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# Electromagnetic Induction - PowerPoint PPT Presentation

Electromagnetic Induction. What’s Next?. Electromagnetic Induction Faraday’s Discovery Electromotive Force Magnetic Flux Electric Generators Lenz’s Law Self-Inductance Transformers. What do we know?. Hans Christian Oersted showed that moving charges create a magnetic field.

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### Electromagnetic Induction

• Electromagnetic Induction

• Electromotive Force

• Magnetic Flux

• Electric Generators

• Lenz’s Law

• Self-Inductance

• Transformers

• Hans Christian Oersted showed that moving charges create a magnetic field.

• If moving charges produced a magnetic field, could a moving or changing magnetic field produce a current?

• Faraday discovered that he could induce current by moving a wire loop through a magnetic field or moving the magnetic field through a wire loop.

• Faraday’s Discovery is known as Electromagnetic Induction

x x x x x x x x x

x x x x x x x x x

+

-

L

v

F

Electromotive Force

• Last week we learned the Lorentz Force.

FB = qvB sinθ = ILB

• When a conductor moves through a magnetic field, a force is exerted on these charges causing them to separate, inducing an EMF.

• We know: W = Fd and V = W/q.

V = Fd/q

Using algebra and solving for F:

F = Vq/d

F = qvB

Set these two relationships equal to one another and then solve for V, which will now be represented as EMF:

EMF (V) = vBL

Where: L is the length of a conductor passing through a magnetic field.

EMF = Electromotive Force (Volts)

+

x x x x x x x x x

x x x x x x x x x

x x x x x x x x x

x x x x x x x x x

I

F

v

-

Electromotive Force

• The EMF results when the conductor has a velocity component perpendicular to the magnetic field.

• Use RHR #1 where the thumb points in the direction of the velocity. The force on the bar is opposite the velocity.

I

A segment of a wire loop is moving downward through the poles of a magnet, as shown. What is the direction of the induced current?

a. The current direction is out-of the page to the left.

b. There is no induced current.

c. The current direction is into the page to the right.

• The drawing shows three identical rods (A, B, and C) moving in different planes in a constant magnetic field directed along the +y axis. The length of each rod and the speeds are the same, vA = vB = vC. Which end (1 or 2) of each rod is positive?

• Rod A:

• 1 b. 2 c. neither

• Rod B:

• 1 b. 2 c. neither

• Rod C:

• 1 b. 2 c. neither

• Why is it important?

• Motors

• Generators

• Transformers

• Convert mechanical energy into electrical energy.

• Similar to an electric motor, but function in an opposite manner.

• Electrical power generation is the foundation by which electricity is supplied to homes and businesses around the world.

• Electricity is generated in many ways - hydroelectric, nuclear, coal, gas, oil fired, wind solar, geothermal.

What is magnetic flux?

• Like electric flux

• A measure of the strength of the magnetic field, B, passing through a surface perpendicular to the field.

• For a bar magnet, the flux is maximum at the poles.

• The more magnetic field lines, the higher the flux.

=BAcos

EMF = vBL

• v = Δx/Δt = (x – xo)

(t – to)

• EMF = (Δx/Δt)BL = (xL – xoL) B = (BA) – (BAo)

(t – to) (t – to)

EMF = -ΔΦ/Δt Where:

Φ = BA cos and

• = the angle the normal

to the surface makes

with B (in this drawing it

is 0o).

I

+

x x x x x x

x x x x x x

x x x x x x

x x x x x x

F

I

v

-

• In the drawing on the previous slide, there is only one loop in the circuit.

• When there is more than one loop in a circuit, as in the coil of a solenoid, the EMF induced by a changing magnetic field will increase by a factor equal to the number of loops in the coil.

EMF = -N ΔΦ/Δt

Where N = the number of loops in the coil.

• Note: The units for Φ are Webers (Wb) or 1 Tm2

I

x

x

x

Magnetic Flux & Generators

Direction of Rotation

v

v

v

w

B

v

v

v

Zero Current

Min Change in Flux

Max Current

Max Change in Flux

Axis of Rotation

• When the armature is at 90o with the magnetic field, the current will be zero because the rate of change in magnetic flux through the coil will be at a minimum.

