faraday’s and lenz's law

Faraday’s Law

Faraday’s lawThe induced emf in a closed loop equals the negative of the time rate of change of magnetic flux through the loop

ℰ=−𝑑Φ𝐵

𝑑𝑡

Faraday’s law

Example

The magnetic field between the poles of the electromagnet is uniform at any time, but its magnitude is increasing at the rate of 0.020-T/s. The area of the conducting loop in the field is 120cm2, and the total circuit resistance, including the meter, is 5.0Ω.

Example

(a) Find the induced emf and the induced current in the circuit. 2.4 x 10-4 V

Example

(b) What is the corresponding induced current? 4.8 x 10-5 A

Direction of Induced EMF

1. Define a positive direction for the vector area A.

2. From the directions of A and B, determine the sign of the magnetic flux and its rate of change.

3. Determine the sign of induced emf or current. If the flux is increasing, so Ɛ is positive, then the induced emf or current is negative.

Direction of Induced EMF

4. Finally, determine the direction of the induced emf or current using your right hand. Curl the fingers of your right hand around the A vector, with your thumb in the direction of A. If the induced emf or current in the circuit is positive, it is in the same direction as your curled fingers, otherwise, opposite.

Lenz’s Law

• It is a convenient alternative method for determining the direction of an induced current or emf

• It states that the direction of any magnetic induction effect is such as to oppose the cause of the effect.

Lenz’s Law

Lenz’s Law

Lenz’s Law

Generator and Transformer

Terms• Generator- a device which converts

mechanical energy to electrical energy• Commutator- allows the electric current

to flow in one direction• Armature/rotor- carrier of the coil• Stator- constituted by the stationary

parts of a generator

Terms• Alternator- electromagnets in an AC

generator• Dynamo- electromagnets in a DC

generator• Transformer- a device that converts

voltage from one magnitude to another

Terms• Step-down transformer- converts

high voltage rating to a lower value• Step-down transformer- converts a

low voltage rating to a higher value

AC Generator

Transformer

V1/V2= N1/N2

For ideal transformers

I1/I2=V2/V1

Example

A 12-V cassette recorder can be operated from a 220 volt AC line using a transformer.a) How many turns should the secondary coil of

the transformer have if the primary coil has 550 turns?

b) What kind of transformer is found in the cassette recorder?

30 turns; step-down

B

+x

I

I

+z

+y

Practice using the RHR

Sources• Young, Hugh D., and Freedman, Roger A. 2002, Reprinted. University

Physics with Modern Physics. 10th ed. Singapore: Pearson Education Asia Pte Ltd. Copyright © 2000 by Addison-Wesley Publishing Longman, Inc.

• Giancoli, Douglas C. 2005, Reprinted. Physics. 5th ed. Singapore: Pearson Education Asia Pte Ltd. Copyright © 1998, 1995, 1991, 1985, 1980 by Prentice-Hall, Inc.

• Hewitt, Paul. 2008. Physical Sciences. 4th ed. New York: McGraw-Hill Companies, Inc.