Unit 5: Magnetism and Electromagnetic Induction

Magnetic Field

The space surrounding a magnet is called a magnetic field.

Notation of magnetic field

• We use (x) when the magnetic field goes into the plane.

• We use (.) when the magnetic field goes out of the plane.

Bar Magnet

A permanent bar magnet creates a magnetic field that closely resembles the magnetic field produced by a circular loop of current-carrying wire.

Magnetic Force

F = qv x B

with magnitude:

F = qv B sin theta

• F = force

• q = charge

• v = velocity

• B = magnetic field

Right-Hand Rule

Whenever you use the right-hand rule, follow these steps:

• Orient your hand so that your thumb points in the direction of the velocity v.

• If the charge is negative, turn your thumb by 180 degrees.

• Point your fingers in the direction of B.

• The direction of FB will then be perpendicular to your palm.

Magnetic force on a current carrying wire

F = ILB

with magnitude: F = BIL sin theta

• F = force

• B = magnetic field

• I = current

• L = length of conductor

Magnetic field created by current-carrying wire

B= μo I / 2πr

• B = magnetic field

• I = applied current

• μo = permeability of free space

• r = the distance from the wire where the magnetic field is calculated

Lenz’s Law

The induced current will always flow in the direction that opposes the change in magnetic flux that produced it.

Emf = -N (ΔΦ/ Δt)

• ‘Emf’ = Induced voltage or electromotive force.

• ‘N’ = The number of loops.

• ‘Δϕ’ = Change within magnetic flux.

• ‘Δt’ = Change in time

Faraday’s Law of electromagnetic induction

Whenever a conductor is placed in a varying magnetic field, an electromotive force is induced. If the conductor circuit is closed, a current is induced, which is called induced current.

The induced emf in a coil is equal to the rate of change of flux linkage.

emf = − dΦ/ dt

• emf = electromotive force

• dΦ = change in magnetic flux

• dt = change in time

Induced current

It is created in three ways:

• Changing the area of the loop of wire in a stationary magnetic field.

• Changing the magnetic field strength through a stationary circuit.

• Changing the angle between the magnetic field and the wire loop.

Motional Emf

Motional emf is the electromotive force generated by the motion of a conductor through a magnetic field.

It is given by the equation

emf = Blv, where

• B is the magnetic field strength

• l is the length of the conductor

• v is the velocity of the conductor

This phenomenon is used in various applications, such as electric generators and motors.

Solenoid

Solenoid is a device that is constructed by a series of coaxial wires through which a continuous current flow.

When a change of current in one circuit induced a voltage into another circuit, this process is known as:

mutual induction

Increasing the current through a conductor will cause the magnetic field around the conductor to:

increase in strength

Current flow in a conductor produces:

a magnetic field

Whenever there is motion between a conductor and a magnetic field:

a voltage is generated in that conductor

What is a magnetic material?

Any material that is attracted to a magnet and has magnetic domains.

What is a magnetic pole?

A place on the magnet where the force of attraction is greatest.

Describes it to behaviors of magnetic poles when they are placed next to each other.

a) like poles repel each other

b) unlike poles attract each other

In what direction do the imaginary magnetic lines of force of a closed loop flow?

From the north pole to the south pole.

electromagnet

Device that uses current flow through conductors around a core to create magnetism.

flux

The total lines of force surrounding a magnetic material. The symbol is Φ.

permeability

The ability of a material to concentrate flux as compared to air. The symbol is μ.

What three factors affect the strength of the magnetic field around the coil?

a) The amount of current through the coil

b) The number of turns on the coil

c) The core material

State the three factors that determine them amount of induced emf.

a) flux density

b) the number of turns in a conductor

c) The rate at which lines of force are cut by the conductor which is determined by the velocity and the angle of cut

mutual induction

Mutual induction takes place when a change in current and one circuit causes a voltage to be induced into another separate circuit.

State two applications that use the principle of mutual induction.

a) ignition coils

b) transformers

Inductance is symbolized by \__ and is measured in __, symbol __.

L, henry, H