1. Electromagnetic induction
Generators, Motors, and Mutual Inductance

2. Generators and Alternating Current
Generator – a device that uses induction to convert mechanical energy to electrical energy
Commonly uses rotational energy by having steam or running water turn a turbine
Steam may be generated by a coal or natural gas fire or from geothermal heat sources
The rotation of the turbine causes a wire loop to rotate in a magnetic field

3. Generators and Alternating Current
When a loop is parallel to a magnetic field, the charges are perpendicular to the magnetic field
Current is maximized, induced emf is maximized
When a loop is perpendicular to a magnetic field, the charges are parallel to the magnetic field
Current is zero, induced emf is zero
Induced emf versus time graphs as a sine curve
emf for a generator = number of loops * cross sectional area of the loops * magnetic field strength * angular frequency of rotation of loops * time
emf = NABt

4. Generators and Alternating Current
When the loop and the magnetic field are parallel, we may calculate the maximum emf
Maximum emf for a generator = number of loops * cross sectional area of the loops * magnetic field strength * angular frequency of rotation of loops
emfmax = NAB
Angular frequency = 2*pi*frequency
 = 2f

5. Generators and Alternating Current

6. Generators and Alternating Current

7. Generators and Alternating Current
Sample Problem: A generator consists of exactly eight turns of wire, each with an area A = 0.095m2 and a total resistance of 12. The loop rotates in a magnetic field of 0.55T at a constant frequency of 60.0Hz. Find the maximum induced emf and maximum current in the loop.
f = 60.0Hz A = 0.095m2 R = 12
B = 0.55T = 0.55V*s/m2 N = 8
emfmax = ? Imax = ?
 = 2f
emfmax = NAB
I=emf / R

8. Generators and Alternating Current

9. Generators and Alternating Current
Alternating current (ac) – an electric current that changes direction at regular intervals
Typically produced in generators
Is reflected in the sinusoidal nature of the graph
In the US, Canada, and Central America the current reverses itself at a frequency of 60Hz or 60 reversals/second
In Europe and most of Asia and Africa, the frequency is 50Hz

10. Generators and Alternating Current
Alternating current can be converted in to direct current
The conducting loop in an ac generator must be free to rotate while remaining part of the circuit at all times
The ends of the conducting loop are connected to conducting rings called slip rings that rotate with the loop
Connections to the external circuit are made by stationary graphite strips called brushes that stay in contact with the slip rings

11. Generators and Alternating Current
Both the loop current and the output current are continuously changing direction
By replacing the two slip rings with a single split slip ring called a commutator, the generator can produce direct current
The brushes change halves of the commutator at the same instant the current reverses so there is a double reversal which cancels out leaving the current flowing in a single direction
By using multiple loops and commutators, the fluctuations from the individual loops are canceled out resulting in an almost constant output current

12. Generators and Alternating Current

13. Generators and Alternating Current

14. Motors Motors – convert electrical energy into mechanical energy
Reverse of a generator
Looks much like a dc generator
The coil of wire is mounted on a rotating shaft and is positioned between the poles of a magnet.
Brushes make contact with a commutator, which alternates the current in the coil.
The alternation of current causes the magnetic field produced by the current to regularly reverse and thus always be repelled by the fixed magnetic field.

15. Motors
The coil and shaft are therefore kept in continuous rotational motion
Back emf – the emf induced in a motor’s coil that tends to reduce the current powering the motor
The induced emf
If this did not occur, Lenz’s law would be violated
The faster the coil rotates, the greater the back emf
The potential difference available to supply current to the motor equals the difference between the applied potential difference and the back emf

16. Motors

17. Mutual Inductance
Faraday used a two coil system to demonstrate electromagnetic induction
Primary circuit is a simple dc circuit with a switch and a wire coil (the primary coil) wrapped around an iron ring
Secondary circuit is a wire coil (secondary coil) wrapped around another part of the same iron ring connected to a galvanometer
By opening and closing the switch in the primary circuit, an emf is induced in the secondary circuit
A steady current will not produce an emf in this manner, only a variable current
Faraday’s law can be rewritten for a changing primary current

18. Mutual Inductance emf = - N * ΔΦM / Δt = -M * ΔI / Δt
M is a constant called mutual inductance
The ability of one circuit to induce an emf in a nearby circuit in the presence of a changing current
The secondary circuit can also induce an emf in the primary circuit
Induction of Current by a Fluctuating Current
Changing the number of turns in the coil is the basis of the transformer