P15: Electromagnetism (1/2, motor effect)

What are the 2 ends of a bar magnet called?

Poles

2 types of poles

• North

• South

Where are the magnetic forces strongest?

At the poles of a magnet

What happens when 2 magnets are brought close tog?

They exert a force on each other

Force when 2 like poles are brought tog

Repulsion

• Eg 2 North poles close tog

What does the force of repulsion between 2 like poles do?

Forces the magnets apart

Force when 2 unlike poles are brought together

Attraction

• N + S pole close tog

What type of forces are the attraction + repulsion of 2 magnetic poles and why?

Non contact forces

• Magnets don’t have to touch to experience the force

Permanent magnet

Produces its own magnetic field

Example of permanent magnets

Bar magnets

What happens when 2 permanent magnets are brought close together?

Attract or repel each other depending on the direction

Induced magnet

Material that becomes a magnet when it is placed in a magnetic field

• The object isn’t magnetic

What force does induced magnetism always cause?

Attraction

What happens when the permanent magnet is removed from an induced magnet?

(when the induced magnet is removed from the magnetic field?)

The induced magnets lose most / all of its magnetism quickly

How to create an induced magnet?

1. Place a permanent magnet betw 2 non-magnetic objects

2. There is a MF around the permanent magnet

3. MF causes the 2 objects to become magnets

1. Like poles…

2. Unlike poles…

1. Repel

2. Attract

4 types of magnetic material

1. Iron

2. Steel (alloy of iron)

3. Cobalt

4. Nickel

What types of magnets can the 4 magnetic materials be made into

• Permanent

• Induced

Magnetic field

A region around a magnet where a force acts on another magnet / magnetic material

What happens if a magnetic material eg iron is placed into this field?

1. Becomes an induced magnet

2. Will experience a force of attraction towards the magnet

What does the strength of a magnetic field depend on?

Distance from the magnet

Is the force of attraction between 2 unlike poles strongest as the magnets are closer or further apart?

Closer

Where is the magnetic field strongest at?

Poles of the magnet

What equipment can you use to find the direction of a magnetic field?

Compass

What does a compass contain?

Small bar magnet

How to plot the magnetic field of a bar magnet using a compass placed near a bar magnet

1. Place compass near the N pole of the bar magnet

2. Draw a cross at the N pole of the compass

3. Move the compass so the S pole of the compass is on the cross

4. Draw another cross at the N pole of the compass

5. Continue doing so until a complete magnetic field line is plotted

6. Connect all dots with a line

7. Show direction of the field line using an arrow

8. Repeat all steps starting at different points around the N pole of the bar magnet

What directions do field lines run in?

North pole to South pole

When plotting field lines, where is the field strongest?

Where the lines are closer tog

What happens when you hold a compass away from any magnets?

Needle always points in the North - South direction

What does it show if the compass needle always points in the N-S direction, even when away from a magnet?

The Earth has its own magnetic field

What is the Earth’s magnetic field due to?

• Compass always points in direction of Earth’s MF

The Earth’s core

The force between a magnet and magnetic material is always…

Attraction

The direction of the magnetic field at any point is given by the…

Direction of the force that would act on another north pole placed at that point

The direction of a magnetic field line is from the north pole of a magnet is to the…

South pole of a magnet

A compass needle points in the…

Direction of the Earth’s magnetic field

What happens when a switch is closed in a circuit?

Electric current flows around the circuit

What happens when current flows through a conducting wire?

Magnetic field produced around the wire

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

1. Current turned off = compass needles lines up with Earth’s magnetic field

2. Current turned on = compass needle deflects

   • Proof of MF around wire

What does the strength of the magnetic field by a wire depend on?

1. Size of current thru wire

2. Distance from wire

Larger current produces…

Stronger magnetic field

Move further from wire…

Strength of MF decreases

What happens to the MF if the direction of current is changed?

Direction of MF is changed

• Reverse direction of CC = reverse direction of MF

Conventional current

Flow of current from the positive terminal to negative

What happens if you place a compass near a wire that has had the direction of current reversed?

Deflects in the opposite direction to before

How to work out the direction of the MF produced by a wire?

Right hand rule

• Place right hand so thumb is pointing in the direction of conventional current

• Fingers are pointing in the direction of MF

Shaping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire. The magnetic field inside a solenoid is strong and uniform

Solenoid

The shape of a coiled wire

How to increase the strength of a MF created by current through the wire regarding the shape of a wire?

Shaping the wire in a coil

• To form a solenoid

Features of the MF created when current is turned on in a solenoid?

Strong + uniform MF inside solenoid

What creates a MF in a circuit?

