Magnetic Fields Magnetic Fields and Induced EMFsand Induced EMFs

Electromagnetic Electromagnetic InductionInduction

Just as a magnetic field can be formed Just as a magnetic field can be formed by a current in a circuit, a current can be by a current in a circuit, a current can be produced using an external magnetic produced using an external magnetic field.field.

The process of inducing a current in a The process of inducing a current in a circuit with a magnetic field is called circuit with a magnetic field is called electromagnetic inductionelectromagnetic induction

But How???But How???

Remember, a moving charge in a Remember, a moving charge in a magnetic field experiences a force.magnetic field experiences a force.

Well… a charge that is still will also Well… a charge that is still will also experience a force if the magnetic field is experience a force if the magnetic field is moving around it.moving around it.

The moving magnetic field causes the The moving magnetic field causes the force on charges which will cause them force on charges which will cause them to go around the circuit with a current.to go around the circuit with a current.

Electromagnetic Electromagnetic InductionInduction

A current can only be produced if the A current can only be produced if the magnetic field is magnetic field is changingchanging, or what , or what physicist call, physicist call, a magnetic fluxa magnetic flux

There are three ways that a magnetic flux There are three ways that a magnetic flux can be produced…can be produced…

Induced Current method Induced Current method #1#1

The circuit is moved into or out of a The circuit is moved into or out of a magnetic field.magnetic field.

This is relative, so the magnetic field This is relative, so the magnetic field could be moving, the circuit could be could be moving, the circuit could be moving, or both could be moving.moving, or both could be moving.

Induced Current method Induced Current method #2#2

The circuit is rotated in the magnetic The circuit is rotated in the magnetic field.field.

The strength of a magnetic field is The strength of a magnetic field is dependent on the angle to the circuit.dependent on the angle to the circuit.

By rotating the circuit, the strength of the By rotating the circuit, the strength of the magnetic field is changing and therefore magnetic field is changing and therefore a current can be induced.a current can be induced.

Induced current #3Induced current #3

The intensity of the magnetic field is The intensity of the magnetic field is varied.varied.

This change in the strength of the This change in the strength of the magnetic field will cause a current to be magnetic field will cause a current to be induced. induced.

Faraday’s LawFaraday’s Law

The emf produced is proportional to the The emf produced is proportional to the change in the magnetic flux per unit of time change in the magnetic flux per unit of time and the number of loops in a circuit.and the number of loops in a circuit.

OrOr

tNemf

Magnetic FluxMagnetic Flux

ΦΦ is the magnetic flux, or how much is the magnetic flux, or how much magnetic field is going through an area.magnetic field is going through an area.

Picture a hula-hoop in the rain…Picture a hula-hoop in the rain…

Hula-hooping in the rainHula-hooping in the rain

The harder the rain, the more water that The harder the rain, the more water that will go through the hoop. will go through the hoop.

The bigger the hula-hoop, the more water The bigger the hula-hoop, the more water that will go through the hoop.that will go through the hoop.

Magnetic flux is the same as this. It is a Magnetic flux is the same as this. It is a factor of how much magnetic field there factor of how much magnetic field there is and how much room does it have to is and how much room does it have to flow through.flow through.

What if you turn the What if you turn the hoop?hoop?

Let’s say you are hula-hooping around Let’s say you are hula-hooping around your arm. Can any rain get through?your arm. Can any rain get through?

The angle also determines the amount of The angle also determines the amount of magnetic field that can get through. magnetic field that can get through.

Magnetic FluxMagnetic Flux Therefore…Therefore…

Where, A is the area of the circuit loop, and B is the Where, A is the area of the circuit loop, and B is the magnetic field strengthmagnetic field strength

cosAB

Faraday’s Law againFaraday’s Law again

In total, Faraday’s Law is…In total, Faraday’s Law is…

t

ABNemf

)cos(

SummarySummary

The things that effect electromagnetic The things that effect electromagnetic induction according to Farday’s law are:induction according to Farday’s law are:

1) The angle between the magnetic field and 1) The angle between the magnetic field and the circuitthe circuit

2) The strength of the magnetic field2) The strength of the magnetic field 3) The area of the loop of the circuit3) The area of the loop of the circuit 4) The number of loops in the circuit4) The number of loops in the circuit 5) How quickly the magnetic field changes5) How quickly the magnetic field changes

simulation

Lenz’s LawLenz’s Law

What direction is the induced current?What direction is the induced current?

Lenz’s Law states:Lenz’s Law states: ““The magnetic field of the induced The magnetic field of the induced

current opposes the change in the current opposes the change in the applied magnetic field”.applied magnetic field”.

Right-hand rule #1 AgainRight-hand rule #1 Again

Remember, a current can induce a Remember, a current can induce a magnetic field of it’s own.magnetic field of it’s own.

The current that it induces will oppose The current that it induces will oppose the direction of the magnetic field flux.the direction of the magnetic field flux.

ExampleExample

A circuit of wire is pulled through a A circuit of wire is pulled through a magnetic field pointed out of the smart magnetic field pointed out of the smart board. What is the direction of the board. What is the direction of the induced current?induced current?

SolutionSolution

The current induced will be such that it The current induced will be such that it will create a magnetic field to go back will create a magnetic field to go back into the smart board. into the smart board.

Therefore, the direction of the current will Therefore, the direction of the current will be be clockwiseclockwise

Door BellsDoor Bells

Ever wonder why there is a small light Ever wonder why there is a small light behind the button of a door bell??behind the button of a door bell??

Door bell continuedDoor bell continued

Pushing the door bell disrupts the current Pushing the door bell disrupts the current in the left circuit.in the left circuit.

This disruption causes there to be a This disruption causes there to be a decrease in the magnetic field in the left decrease in the magnetic field in the left circuit.circuit.

According to Farday’s law, this decrease According to Farday’s law, this decrease in the left circuit, causes a current to be in the left circuit, causes a current to be induced in the right circuit.induced in the right circuit.

Door bell continuedDoor bell continued

The induced current causes a magnetic The induced current causes a magnetic field in the right circuit directed through field in the right circuit directed through the middle to the coil.the middle to the coil.

The iron bar is pushed outwards and hits The iron bar is pushed outwards and hits the chime. the chime.

Door BellDoor Bell