Magnetic fields and electromagnetism

Define a magnetic field.

Magnetic field is a field which exists wherever there is a force on a magnetic pole. Magnetic field = field of force.

What are 2 ways to create a magnetic field?

1. Using a permanent magnet.

2. Using the movement of electric charges by having an electric current.

How will north and south pole interact when they meet?

They will attract eachother.

How will north and north/south and south pole interact when they meet?

They will repel eachother.

What are rules for drawing magnetic field lines?

1. Come out of north pole and go into south pole

2. The direction of a field line at any point in the field shows the direction of the force that an N particle would experience

3. The field is strongest where lines are close together

What is an electromagnet?

A magnet that makes use of the magnetic field created by electric current.

How is an electromagnet created?

1. A coil is used because it concentrates the magnetic field

2. First end becomes a north pole, the second end becomes a south pole

3. Coil is called a solenoid.

What is a solenoid?

A long current-carrying coil used to generate a uniform magnetic field within its core. The magnetic field is increased by the use of an iron core (ferrous core).

What is a ferrous core?

An iron rich core.

How does a solenoid look like?

How can a magnetic field get weaker?

If the electromagnet is unravelled.

For the following picture, how does the view from A look like?

current - out of the page

magnetic field lines - anticlockwise

For the following picture, how does the view from B look like?

current - into the page

magetic field lines - clockwise

How is a magnetic field created?

It is created by moving charges (electrons in a wire).

What are moving charges in a permanent magnet? Why is the iron best metal for an electromagnet?

• For permanent magnets, the magnetic field is produced by moving electrons within the atoms.

• Electrons = tiny current because it circulates around in the atom. This current creates a magnetic field.

• For iron, electrons create weak fields in one atoms, but since there are a lot of atoms, all the weak fields combine and produce one strong magnetic field together.

Why non-magnetic materials produce no magnetic fields?

Because the fields produced by the electrons cancel each other out.

What is the right-hand grip rule?

Right-hand grip rule - a rule for finding the direction of the magnetic field inside a solenoid.

Thumb - the direction of magnetic field

Fingers - current

THUMB ALWAYS SHOWS THE NORTH POLE!!

What is the right-hand rule?

Right-hand rule - a rule for finding the direction of the magnetic field around a straight, current-carrying wire.

Thumb - current

Fingers - the direction of magnetic field

What is the Fleming’s left-hand rule?

Fleming’s left-hand (motor) rule: rule is used to predict the force experienced by a current-carrying conductor placed in an external magnetic field.

Thumb - force (F)

First finger - magnetic field (B)

second finger - current (I)

How can a magnetic force be explained?

The magnetic force is a product of combined magnetic fields. The magnetic field of magnets and magnetic field of the current-carrying conductor combine to produce a force.

What is a motor effect?

When a current-carrying wire in the presence of a magnetic field experiences a force.

This force is a simple product of combination of magnetic fields.

How is strength of electric/gravitational fields represented?

The strenght is represented by the separation between the field lines. Field is strongest where field lines are closest together.

What is magnetic flux density?

Magnetic flux density is strength of a magnetic field.

Magnetic flux density (B) - the force acting per unit current per unit length on a wire placed at right-angles to the magnetic field.

MFD - number of magnetic field lines passing through a region per unit area.

Where is the magnetic flux density strongest?

It is strongest at the poles of bar magnet.

What is the formula for force on a condctor?

F = BIL

F - force experienced by a current-carrying conductor (Fleming’s left hand rule)

B - magnetic flux density

I - current in the conductor'

L - length of the conductor in the magnetic field

What is the unit of magnetic flux density?

The Tesla (T)

The formula for force on he conductor can be used only when??

Only when the field is at right angles to the current.