Starter

A motor converts ______________energy into __________energy.

Induced Voltage:Three Cases

• The B Field can change :

V= NA(DB /Dt )

• The area can change (motional emf):

V= BLv

• The angle can change (generator):

V = wNBA sinwt

The B Field can change :

V= NA(DB /Dt )

Example: A magnet is moved into a coil with 100 turns and an area .10m2. The field in the loop changes from zero to 2T in .1 seconds. 1. What is the induced voltage? V = NA(DB /Dt ) = (100)(.1)( 2/.1) = 200Volts2. If the coil has a resistance of 100 Ohms, what’s the current in the coil?

I = V/R = 200/100= 2 Amps

The area can change: V= Blv

Example: A metal bar moves to the right with a velocity of 10m/s through a magnetic field of 3T. The loop height, L, is .5m.

1. What is the induced voltage?

2. If the circuit has a resistance of 2 Ohms, what’s the current in the circuit?

V = BLv = (3)(.5)( 10) = 15Volts

I = V/R = 15/2= 7.5 Amps

The angle can change: V= wNBA sinwt

Example: A generator turns at 100rad/sec. It has 500 turns, and a magnetic field of .5T. The loop area is.01 m2. What is the maximum voltage?

This is used for the generator. The maximum voltage is:

Vmax = wNBA.

Vmax = wNBA = 100(500)(.5)(.01) = 250Volts.

Lenz’s Law

When an external magnetic field changes through a conducting loop, a current is induced that produces another magnetic field that opposes the external change.

Lenz’s Law

As the magnet moves, the current in the resistor goes from….

A to B

Lenz’s Law

As the magnet moves, the current in the resistor goes from….

B to A

Lenz’s Law

If the B field points out and increases, the current in the loop is…………..

Clockwise

Lenz’s Law

If the B field points in and increases, the current in the loop is…………..

Counter - Clockwise

STARTER

If the B field points into this conducting loop of wireand increases with time, the direction of the current in the loop is…………..

Counter-Clockwise

Transformers

Transformer EquationsN1 = number of turns on the primary side

N2 = number of turns on the secondary side

V1 = AC voltage applied to the primary side

V2 = AC voltage on the secondary side

I1 =current in the primary windings

I2 = current in the secondary windings

From Faraday’s Law : V2 = V1 (N2/N1)

From conservation of energy: V1 I1 = V2 I2

Then: I2 = I1 (N1/N2)

Transformer Equations

Step Up Transformer N2>N1

Example: A transformer has 5000 turns in the secondary and 1000 turns in the primary.

The primary voltage is 100V (AC) and the primary current is 25 Amps.

1. What is the secondary voltage?2. What is the secondary current?3. What is the power in the primary and

secondary?

Solve the Problem

Givens: V1 = 100 N1 = 1000 N2= 5000

I1 = 25

1. Solve for V2 :

V2 = V1 (N2/N1) = 100( 5000/1000) = 500 Volts

2. Solve for I2:

I2 = I1 (N1/N2) = 25( 1000/5000) = 5 Amps.

3. P1 = P2 : V1 I1 = V2 I2 = 2500 Watts

Step Down Transformer N2<N1

Example: A transformer plugged into the wall ( 120V) charges a 9 volt battery.

1. What is the turn ratio (N1/N2) ?

2. What is the secondary current if the primary current is .5A?

3. What is the power in the primary and secondary?

Solve the Problem

Givens: V1 = 120 V2 = 9 I1 = .5

1. Solve for (N2/N1) :

V2 = V1 (N2/N1) so (N2/N1) = V2 /V1 = 9/120 = .075

2. Solve for I2:

I2 = I1 (N1/N2) = .5 ( 1 /.075) = 6.67Amps.

3. P1 = P2 : V1 I1 = V2 I2 = 60 Watts

AC Voltage and Current

Example

What is the peak voltage for household voltage of 120V(rms)?

V(peak) = 1.41(120) = 169 Volts