Chapter 21 - Electromagnetic Induction

varying magnetic field with time, an electric field could be generated, this phenomenon is known as

electromagnetic induction

electromagnetic induction - with the switch open

• no current through coil X

• no magnetic field in Ron core

• no current through coil Y

electromagnetic induction - immediately after the switch is closed

• a current begins to flow through coil X

• this current creates a magnetic field within coil X, the iron core, and also coil Y 

• a current starts to flow through coil Y 

electromagnetic induction - later, after the switch has been closed a while:

• a current still flows through coil X

• there is still a magnetic field within the coils and iron ocre

• current no longer flows through coil Y 

electromagnetic induction - if the switch is now opened:

• the current in coil X stops flowing

• the magnetic field dies off 

• for a brief moment, a current flows through coil Y 

a constant magnetic field produces no

current in a conductor

a banging magnetic field can produce

an electric current

• such a current is called an induced current 

when the magnetic field through coil Y changes, a current occurs in Y as if there were a source of electromotive force (emf), meaning electric potential difference, in circuit Y:

• a changing magnetic field (not magnetic field itself) induces an emf (a potential difference)

• and the faster the field changes, the larger the induced emf 

faraday showed that no current is registered in the galvanometer when

bar magnet is stationary which respect to the loop

However, a current is induced in the loop when a

relative motion exists between the bar magnet and the loop

Faraday’s experiment demonstrates that an

electric current is induced in the loop by changing the magnetic field

• the coil behaves as if it were connected to a voltage course

experimentally, it is found that the induced voltage depends on

the rate of change of magnetic flux through the coil

to produce an induced current you need

1. a closed conducting loop

2. an external magnetic flux through the loop that is changing in time 

Faraday’s Law describes the effect:

the magnetitude of the induced emf in the coil is proportional to the rate of change of magnetic flux through the loop 

the induced current as a result of the induced emf is proportional to

the resistance of the conductive loop

the minus sign in induced emf equation means that the induced emf always

opposes the change in flux

an induced current produced by an induced emf moves in a direction so that the magnetic field created by that current 

opposed the original change in flux 

an induced emf is always in a direction that 

opposes the original change in flux that caused it 

be aware that we are now discussing two distinct magnetic fields:

1. the changing magnetic field or flux that induces the current and 

2. the magnetic field produced by the induced current

   1. (all current produce a magnetic field)

   2. the second (induced) field opposes the change in the first 

the magnetic field due to the induced current

1. points in the same direction as the external field is the flux is decreasing

2. points in the opposite direction from the external field if the flux is increasing

3. is zero if the flux is not changing 

when a changing current passes through a coil or solenoid, a

changing magnetic flux is produced inside the coil, and this in turn induces an emf 

a changing current in solenoid 1 will induce an

emf in the solenoid 2

the metal inductance is defined as the

proportionally constant between the induced emf and the rate of change of the current

the unit of metal inductance (M) is the

Henry (H)

mutual inductance (M) is a “constant”

• it does not depend on I 

• it depends on “geometric”

  • size, shape, number of turns, and relative positions of the two coils, and also on whether iron (or other ferromagnetic material) is present 

inductance idea applies the same to a 

single solenoid

self-inductance, also kwon simply as inductance has the symbol

L and is also measured in Henrys

a device that is designed to have an iductance is known as

inductor 

an inductor store energy in its

magnetic field

the magnetic field in an inductor is the source of energy for the

induced emf when the current changes

an inductor can store energy in its magnetic field din the same way a

capacitor stores energy in its electric field

we can also find the energy density in any magnetic field which only depends on

the strength of the magnetic field

the symmetry between the electric and magnetic fields is reflected in the similar roles played by

capacitors and inductors

Air filled parallel plate capacitor

• constant electric field

• capacitance only depends on geometry

• energy stores in electric field

air filled solenoid inductor

• uniform magnetic field inside the solenoid

• inductance only depends on geometry

• energy stored in magnetic field