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ES Lesson 7: AC electricity
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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?
© 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.
BE demo 1-2
Make: Electronics, O’reilly [Publishing], Charles Platt, 2009
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
IQT Corporation, Electric Pizza, An in house course, 2010
7
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.
BE demo 1-4
~ 1000
turns
~ 142
turns
AC VOM
120 Volts AC
Oscilloscope
AC Sine Wave
1 Megohm
resistor
AC VOM
~ 17 Volts AC
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
[A.O.Smith:
The AC's and DC's of Electric Motors]
[Intro to College Physics, Appleton-Century-Crofts, R.D. Rusk, 1960]
Mutual coupling between electromagnetic fields
(a) strong coupling
(b) no coupling
(c) weak coupling
[Intro to College Physics, Appleton-Century-Crofts, R.D. Rusk, 1960]
One way to measure input and output currents
VOM
Amps
VOM
Amps
AC
Generator
(Volts)
LED
Constructing a transformer with a circular magnet
Second method for measuring the currents
VOM
V ac
VOM
V ac
AC
Generator
(Volts)
Light