• When the windings of the armature are aligned with the direction of the magnetic field, the current will be at a maximum because the rate of change in magnetic flux will be at a maximum.

• The armature turns such that the coils of wire cut through the magnetic field inducing an EMF in the coil.

• The magnetic field or the conductor need to be moving in order for an EMF to be generated.

• The greater the change in magnetic field, the greater the EMF, ie. the faster the armature turns, the greater the power produced.

• Use RHR #1 to determine the direction of current through the coil.

• Generator

• The induced EMF resulting from a changing magnetic flux has a polarity that leads to an induced current whose direction is such that the induced magnetic field opposes the original flux change.

• If the magnetic field is increasing, a current will develop to oppose the increasing magnetic field.

• If the magnetic field is decreasing, a current will develop to create a magnetic field in the same direction as the one that is decreasing.

• A current will form that attempts to keep the magnetic field constant.

• Lenz’s Law abides by the laws of conservation of energy.

Lenz's Law

Induced

Current

x x x x x x

x x x x x x

x x x x x x

x x x x x x

No Current

Induced

Current

No Current

Lenz’s Law

Current will be induced in the copper ring when it passes through a region where the magnetic field changes. When the magnetic field is constant or absent, their will be no induced current.

• Eddy current balances.

• Eddy current dynamometer.

• Metal detectors (Lenz's Law)

• Braking systems on trains.

• What are Eddy currents?

• Eddy currents are currents created in conductors to oppose the changing magnetic fields they are exposed to.

• Eddy currents respond to the changes in an external magnetic field.

• Eddy currents can form in conductors even if they are not capable of being magnetized.

• When a current carrying wire moves in a magnetic field, an EMF is produced called the back EMF.

• The back EMF opposes the current in the motor resulting in a decrease in the total current through the motor.

• As the motor slows down, the current will increase.

• Back EMF’s may cause sparks at outlets and switches when circuits are disconnected while in use.

• Both motors and generators consist of coils that rotate in a magnetic field.

• There are two sources of EMF:

• An applied EMF to drive the motor.

• An EMF induced (back EMF) by the generator like action of the coil that opposes the applied EMF.

EMFnet = Vapplied – EMFinduced

I = (Vapplied – EMFinduced)/R

• An increasing current in a coil will induce an EMF that is opposing to the current in the coil.

NΦ = LI

Where L is a constant called self-inductance.

Substituting into Faraday’s Law of induction:

EMF = -L ΔI/Δt

Note: the negative sign shows that the EMF is always opposing the change in current.

Note: the faster the change in current, the greater the EMF.

• Transformers are used to increase or decrease AC voltage.

• Transformers that increase voltage are called step-up transformers.

• Transformers that decrease voltage are called step-down transformers.

• Transformers efficiently change voltages with little loss of energy.

• Transformers consist of two windings wrapped around an iron core.

• The iron core is easily magnetized and will enhance the magnetic field.

• Mutual Inductance: The changing current in one coil (primary) will induce an EMF in the other coil (secondary).

Transformers (cont.)

• The EMF induced (secondary voltage, Vs) in a secondary coil is proportional to the primary voltage (Vp).

• The EMF induced is also proportional to the number of windings (Ns) in the secondary coil.

• The EMF is inversely proportional to the number of windings in the primary coil (Np).

Vs/Vp = Ns/Np

Pp = Ps

VpIp = VsIs

Rearranging: Is/Ip = Vp/Vs = Np/Ns

• Electromagnetic induction: is the process by which current is generated by moving a conductor through a magnetic field or a magnetic field through a conductor.

• The induced current is maximum when the relative motion of the conductor is perpendicular to the magnetic field.

• The induced voltage is called EMF (=vBL).

• Magnetic flux is a measure of the strength of the magnetic field passing through a surface.

• A generator is a device that converts mechanical energy into electrical energy.

• Generators are similar to motors.

• Lenz’s Law: The induced EMF resulting from a changing magnetic flux has a polarity that leads to an induced current whose direction is such that the induced magnetic field opposes the original flux change.

• Self-Inductance: A changing current in a coil will induce an EMF that opposes the change in current.

• Transformers convert high voltage/low current electrical energy to low voltage/high current electrical energy.

• Transformers consist of two coils (primary and secondary) wrapped around a common iron core.