current

[Intro to College Physics, Appleton-Century-Crofts, R.D. Rusk, 1960]

How’d those electrons get into your outlet?

ES demo 7-2. Use a toy generator to produce AC and DC voltages. Display output using a multimeter and oscilloscope.

BE demo 7-1

ES demo 7-3. Transformer operation using two AC voltmeters and an oscilloscope.

~ 1000 turns

~ 142 turns

AC VOM

120 Volts AC

AC VOM

~ 17 Volts AC

Oscilloscope

AC Sine Wave

1 Megohmresistor

BE demo 1-4

Output on the secondary coil Output on the secondary coil depends on input voltage and turns ratio

(Vs) = Vp (Ns / Np)

where:

Vs = voltage in secondary

Vp = voltage in primary

Ns = number turns in secondary

Np = number turns in primary

Back up charts

Fields associated with different shaped magnets

[Intro to College Physics, Appleton-Century-Crofts, R.D. Rusk, 1960]

[A.O.Smith:The AC's and DC's of Electric Motors]

Mutual coupling between electromagnetic fields

[Intro to College Physics, Appleton-Century-Crofts, R.D. Rusk, 1960]

(a) strong coupling

(b) no coupling

(c) weak coupling

One way to measure input and output currents

LED

VOM

Amps

VOM

Amps

ACGenerator

(Volts)

Constructing a transformer with a circular magnet

Second method for measuring the currents

Light

VOM

V ac

VOM

V ac

ACGenerator

(Volts)