Generators & Motors

• Textbook Sections 23-6 – 23-9

http://www.walter-fendt.de/ph14e/electricmotor.htmhttp://www.walter-fendt.de/ph14e/generator_e.htm

Physics 1161: Lecture 13

Review: Two uses of RHR’s

• Force on moving charge in Magnetic field – Thumb: v (or I)– Fingers: B – Palm: F on + charge

• Magnetic field produced by moving charges – Thumb: I (or v for + charges)

– Fingers: curl along B field

Palm: out of page.

BI

F

+ v+ + +

I

Review: Induction• Lenz’s Law

– If the magnetic flux (B) through a loop changes, an EMF will be created in the loop to oppose the change in flux

– EMF current (V=IR) additional B-field.• Flux decreasing => B-field in same direction as original

• Flux increasing => B-field in opposite direction of original

• Faraday’s Law– Magnitude of induced EMF given by:

if

if

ttt

A conducting loop is halfway into a magnetic field. Suppose the magnetic field begins to increase rapidly in strength.

1 2 3 4 5

20% 20% 20%20%20%

Which of the following statements is true? 

1. The loop is pushed upward, toward the top of the page. 

2. The loop is pushed downward, toward the bottom of the page. 

3. The loop is pulled to the left, into the magnetic field. 

4. The loop is pushed to the right, out of the magnetic field. 

5. The tension in the wires increases but the loop does not move.

A conducting loop is halfway into a magnetic field. Suppose the magnetic field begins to increase rapidly in strength.

1 2 3 4 5

20% 20% 20%20%20%

Which of the following statements is true? 

1. The loop is pushed upward, toward the top of the page. 

2. The loop is pushed downward, toward the bottom of the page. 

3. The loop is pulled to the left, into the magnetic field. 

4. The loop is pushed to the right, out of the magnetic field. 

5. The tension in the wires increases but the loop does not move.

A current-carrying wire is pulled away from a conducting loop in the direction shown.

1 2 3

33% 33%33%As the wire is moving, is there a cw current around the loop, a ccw current or no current? 

1. There is a clockwise current around the loop.

2. There is a counterclockwise current around the loop.

3. There is no current around the loop.

A current-carrying wire is pulled away from a conducting loop in the direction shown.

1 2 3

33% 33%33%As the wire is moving, is there a cw current around the loop, a ccw current or no current? 

1. There is a clockwise current around the loop.

2. There is a counterclockwise current around the loop.

3. There is no current around the loop.

A square loop of copper wire is pulled through a region of magnetic field as shown in the figure.

1 2 3 4

25% 25%25%25%

Rank in order, from strongest to weakest, the pulling forces,F1, F2, F3, and F4 that must be applied to keep the loop moving at constant speed.

1. F2 = F4 > F1 = F3

2. F3 > F2 = F4 > F1

3. F3 > F4 > F2 > F1

4. F4 > F2 > F1 = F3 5. F4 > F3 > F2 > F1

A square loop of copper wire is pulled through a region of magnetic field as shown in the figure.

1 2 3 4

25% 25%25%25%

Rank in order, from strongest to weakest, the pulling forces,F1, F2, F3, and F4 that must be applied to keep the loop moving at constant speed.

1. F2 = F4 > F1 = F3

2. F3 > F2 = F4 > F1

3. F3 > F4 > F2 > F1

4. F4 > F2 > F1 = F3 5. F4 > F3 > F2 > F1

Motional emfThe lightbulb in the circuit has a resistance of 12 Ω and consumes 5.0 W of power; the rod is 1.25 m long and moves to the left with a constant speed of 3.1 m/s. What is the strength of the magnetic field?

mft

B A

t

B l x

t

mf Blv

To find I: P IV 2I R

mf IR and

PI

R 5.0

12

W

0.645 A

Now … Blv IRIR

Blv

(0.645 ) (12 )

(1.25 ) (3.1 / )

A

m m s

2.00T

Motional emfThe lightbulb in the circuit has a resistance of 12 Ω and consumes 5.0 W of power; the rod is 1.25 m long and moves to the left with a constant speed of 3.1 m/s. What external force is required to maintain the rod’s constant speed?

