Topic 6 - Magnetism

What is the needle of a compass?

A permanent magnet.

What magnetic poles exist on a compass needle?

• North magnetic pole (N)

• South magnetic pole (S).

What does magnetic force cause a compass to do?

The north pole points North and the south pole points South when the magnet is free to move.

How are magnetic forces related to electric forces?

Magnetic forces are closely related to electric forces.

How do magnetic poles behave compared to electric charges?

Their behaviour is similar to like and unlike electric charges.

What happens when unlike magnetic poles interact?

They attract.

What happens when like magnetic poles interact?

They repel.

What surrounds a magnet?

A magnetic field.

What is a magnetic field?

An area where a magnetic force is experienced.

How is the direction of the magnetic field determined?

By the direction indicated by the north pole of a small compass needle placed at that point.

What do magnetic field lines represent?

The force acting on an isolated north pole.

Where do magnetic field lines begin and end?

They start at the north pole (N) and end at the south pole (S).

What symbol is used for magnetic field?

B-field.

What experiment shows magnetic field patterns around magnets?

Iron filings around a bar magnet.

What type of magnet also shows magnetic field lines clearly?

A horseshoe magnet.

What large natural magnetic field exists around Earth?

The Earth’s magnetic field.

Can electric charges experience a force in a magnetic field?

Yes.

What condition must be satisfied for a charge to experience magnetic force?

The charge must be moving.

What direction must the velocity of the charge have relative to the magnetic field?

The velocity must have a component perpendicular to the magnetic field.

What is Right Hand Rule No.1 used for?

Determining the direction of magnetic force on a moving charge.

How do you apply Right Hand Rule No.1?

• Fingers point in direction of magnetic field

• Thumb points in direction of velocity of charge

• Palm faces direction of magnetic force.

For which charge type does RHR-1 predict force direction directly?

Positive charge.

What happens if the moving charge is negative?

The force direction is opposite to the RHR-1 prediction.

What happens to a charged particle in an electric field?

The magnitude of its velocity changes.

What happens to a charged particle in a magnetic field?

The magnitude of its velocity remains constant.

What instrument uses magnetic fields to analyse particles?

A mass spectrometer.

How are charged particles first accelerated in a mass spectrometer?

By an electric field.

For particles with similar charge, what determines speed?

Heavier particles move slower.

What role does magnetic force play in a mass spectrometer?

It acts as a centripetal force.

How do lighter particles move in the spectrometer?

They have smaller radii of curvature.

What happens when the magnetic field strength increases?

Different masses can be focused onto the detector.

What example spectrum is shown in the slides?

The mass spectrum of naturally occurring neon with three isotopes.

What are aurorae?

Visual phenomena caused by charged particles moving through Earth’s atmosphere.

What are the two main types of aurorae?

• Aurora Borealis (Northern Lights)

• Aurora Australis (Southern Lights).

What determines the direction of particle movement in aurorae?

Earth’s magnetic field.

What causes aurorae to occur near the poles?

Charged cosmic particles are directed toward the poles by Earth’s magnetic field.

Why is light emitted during aurorae?

Charged particles collide with air molecules.

What happens to a current-carrying wire in a magnetic field?

Magnetic force pushes the wire sideways.

In the slide example, which direction does the wire move?

To the right.

How do magnetic forces create sound in speakers?

Magnetic force on moving charges causes the speaker cone to move in and out.

What does this movement create in the air?

Compressions and rarefactions (sound waves).

What happens to forces acting on a current loop in a magnetic field?

They have equal magnitude but opposite directions.

What rule shows their direction?

Right Hand Rule No.1.

What principle comes from forces on a current loop in a magnetic field?

The principle of the electric motor.

What type of motor is shown in the slides?

DC electric motor.

What is Right Hand Rule No.2 used for?

Determining the direction of magnetic field around a current-carrying wire.

How is Right Hand Rule No.2 applied?

• Thumb points in direction of conventional current

• Curled fingers show direction of magnetic field.

What happens when RHR-2 is applied to a current loop?

It shows the overall magnetic field pattern of the loop.

What does the magnetic field of a current loop resemble?

The field around a bar magnet.

What device does this principle create?

An electromagnet.

How are electromagnets typically constructed?

• Many windings of wire (coil)

• Iron core to increase magnetic field strength.

What materials can become permanent magnets?

Ferromagnetic materials such as:

• Iron

• Alloys of cobalt

• Alloys of nickel.

What causes permanent magnetic fields in materials?

Orbital motion and spin of electrons.

What happens in ferromagnetic materials at the atomic level?

Neighbouring atoms have aligned magnetic fields.

What are these aligned regions called?

Magnetic domains.

When does a material become a permanent magnet?

When enough domains align in the same direction.