Magnetism & electromagnetism TOPIC 6

Where are the magnetic forces strongest?

At the poles of a magnet

What happens when two magnets are brought close to each other?

• They exert a force on each other

• Two like poles repel each other

• Two unlike poles attract each other

• These are non-contact forces

What does a permanent magnet do?

• Produce it’s own magnetic field

What happens when two permanent magnets are brought close together?

• They attract or repel depending on the direction

What does an induced magnet do?

• It becomes a magnet when it’s placed in a magnetic field?

• Induced magnets always causes a force of attraction.

What happens when permanent magnets (that are attracting induced magnets) are removed?

The induced magnets lose most or all of their magnetism quickly.

What are the 4 types of magnetic material?

• Iron

• Steel (alloy of iron)

• Cobalt

• Nickel

• They can be made into a permanent or induced magnet.

What is a magnetic field?

The region around a magnet where a force acts on another magnet or on a magnetic material.

What does the strength of the magnetic field depend on?

The distance from the magnet.

Where is magnetic field the strongest?

At the poles of the magnet.

What can be used to find the direction of a magnetic field?

A compass

What does a magnetic compass contain?

• A small bar magnet

How to plot the magnetic field pattern of a magnet

• Place bar magnet in the centre of the paper

• Draw around the magnet so you remember its position

• Place compass near the north pole of the magnet

• Mark a door at the tip of the compass needle

• Move compass so that the back of the needle is on the dot you just made

• Repeat process marking new dots and joining them to make a field line

• Continue until the line reaches the south Pole

• Repeat process from different points around the magnet to get multiple field lines

Key features a field pattern:

Arrow go from north to south outside the magnet

Lines never cross

Field is strongest near the polls(lines are the closest together there)

Describe direction of magnetic field

North Pole to South Pole

When lines are closer together on a magnetic field diagram, what does this mean?

The field is stronger (the poles)

How do we know the Earth has it’s own magnetic field?

• If we place magnetic compass away from any magnet then the needle always point in the North South direction

• Field looks like bar magnets field

• Core is made of iron & nickel -both magnetic metals

• Moving molten metal in the outer core creating the field

  • The Earth’s magnetic field is due to the Earth’s core.

What happens when a current flows through a conducting wire?

A magnetic field is produced around the wire.

How can we prove that there’s a magnetic field around the wire?

1. Get a compass and place it next to wire.

2. When the current is turned off, the compass needle lines up with the Earth’s magnetic field.

3. However, if we turn the current on again, then the compass needle deflects proving that there’s a magnetic field around the wire.

What does the strength of the magnetic field depend on?

The size of the current.

What happens when the direction of the current is changed?

• The direction of the magnetic field is changed.

• A compass placed next to this wire would deflect in the opposite direction.

Ways to increase the strength of the magnetic field of a solenoid?

• Coil the wire/use more coils

• Increase the current

• Adding an iron core through the centre of the coils

Solenoid

Coiled wire

Coiling the wire to increase m.f. :

• Coil the wire- solenoid shape.

• When we turn on the current, we get a strong and uniform magnetic field inside the solenoid.

What does the magnetic field around a solenoid resemble?

The magnetic field around a bar magnet

What is an electromagnet?

A solenoid containing an iron core.

Why are electromagnets extremely useful?

• Because we can change the strength of the magnetic field by changing the size of the current.

• We can also turn it on or off.

Why is it dangerous to turn off or on the switch of a high voltage electric circuit?

• Sparking

• Risk of electrocution

• So a relay is used to turn circuits on or off

What is the motor effect?

• A force that is experienced by a wire carrying current and right angles to another magnetic field.

• This force can be calculated.

• If conductor is parallel to magnetic field, it will not experience a force.

Formula for calculating the size of force in motor effect

Force (N)= magnetic flux density (T) x current (A) x length (m)

What is the magnetic flux density?

A measure of the strength of the magnetic field.

Fleming’s left-hand rule

1. Place your thumb, first finger and second finger at right angles.

2. Thumb - Direction of motion/ force

3. First finger- Direction of magnetic field from N to S

4. Second finger- Direction of conventional current + to -

Where can the motor effect be used?

In electric motors

Electric motor wire

• Current running in opposite directions on either side of the loop.

• Place loop of wire in magnetic field.

• Now wire experiences upward force on one LH side and downward force on the RH side (Fleming’s LHR).

• These moments make the loop rotate in clockwise direction.

• However, at 90 degrees the loops will stop rotating (balanced moments).

• If it went beyond 90 degrees, the direction of the current means that the force on left hand side is acting downwards.

• These forces push the loop back to 90 degrees position.

• However, this problem can be solved by switching the direction of the current when the loop passes 90 degrees.

• A split ring commutator is used.