Current through the wire

The magnetic field around a solenoid has a similar shape to…

MF around bar magnet

How to work out the direction of the MF in a solenoid

RH rule

• Place fist of RH so fingers curl in the same direction as conventional current

• Thumb points in direction of North pole (in this eg)

• Thumb = MF, fingers = current

3 ways to increase the strength of a MF produced by a solenoid

1. Increase size of current

2. Increase no. of turns of the coil

3. Place piece of iron inside solenoid

   • Iron core

Electromagnet

Solenoid containing an iron core

Why are electromagnets useful?

• Can change strength of MF by changing size of current

• Can turn electromagnet on or off

What devices contain electromagnets?

• Relay

• Electric doorbell

In high voltage circuit why is using a switch to turn it on + off dangerous?

• Can get sparking

• Risk of electrocution

In high voltage circuits, what is used instead of a switch to turn it on and off?

Relay

What does a relay contain?

2 separate circuits

How are the 2 circuits in a relay different?

• Low voltage circuit containing electromagnet

• High voltage circuit

Why does a relay contain a low voltage circuit with an electromagnet?

Circuit is safe to be switched on + off

What is the switch replaced with on the high voltage circuit in a relay + why?

2 metal contacts

• 1 contact connected to spring → keeps contacts apart

When is a magnetic field produced?

When current moves through a wire

What happens if a wire (w a current through it, ao it has a MF) is placed in another MF?

Motor effect

• MF around the wire interacts with MF between the magnets

• So wire experiences a force (eg upwards)

• Upwards force causes wire to move upwards

Motor effect

When a conductor carrying a current is placed in a MF, the magnet producing the MF + the conductor exert a force on each other

• When the wire is NOT PARALLEL to the lines of the MF

Magnetic flux density

Measure of the strength of a MF

Size of force equation (produced by motor effect)

F = BIl

Which wire does the size of force equation apply to?

Wires at right angles to the MF

Factors affecting the size of the force

1. Magnetic flux density (of MF)

2. Current (in conductor)

3. Length of conductor

How to determine the direction of the force experienced by a wire?

Fleming’s left hand rule

• Place thumb, 1st, + 2nd finger so they are at right angles

• Point 1st finger in direction of MF (from N to S)

• Point 2nd finger in direction of conventional current (+ to -)

• Direction thumb is pointing shows direction of motion (the force)

When will the conductor not experience a force?

When the conductor is parallel to the MF

When will the conductor experience a force (movement)?

When the conductor is at right angles to the MF

Where is the motor effect used?

In electric motors

Basis of an electric motor

A coil of wire carrying a current in a MF tends to rotate (due to motor effect)

Summary of what causes a coil to rotate

As the current flows (from - to +), the force produced in each side of the coil acts in opp directions

Why does the motor stop rotating when the coil reaches 90° position?

Force produced is now parallel to the MF line → would be 0

Why are graphite brushes used in a split ring commutator?

• Conducts electricity

• V slippery = v little friction when in contact with SRC

When current passed through a coil why does it spin?

• Force acts on each side of the coil due to the motor effect

• Force on 1 side is in the opp direction to force on the other side

What does a split ring commutator do every half turn of the coil?

Reverses current around the coil

In the motor effect, how can you increase the size of the force produced?

1. Increase current

2. Use stronger magnet

   • So increased strength of MF

In the motor effect, how can you change the direction of the force produced?

1. Switch MF

2. Switch direction of current

How can you increase motor speed?

1. Increase current

2. Increase magnetic field

What do loudspeakers and headphones do?

Use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves

Where are loud speakers found?

• Speakers on a stereo

• Inside headphones

Describe the set up of a loudspeaker / headphones (3 points)

1. Cone with coil of wire wrapped around 1 end

2. Coil of wire connected to AC electrical supply

3. Permanent magnet which goes inside the coil of wire

How do loudspeakers and headphones work?

1. As an AC passes through the coil, it generates a MF (around the wire)

2. The MF from the coil interacts with the MF from the permanent magnet

   • ie the MFs attract or repel each other

3. → Produces a RF (ME) → causes the cone to move

4. When current switches direction, the direction of the force on the cone reverses

5. Causes the cone to move in and out → generates sound waves

Changing the frequency of the AC supply changes…

The freq the cone vibrates at

Higher frequency produces a …

Higher pitch sound

Increase size of current…

Increases amplitude of the vibration → increases volume of sound

What is this field called?

Uniform magnetic field

• The MF is the space IN BETWEEN the lines, not the lines

Describe a uniform magnetic field

Field lines are:

• Straight

• Evenly spaced

• Parallel

• Arrows point towards south