Electromagnetic Induction

….the grand finale….

Need to include pictures of Itaipu and Three Gorges Dam!!!!!

Check it out….

The main idea behind hydroelectric dams….

Three Gorges-located in China-Largest in the world (22,500 MW) capacity-1.24 million people ‘relocated’

Some big ones….

Some big ones….Itaipu-location: Border of Brazil and Paraguay-2nd biggest in the world (14,000 MW capacity)-90% of Paraguay’s energy needs-19% of Brazil’s needs

Electricity & MagnetismSo far….1. An electric current produces a

magnetic field

2. A magnetic field exerts a force on an electric current or moving charge

Michael Faraday (1791-1867Smart dude alert!!! Chemist, Physicist, Pilosopher

Main Discoveries:-Magnetic Field-Electromagnetic InductionOften referred to as the ‘Best experimentalist in the history of science

Ref: http://en.wikipedia.org/wiki/Michael_Faraday

Electromagnetic Induction

Definition: Generation of an EMF from changing magnetic fields

Electromagnetic InductionKey Points:1. EMF is induced by a changing magnetic field. 2. A changing magnetic field can be obtained by

i) moving a magnet or coil, or ii) changing electric current

3. A steady current or magnet (constant magnetic field) induces no emf

Magnetic Flux (Φ)

Definition: Number of field lines passing through a coil

For a loop of wire in a magnetic field that depends on:1. Magnetic Field Strength2. Area of loop3. Orientation of field

Magnetic Flux

Φ = BAcosΘ Φ: Magnetic Flux (Weber - W) (also Tm2)B: Magnetic Field (T)A: Area of the surface (m2)Θ: Angle between B and the normal to the surface

Faraday’s Law

An EMF is produced by a changing magnetic field, specifically a changing magnetic flux

ε = -N ε: electromotive force (emf)N: # of loopsΔΦ: Change in magnetic fluxΔt: Change in time

The negative sign has to do with Lenz’s Law, which we’ll get to…

Example

A 1.80 m diameter circular coil that contains 50 turns of wire is perpendicular to a 0.250 T magnetic field. If the magnetic field is reduced to zero in a time of 0.100 s what is the average induced EMF in the coil?

Lenz’s Law

Copper pipe and magnet

Quick Review: How long does it take an object to fall 1.5 m if starting from rest?

Lenz’s Law (page 624)

Definition: The induced current will produce a magnetic field (and magnetic flux) that opposes the change in magnetic field (and flux).

Example

Label the disk's induced magnetic poles (North and South) as it reacts to the motion imposed by an outside force:

Answers

Example

What is the direction of the current through each resistor?

Answers

Example

A circular loop of wire radius 2.5 cm is placed in a magnetic field B = 0.020 T (into the page). The field is then reduced to 0.010 T (into the page) in 0.10 s. a) What is the average induced EMF?b) Which direction does the current flow?

Direction of Current

-Induced current will produce a flux to oppose the change-We are decreasing flux into page, so induced current will want to keep this (higher) flux directed into page-RHR: direction of induced current will produce magnetic field -Therefore, conventional current is clockwise

QuestionIf a wire coil with 320 turns is exposed to a magnetic flux decreasing at a rate of 0.03 W/s, find:a) The induced voltageb) Polarity of the solenoid?c) Direction of the current in the solenoid

Moving Conductor

ε = Blv ε: EMF (V)B: Magnetic Field strength (B)l: Length of conductor (m)v: speed of conductor (m/s)

Derivation (page 627)

ε = -N = = = = Blv

Example

A wire is in a magnetic field as shown.In which direction could the wire be moved to induce an emf across the length of the wire?A. to the leftB. up the pageC. into the pageD. down the page

ExampleFour conductors of equal length are each moved through a uniform magnetic field in different directions and with different speeds, as shown.

While the four conductors are being moved through the field, in which conductor will the largest potential difference be induced?A. Conductor AB. Conductor BC. Conductor CD. Conductor D

Example

A conducting rod 25.0 cm long moves perpendicular to a magnetic field (B = 0.20 T) at a speed of 1.0 m/s. Calculate the induced EMF in the rod.

Example 2

A conducting rod 15 cm long moves at a speed of 2.0 m/s to the right in a 0.30 T magnetic field directed out of the page. If the resistance of the circuit is 4.0 Ω:a) What is the magnitude of the current

through the circuit?b) What direction is the current

Try these….

Page 654 (8, 9, 10, 12, 14, 15)

Motor

Spinning wires in a magnetic field

Need more explanation of motors and back emf.

Back EMF

Motors: Electric NRG into Mechanical NRG

Generators: Mechanical NRG into Electrical NRG

Generator

TransformersFrom the dam to our homes…..

-When we generate power we ramp up voltage for transmission (up to 100 000 V)-When it arrives at homes, we ramp it back down

Transformers

Read pgs. 633-636In notes: -What is a transformer?-Draw a transformer in your notes (Fig. 21-21)-State the two transformer equations-What is the difference between a step up and a step down transformer?

Transformers

A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF), or "voltage", in the secondary winding. This effect is called inductive coupling.

TransformersComposed of two different coils of wire around opposite sides of an iron core. Passing an alternating current through a coil of wire that surrounds a metal core induces a varying magnetic field in the core. This field will cause a responding current flow in a secondary coil wound around the opposite side.

Transformers

Examples

#1: A step-up transformer is used to convert 120 V to 15000 V. If the primary coil has 24 turns, how many turns does the secondary coil have? (3000)

#2: A step-down transformer reduces the voltage from 120 V to 12.0 V. If the primary coil has 500 turns and draws 0.030 A:a) What is the power delivered to the secondary coil?

(3.60 W)

b) What is the current in the secondary coil? (0.300 A)