Chapter 20

Electromagnetic Induction

Electromagnetic Induction

• Electricity and magnetism• Generators, motors, and transformers

Current

Previously, we said three things were required for a current to flow in a circuit:

1. Potential difference2. Closed circuit3. Charges free to move

Electromagnetic Induction

electromagnetic induction = the process of creating a current in a circuit loop by changing the magnetic field that passes through the circuit

(Inducing an emf)

Linear Induction Flashlight / Faraday Flashlight / “Shake Flashlight”

Lenz’s LawLenz’s Law describes the rule for finding the direction of the induced current:

Note: the field of the induced current does not oppose the applied field, but the change in the applied field (so that the total field strength remains constant).

The magnetic field of the induced current is in a direction to produce a field that

opposes the change causing it.

Circuit is moved into the magnetic field

Circuit is rotated in the magnetic field

Intensity of the magnetic field is varied

Magnetic Flux

magnetic flux = the number of field lines that cross a certain area at right angles to that area

Units: Weber (Wb)

ΦM=𝐴𝐵cos 𝜃(magnetic field)(surface area)×

magnetic flux =

(cosine of the angle between B and the normal to the plane of the loop)

×

Faraday’s Law of Magnetic Induction

For a single loop,

* Minus sign indicates the polarity of the induced emf (reflects Lenz’s Law)

𝑒𝑚𝑓=−ΔΦ𝑀

Δ𝑡

PracticeA circuit is stretched over a period of 0.25 seconds in a magnetic field of 0.050 T. The circuit dimensions change from 4.0 m x 4.0 m to 2.0 m x 6.0 m, as shown below. Determine the emf induced in the circuit.

�⃗�

start end

4.0 m

4.0 m 6.0 m

2.0 m

Faraday’s Law of Magnetic Induction

For multiple loops,

N = the number of loops in the circuit

𝑒𝑚𝑓=−𝑁ΔΦ𝑀

Δ𝑡

PracticeA coil with 25 turns of wire is wrapped around a hollow tube with an area of 1.8 m2. Each turn has the same area as the tube. A uniform magnetic field is applied at a right angle to the plane of the coil. If the field increases uniformly from 0.00 T to 0.55 T in 0.85 s, find the magnitude of the induced emf in the coil. If the resistance in the coil is 2.5 Ω, find the magnitude and direction of the induced current in the coil.