eleanor roosevelt high school chin-sung lin lesson 21

Electromagnetic Induction

Eleanor Roosevelt High SchoolChin-Sung Lin

Lesson 21

Electromagnetic Induction & Faraday’s Law

In 1831, Michael Faraday (England) and Joseph Henry (US) independently discovered that magnetism could produce current in a wire


Faraday’s law— Electromagnetic induction

Electromagnetic induction— any change in the magnetic field around a conductor induces a voltage (or emf)

Faraday’s law— The induced voltage (or emf) in a coil is proportional to the product of the number of loops and the rate of change of the magnetic field within those loops


How can we change the magnetic field around a conductor to induces a voltage (or emf)?


The change could be produced by

◦ relative motion of a wire with respect to the magnetic field



◦ moving the coil into or out of the magnetic field



◦ rotating the coil relative to the magnet



◦ changing the magnetic field strength


A magnet moving past a stationary conductor, or

A conductor moving through a stationary magnetic field


The work done to the magnet is equal to the energy generated in the circuit to which the coil is connected

Wmechanical = Welectric


Induced voltage depends on:

◦Speed of the wire traversing the magnetic field lines. Quicker motion induces a greater voltage (V ~ v)

◦Number of loops of wire that moves in a magnetic field. The voltage is proportional to the number of loops (V ~ N)

Induced Voltage

If the coil does not form a complete circuit, what will happen?

Induced Voltage

Induced voltage without current, no work to plunge the magnet into the coil

Induced Voltage

-+

If the coil forms a complete circuit, what is the direction of the induced current?

Induced Current

A

Ammeter

The induced magnetic field is repelling, the current will flow in a way to create such a repelling field

Induced Current

A

Ammeter

The more loops of the coil, the more voltage induced (V ~ N)

The more voltage induced in the coil, the more current through the resistor in the circuit (I ~ V)

The more current through the coil, the stronger the magnetic field it generated (B ~ I)

The stronger the magnetic field generated, the stronger the repelling force acting back to your magnet (F ~ B)

A coil with more loops is a stronger electromagnet and push back harder

Induced Current

What factors will affect the induced current?

Induced Current

A

Ammeter

Induced current depends on ◦the induced voltage ◦the resistance of the coil and the◦the “reactance” of the coil

Induced Current

Ammeter

A

Reactance

◦similar to resistance ◦depends on the number of loops in the coil the frequency of the AC source

Reactance

Reactance

The counter-emf is the source of the opposition to current flow change

◦A constant DC current has a zero rate-of-change, and sees an inductor as a short-circuit

◦An AC current has a time-averaged rate-of-change that is proportional to frequency, this causes the increase in inductive reactance with frequency

Reactance

Induced voltage of a moving conductor in a magnetic field

V = v B L

Moving Conductor in a Magnetic Field

Generators &Alternating Current

The movement of a magnet is alternating, the induced voltage alternates on direction

The greater the frequency of the field change, the greater the induced voltage

Generator & Alternating Current

The frequency of the induced alternating voltage equals the frequency of the alternating magnetic field within the loops


High Frequency

Low Frequency

Generator— a device that converts mechanical energy to electrical energy

Motor— a device that converts electrical energy to mechanical energy


When a closed conductor loop is moved in a magnetic field, an induced current flows through it

The direction of induced current is given by the Flemming's right hand generator rule

Flemming’s Right Hand Generator Rule


What’s the direction of the induced current?

N

S


Given by the Flemming's right hand generator rule

N

S


As the number of magnetic field lines within the loop changes, the magnitude and direction of the induced voltage and current change


One complete rotation of the loop produces on complete cycle in voltage and current


The voltage induced by the generator alternates, and the current produced is alternating current (AC)

The standard alternating current is 60 Hz

Generator Example

Hydro power generators

Aim: Transformer & Electromagnetic WaveDoNow: Draw the voltage waveform

One complete rotation of the loop produces on complete cycle in voltage and current

Transformers

Transformer Definition

A a static device that transfers electrical energy from one circuit to another through inductively coupled conductors

A static device that transfers electrical energy to magnetic energy, and to electric energy again

A device with which we can raise (for transmission) and lower (for use) the AC voltage in a circuit

Transformer only works for AC

Transformer Principle

Primary

Secondary

Primary and secondary coils Use AC voltage source (primary coil) AC voltage is induced (secondary coil) Frequency AC voltage source = Frequency Induced AC voltage


Iron core (high permeability) is inserted into the coils to intensify the magnetic field

Iron core forms a complete loop to guide all magnetic field lines through the secondary


Transformer Symbol:


Np no. of turns of primary coil

Ns no. of turns of secondary coil

Vp voltage of primary coil

Vs voltage of secondary coil

Ip current of primary coil

Is current of secondary coil

IPIS


IP

IS

VP VS

NP NS

=


Step-up transformer◦NP < NS

◦VP < VS

Step-down transformer

◦NP > NS

◦VP > VS


IP

IS

PP = PS

IP VP = IS VS

VP / NP = VS / NS

IP VP = IS VS

VP / VS = IS / IP = NP / NS

VS = VP (NS / NP )

IS = IP (NP / NS )


VP IS NP

VS IP NS

= =

NS

NP

VS = VP

NP

NS

IS = IP

Application - Power Transmission

Electromagnetic Waves

Induction of Electric Field Faraday’s law:

◦ An electric field is created in any region of space in which a magnetic field is changing with time

◦ The magnitude of the created electric field is proportional to the rate at which the magnetic field changes

◦ The direction of the created electric field is at right angles to the changing magnetic field

Induction of Magnetic Field Maxwell’s law:

◦ A magnetic field is created in any region of space in which an electric field is changing with time

◦ The magnitude of the created magnetic field is proportional to the rate at which the electric field changes

◦ The direction of the created magnetic field is at right angles to the changing electric field

Electromagnetic Waves In 1861 Scottish physicist

James Clerk Maxwell discovered the theory of electromagnetism

Maxwell united all previously unrelated observations and equations of electricity, magnetism and optics into a consistent electromagnetic field theory

Electromagnetic Waves German physicist Heinrich

Rudolf Hertz was the first to satisfactorily demonstrate the existence of electromagnetic waves by building an apparatus to produce and detect VHF or UHF radio waves

Electromagnetic Waves A charge oscillates back and forth in empty

space will produce electromagnetic waves in space where vibrating electric and magnetic fields regenerate each other

Electromagnetic Waves Electric field is perpendicular to the

magnetic field, and both are perpendicular to the direction of the motion of the wave

Electromagnetic Waves No medium is required The speed— the speed of light The wave is continuously self-reinforcing. The

changing electric field induced a magnetic field. The changing magnetic field acts back to induce a electric field

The End

eleanor roosevelt high school chin-sung lin lesson 21

Documents

coil slide

induced voltage slide

magnet slide

wire slide

frequency slide

ammeter slide

induced magnetic field

n slide