Magnetism

Aurora

Where can we find a magnetic field?

• Around a permanent magnet• Around the conductor which

carry currente.g. Electromagnets

All magnets have two poles, named the “north pole”and the “south pole”.

What’s the effect for magnetic field?

• Attract certain types of material(ferrous), like iron 、 cobalt or nickel alloys

• Attract or repel another magnet.“like pole repel and unlike pole attract.”

• Others……….

Magnetic Field

• It is a region in which a particle with magnetic properties experiences a force, and in which a moving charge experiences a force.

How to know the strength and the direction of the magnetic field?

The greater the magnetic force experienced by the test magnet, the stronger the magnetic field strength.

the direction of the magnetic field at a point is taken as the direction that a north pole would tend to move if it were at that point

The forces increase as poles get closer together.

The Earth's magnetic field

• The Earth is a gigantic magnet with its magnetic south pole is close to its geographic north pole, and its magnetic north pole is close to its geographic south pole.

When a magnet is let to move freely, it will align itself with its north pole pointing to the north. This is how a magnetic compass works.

How to describe a magnetic field?

•Field line ： outside of the magnet,

start from N pole and terminate at S pole

5 Describe an experiment to identify the pattern of field lines round a bar magnet

Place a small compass (plotting compass) near the north end of the bar magnet on a sheet of paper ,Use a pencil to mark on the paper where the arrow head points ,

Move the compass so that the tail of the arrow is on the pencil mark ,Repeat steps 1-3 until you reach the magnet again ,

Repeat the whole process from different starting points until the field is complete as in the diagram above .

Field line for a bar magnet

• Magnetic field strength, B

• Magnetic field strength is often called magnetic flux density and is given the symbol 'B' .

• Magnetic field strength is measured in tesla, T.

Measuring Magnetic Field Strength

Measuring Magnetic Field Strength

• The strength of a magnetic field can be found using a device called the Hall probe.

Magnetic field strength on Es about 0.00005Telsla / 5×10 － 6T

2 What is induced magnetism? • A piece of iron or steel becomes magnetised

when a magnet is brought near to it.• Microscopic ‘domains’ inside the ferrous

material are all lined up by the nearby magnet

19/04/23

Why are Material MAGNETIC

Unmagnetised Material Most materials look like this

Magnetised Material Only magnetic materials can

do this

Breaking a magnet forms two new magnets.

3 Describe how a piece of ferrous metal can be magnetised

• Stroking Method: • Stroke the metal with a magnet, repeating often, using

the same end of the magnet and always in the same direction

• Electrical Method: • Wind a wire round the metal and connect the wire to a

direct current source (e.g. a battery). This create an electromagnet, but some magnetism remains after the battery is disconnected

4 Describe how a magnet can be demagnetised •Hammering MethodHit the magnet repeatedly with a hammer

•HeatingHeat the magnet in a Bunsen flame until it is red hot

•Alternating Current MethodWind a wire round the magnet and connect it to an alternating current

6 Distinguish between the magnetic properties of iron and steel

• Iron is known as a SOFT magnetic material. It is easily magnetised and easily loses its magnetism.

• Steel is known as a HARD magnetic material. It is difficult to magnetise and keeps its magnetism well

7 Describe the design and use of permanent magnets and electromagnets

• Permanent magnets are made of hard magnetic materials (steel) so that they do not lose their magnetism. They are used in compasses, electric motors, fridge doors, etc.

• Electromagnets are made of soft magnetic materials (iron) so that they can be switched on and off easily. They are used in junk yard cranes to move old vehicles and also in electronic relays, burglar alarms, etc.

MAGNETIC FIELDS DUE TO CURRENTS

MAGNETIC FIELDS DUE TO CURRENTS

• The ‘right-hand rule’ can be used to give the direction of the magnetic field lines produced by a current in a long straight wire:

•The wire is gripped with the right-hand, with the thumb pointing in the direction of the current, and the fingers then curl in the direction of the field lines .

Describe the magnetic field due to a current in a straight wire

• The magnetic field round a straight wire is in the form of circles.

• The right hand grip rule can be used to predict the direction of the field.

• EXTENDED: the field is strongest nearest to the wire and stronger for bigger currents

Direction of current

•Imagine a wire going directly into this screen.

•A dot would show a current coming outwards.

•A cross would show a current going inwards.

Draw out the direction of the magnetic field line.

I IB B

Draw out the direction of the current by using“.”or “×” in the center.

I I

B

B

Draw out the direction of the current in the wire.

Draw out the direction of the magnetic field line and the north

pole of the compass.

2 Describe the magnetic field due to a current in a solenoid• The field near a solenoid is

like that of a bar magnet

• The poles can be predicted using the diagram above

• EXTENDED: the field is strongest near the centre of the solenoid, and stronger for bigger currents

Right Hand Grip Rule •Grasp the coil or solenoid in your right hand with the fingers pointing in the direction of the current. The thumb points towards the direction of the magnetic field inside the solenoid.

Compare magnetic field between bar magnet and solenoid with current.

• Inside the magnet, the field lines continue from the south pole to the north pole to complete a loop.

1. Relay

An advantage of using a relay as a switch is that:

• a small current in one circuit can safely control a larger current in a second circuit

3 Draw a circuit for an electric bell and describe how it works.

3 Draw a circuit for an electric bell and describe how it works.

• When the circuit is completed, by pressing the bell-push, the current flows through the coil and creates an electromagnet

• The electromagnet attracts the armature, which causes the hammer to hit the bell

• At this point the circuit is broken at X, so the electromagnet is turned off

• As the armature is no longer attracted to the electromagnet, the springy metal strip pulls it back

• This creates contact at X again and the whole cycle repeats until the bell-push is released

4 List some other applications of the magnetic effect of current

• Electromagnetic Door Locks

• Electromagnetic Relays

• Loud Speakers

• Ammeters and Voltmeters