Physics: Magnetism & Electromagnetism

What is a permanent magnet?

Produces its own magnetic field

They repel when like poles are together

Attract when different poles are together

What is an induced magnet?

A temporary magnet

When put near a permanent magnet, the material becomes magnet due to the surrounding magnetic field.

Always cause a force of attraction

Lose magnetism quickly when not in a magnetic field

What are the 4 magnetic materials?

• Iron

• Steel (alloy of iron, not an element)
  Cobalt

• Nickel

Can be made into a permanent or induced magnet

What is a magnetic field?

Strength of magnetic field depends on the distance between magnet and material. Strongest at the poles.

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

How to find the magnetic field RP

1. Draw around the bar magnet

2. Place compass at north pole of magnet

3. Draw a cross where the N arrow points

4. Place the S arrow on the N cross

5. Keep plotting until you reach the other pole.

6. Draw a line through the crosses

7. Add arrows (N to S) to show which direction a North pole would travel.

Why does a compass work?

Compass always points towards North because of the earths natural magnetic field.

1. Needle (iron)

2. Floats in water

3. North side is attracted to South pole

What is a solenoid?

Coil of wire

How can you prove there is a magnetic field around a wire?

When current is turned on

There is a magnetic field around wire

Which causes the needle in a compas to deflect

WHat happens when you change the direction of the current in a circuit?

The direction of the magnetic field changes / reverses

What is Flemings left hand rule?

FBI

Thumb - F - Force

Index finger - B - Magnetic field (N to S)

Middle finger - I - current (Positive to negative)

How can you make the magnetic field stronger?

1. Coil the wire / more turns in the coil

2. Stronger current

3. Place iron core inside solenoid (electromagnet)

How does a relay work?

Using a switch in a high voltage circuit is dangerous (sparks)

Use relay instead

• Two separate circuit

• One low voltage with electromagnet

• other high voltage, replace switch with 2 metal contacts, one contact connected to spring keeping contact s apart & iron block next to spring

• When low voltage circuit is switched on the current produces a magnetic field around the electromagnet. This attracts the iron block and makes the contacts touch.

How does a doorbell work?

• Switch is closed when buzzer is pressed

• Current flows through

• Makes magnetic field around electromagnet

• Attracts the iron contact

• Clapper hits the bell

What is the motor effect?

A current carrying wire experiences a force when in a magnetic field.

• Only experiences force when current flows

• Can reverse direction of force by switching poles of magnet o r changing the direction of the current.

• Force, Magnetic flux density & current are all at right angles to each other.

• If the magnets are parallel to the conductor, no force

How does an Electric motor work?

1. Loop of wire carrying a current

2. The current is back → front on the left, front → back on right side

3. This means the force is up on the left and down on the right (Moment)

4. Causing the loop to rotate.

5. At 90º the loop stops rotating.

6. If it goes over 90º, The current makes left side force down and right side force up which pushes it back to 90º

7. The current can be switched direction at 90º so it rotates all the way.

8. Use a split-ring commutator → connected to brushes (allow electric current to pass onto ring) so the motor can keep rotating in same direction

How do speakers and headphones work?

1. Coil of wire around one end of cone

2. Connected to AC

3. Permanent magnet in solenoid

4. Current through coil, generates Magnetic field

5. MF of solenoid and magnet interact (attract or repel)

6. Resultant force. Cone moves in and out when current reverses → sound waves

7. Changing freq of AC supply, change freq that the cone vibrates.

8. Higher freq = higher pitched sound vice versa.

9. Increase size of current, increase amplitude of vibration → increases volume

What is the generator effect?

Opposite of motor effect

Kinetic energy → electrical energy

• Wire below magnetic field → moved up through magnetic field, induces PD across ends of wire

• When wire stops moving PD is lost

• When moved down, induce PD again but reversed

• This is called induced potential

• If it is a complete circuit, we induce current - the generator effect

Works the same is the wire is still and the magnetic field moves through.

How can we increase the induced PD and current?

1. Use stronger magnetic field

2. Move wire more rapidly

3. Shape wire into coils & add more turns

What happens when you move a magnet in and out a coil of wire?

• Magnet moving in and out coil of wire

• Direction of current changes when direction of magnet changes

• Magnet in wire induces current which always makes its own magnetic field which opposes the movement of the magnet.

• When N side is in the coil, the coil becomes N. Repelling the magnet

• When N is coming out of coil, coil becomes S. Attracting the magnet

• This struggle to move the magnet is doing work

• Transfers energy from movement of magnet into movement of the current.

What is the alternator effect?

1. Wire rotating in magnetic field, inducing PD

2. Coil connected to two metal rings (commutators)

3. Red - ring A (down), Orange - ring B (up)

4. Horizontal - max PD, Vertical - no PD

5. When 2 sides of coil changes direction, PD reverses

6. Produces AC

How can you increase the size of the alternating current?

• Increase number of turns in the coil / area of coil

• Increase strength of magnetic field

• Increase rotation speed - increases size and freq of AC

What is a dynamo?

1. DC, coil rotates in magnetic field, direction of coil never changes

2. Split-ring commutator (2 sides separated by gap)

3. A - down, B - up

4. Vertical - no PD

5. Induces PD and current

How does a microphone work?

1. Diaphragm attached to solenoid, on permanent magnet

2. Diaphragm vibrates when sound waves hit

3. Wire moves in and out through MF

4. Induces PD across ends of wire

5. Freq of PD = Freq of sound waves, passed through an amplifier → speaker (increase volume)

What is a transformer?

• 2 Separate coils (current can’t pass between them) with equal amount of turns

• Coils wrapped around an iron core

• Primary → AC, current flows, generates changing magnetic field

• Magnetic field is transmitted along the iron core & passes through secondary coil, induce PD

Why is an iron core used in a transformer?

• Iron is easily magnetised

• It increases the strength of the magnetic field

• Transformers only work with AC (need a changing magnetic field to induce PD)

• DC produces constant magnetic field so wouldn’t work

What is a step-up / step-down transformer?

The coils of wire have a different number of turns. This changes the PD along the transformer

Twice the tuns in secondary coil = Twice the PD in secondary coil

Step-down transformer has less turns in the secondary coil which decreases the PD.

How are transformers used in power transmission?

High current in transmission cables waste lots of energy as heat. We use high PD instead.

Step up transformers increase PD

Step down transformers decrease PD to go into homes.

Transformer calculations (equation given)

Power of primary coil = Power of secondary coil