F dP

t

F v 5.0

1.613.1

P WF N

v m s

Review: Rotation Variablesv, , f, T

• Velocity (v):– How fast a point moves. – Units: usually m/s

• Angular Frequency ():– How fast something rotates. – Units: radians / sec

r

vv

v = r

f = / 2

T = 1 / f = 2 /

• Frequency ( f ):– How fast something rotates.– Units: rotations / sec = Hz

• Period (T):– How much time one full rotation takes.– Units: usually seconds

Generators and EMF

rL = A

side 1 = r B L sin()

side 2 = r B L sin()

loop = side 1 + side 2

2r B L sin()

loop = A B sin()

loop = A B sin(t)

vv

•

x

r

1

2

t

AB

AB

EMF is voltage!

side 1 = v B L sin()v = r

At which time does the loop have the greatest emf (greatest / t)?

1 2 3

0% 0%0%

1. 12. 23. 3

At which time does the loop have the greatest emf (greatest / t)?

1 2 3

0% 0%0%

1. 12. 23. 3

1) Has greatest flux, but = 0 so = 0.

2) (Preflight example) 30 so AB/2.

3) Flux is zero, but = 90 so = AB.

Comparison:Flux vs. EMF

Flux is maximum– Most lines thru loop

EMF is minimum– Just before: lines enter from left– Just after: lines enter from left– No change!

Flux is minimum – Zero lines thru loop

EMF is maximum – Just before: lines enter from top. – Just after: lines enter from bottom.– Big change!

•

x

•x

CheckpointRotating Loop

Flux is _________ at moment shown.

Increasing

decreasing

not changing

When =30°, the EMF around the loop is:

increasing

decreasing

not changing

Which of the following graphs is the correct graph of EMF vs. angle for the loop shown above?

Flux is decreasing at moment shown.

When =30°, the EMF around the loop is:

increasing

decreasing

not changingEMF is increasing!

CheckpointRotating Loop

Generator

Generators and Torque

vv

•

x

r

= A B sin()

Recall:

= A B I sin()

= A2 B2 sin2()/RTorque, due to current and B field, tries to slowspinning loop down. Must supply external torque to keep it spinning at constant

Voltage!

Connect loop to resistance R use I=V/R:

I = A B sin() / R

Generator

vv

•

x

A generator consists of a square coil of wire with 40 turns, each side is 0.2 meters long, and it is spinning with angular velocity = 2.5 radians/second in a uniform magnetic field B=0.15 T. Determine the direction of the induced current at instant shown. Calculate the maximum emf and torque if the resistive load is 4.

= NA B sin() Units?

= NI A B sin() Units?

Generator

vv

•

x

A generator consists of a square coil of wire with 40 turns, each side is 0.2 meters long, and it is spinning with angular velocity = 2.5 radians/second in a uniform magnetic field B=0.15 T. Determine the direction of the induced current at instant shown. Calculate the maximum emf and torque if the resistive load is 4.

= NA B sin()

= NI A B sin()

Note: Emf is maximum at =90

Note: Torque is maximum at =90

= (40) (0.2m)2 (0.15T) (2.5 radians/s)

= 0.6 Volts

= 40*I0.15A*(0.2m)2 * 0.15 T* 1

= 0.036 Newton-meters

maxmax

emfI

R

0.60.15

4

VA

Power LinesCheckpoint

• Power is transferred from the power plant to your house through high voltage power lines because:

• Generators at power plants operate at high voltages.• It will decrease power loss.• The power company wants to discourage people

from climbing on the lines.

MotorAn electric motor is exactly the opposite of a generator – it uses the torque on a current loop to create mechanical energy.