Magnets and current in a magnetic field

What does a magnet do?

A magnet will attract certain materials (such as Iron, Steel, Nickel and Cobalt) to itself

Where is a bar magnet (permanent magnet) strongest?

A bar magnet is strongest at each end or it’s Magnetic Poles

How do Magnetic Poles work?

Magnetic Poles exist in pairs, called the North Pole and South Pole. Like poles attract and unlike poles repel.

Magnetic Field

Any region of space where magnetic forces can be felt.

Magnetic Field Lines

A line drawn in a magnetic field so that the tangent to it at any point shows the direction of the magnetic field at that point.

Magnetic field lines of a bar magnet

Remember, magnetic field lines always go from North Pole to South Pole.

Magnetic field lines of unlike poles

Remember, magnetic field lines always go from North Pole to South Pole.

Magnetic field lines of like poles

Remember, magnetic field lines always go from North Pole to South Pole.

Diagram of a solenoid

Demonstration of a magnetic field due to a current in a long straight wire (important)

• Each end of the wire is connected to a d.c. power supply with high current.

• Note direction of the current

• Iron filings can be used to show the magnetic field.

• Plotting compasses are used to show the direction of the field.

Demonstration of a magnetic current in a loop

Demonstration of a magnetic field due to a current in a solenoid

A solenoid is a coil of wire whose length is much longer than its radius.

How to determine the direction of a magnetic field due to current in a straight wire

Use the right hand grip rule: Grip the conductor with your thumb pointing in the direction of the current; your fingers now indicate the direction of the magnetic field lines.

Determining the direction of a magnetic field due to current in a solenoid

Clockwise is South Pole. Anti-clockwise is North Pole.

To demonstrate the magnetic effect of an electric current: the electromagnet

An electromagnet consists of a soft iron core in a solenoid. When the current is switched on, the core acts as a magnet and can be used to pick up nails.

Uses of electromagnets

1. Electromagnets can be used in junkyards to lift cars.

2. They are used in electric motors, loudspeakers and electromagnetic relays.

Properties and uses of the earths electromagnetic field.

• The earths magnetic field can be used for accurate navigation, both by man and animal.

• Flipping the earths magnetic field is possible.

• Magnetic fields protect the earth from dangerous radiation from the sun.

Demonstration of a current carrying conductor in a magnetic field experiencing a force

1. Place a strip of tinfoil between two magnetic poles, as shown in the diagram.

2. Pass direct current through the tinfoil in the direction shown.

3. The tinfoil experiences a force.

4. Reverse the current making it an a.c.

5. The tinfoil experiences a force in the opposite direction.

What is a current carrying conductor experiencing a force in a magnetic field used for?

1. The electric motor

Size of the force experienced by a current carrying conductor formula

F = BIL

B = magnetic flux density

I = current

L = length of conductor

What is the standard unit for magnetic flux density?

The Tesla (T)

Definition of a Tesla

Magnetic flux density that causes a current carrying conductor to experience a force of 1N per amp per metre I.e. B=F/IL

Direction of the force on a current carrying conductor in a magnetic field.

Force on a moving charge in a magnetic field at velocity v

F = Bqv

B = magnetic flux density

E/q = charge

v = velocity

Give an expression for the momentum of a particle in the cyclotron (magnetic force leading to circular motion)

Centripetal force = magnetic force

mv²/r = Bqv

mv = Bqr

Definition of the ampere

That constant current which, if flowing in two indefinitely long parallel wires one metre apart in a vacuum will experience a force of 2 × 10^-7N per metre length.

To demonstrate the principle on which the definition of the ampere was based

1. Connect two parallel conductors in a circuit as shown.

2. Complete the circuit to switch on the current

3. The strips will either move towards each other or repel each other, depending on the direction of the current.