Physics: Magnetism (paper 2)

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 is a compass used to find the direction of a magnetic field?

• Place compass near the North pole of the bar magnet.

• Draw a cross at the north pole of the compass.

• Move the compass so the South pole is on the cross and draw another pole at the North point of the compass.

• Continue doing this until you have a complete magnetic field line.

• Connect all the dots with a line and show the direction of line using an arrow.

• This direction is from North Pole to South Pole.

• Repeat this at different points around the North Pole of the bar magnet.

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

  • 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

Describe a relay

• Contains 2 separate circuits.

• Low voltage circuit containing an electromagnet (safe) and a high voltage circuit in which the switch has been replaced with 2 metal contacts.

• One of the contacts is connected to a spring which keeps the contacts apart.

• There’s also an iron block next to spring.

• When low voltage is switched off, no current is flowing so there’s no magnetic field.

• High voltage is also turned off because the contacts are not touching.

• When low voltage circuit is switched on, a current flows around the circuit and a magnetic field is produced around the electromagnet.

• The magnetic field attracts the iron block and causes the contacts to close and switches on the high voltage circuit.

What’s another appliance that uses an electromagnet?

A doorbell

Explain doorbell

• Switch closes when buzzer is pressed, causing a current to flow through the circuit.

• This causes a magnetic field to be produced around the electromagnet.

• The iron contact is now attracted towards the magnetic field.

• When the contact moves towards the magnetic field, the clapper now hits the bell.

• At the same time, this breaks the circuit so there’s no current flowing through and no magnetic field around the electromagnet.

• The iron contact now springs back to it’s original position.

• Now the circuit is complete again, so a current flows around the circuit and the process is repeated.

What would happen if a wire with current flowing through it is placed in another magnetic field?

• The magnetic field around the wire now interacts with the magnetic field between the magnets, meaning that the wire now experiences a force.

• This force is only experienced when the wire carrying current is at right angles to another magnetic field.

• This is called the motor effect.

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.

Split ring commutator

• Split metal ring connected to conducting brushes.

• Brushes allow electric current to pass onto the ring.

• The current then produces a turning force on the motor, making it rotate in the clockwise direction.

• At 90 degrees, the currents broken for a tiny fraction of a second.

• However, the wire keeps turning due to momentum.

• Now the current switches direction which means LHS still has a force acting downwards and RHS still has a force acting upwards, meaning the motor keeps rotating in the same direction.

What does a split ring commutator do (simple terms)?

Swaps the positive and negative connection every half turns, changing the direction of the current ever half turn so the forces acting on the coil will always be in the same direction so the coil always rotates in the same direction.

What are split ring commutators used in?

• Fans

• Vehicles

• Hard drives

How to increase speed of rotations?

• Increase the current

• Add more turns to the coil

• Increase magnetic flux density by using more powerful magnets.

How is the motor effect used in loudspeakers (and headphones)?

• Get a moving coil loudspeaker.

• (These are found on speakers on a stereo)

• Within this, there’s a cone which has a coil of wire wrapped around one end.

• The coil of wire is connected to an AC electrical supply.

• There’s also a permanent magnet which goes inside the coil of wire.

• As current passes through the coil, it generates a magnetic field.

• Magnetic field from the coil now interacts with the magnetic field from the permanent magnet.

• These magnetic fields either attract or repel each other.

• This produces a resultant force which causes the cone to move.

• When the current switches direction (AC), the direction of the force on the cone reverses, causing the cone to move in and out, generating sound waves.

What does changing the frequency of the AC supply do?

Changes the frequency that the cone vibrates

• High frequency= higher pitch sound

• Lower frequency= lower pitch sound

What does changing the size of the current do?

Changes the amplitude of the vibration

• Increasing current= Increasing amplitude= Increasing volume

• Decreasing current= Decreasing amplitude= Decreasing volume

How is potential difference induced?

• When a conducting wire moves up through a magnetic field.

• This potential difference is induced across the ends of the wire.

• However, when the wire stops moving, the p.d. is lost.

• If the wire is moved back down through the magnetic field we regain p.d. but in the opposite direction.

• This is called the induced potential.

How do we induce a current?

• When we move a complete circuit through a magnetic field.

• This is called the generator effect.

• The direction of current switches when the direction of movement switches.

• If the movement stops, the current also stops.

What is the generator effect?

The induced voltage produces an induced current if the conductor is connected in a complete circuit.

How else can we get an induced potential and induced current?

• If we keep the wire still but move the magnetic field.

• We only get induced potential or current when the wire moves through the magnetic field (up and down) not along the magnetic field (backwards and forwards).

How to increase the induced potential and current?

• Use a stronger magnetic field

• Move the wire more rapidly

• Shape the wire into a coil and increase the number of turns on the coil.

Magnet moving in and out of a coil of wire

• This produces an induced current.

• This current changes direction when direction of movement changes.

• The current also changes direction when we switch the poles of the magnet.

• This induced current creates it’s own magnetic field which opposes the movement of the magnet.

• When North pole is inserted into coil, that end also becomes North pole, repelling the magnet, making it harder to push in.

• When we pull the North pole out, that end of the coil becomes a South pole, attracting the magnet and making it harder to pull out.

• Because the induced current makes it harder to move the magnet, this means work is being done (we’re transferring energy from the movement of the magnet into the movement of the current.)

What is an alternator?

A coil of wire rotating in a magnetic field.

How does an alternator generate an alternating current?

• Get an alternator which is a coil of wire rotating in a magnetic field.

• The coil is connected to 2 metal rings called commutators.

• The commutators allow current to pass out of the coil.

• A p.d. is induced when the wire passes through the magnetic field.

• The red side of the wire always connects onto ring A and the orange side of the wire always connects onto ring B.

• The maximum p.d. is when the coil is horizontal.

• At the point the wire is sweeping directly through the magnetic field lines at the fastest possible rate.

• Red side is moving downwards and orange is moving upwards so the wire turns in the anticlockwise direction and is now vertical.

• When this happens, the p.d. falls to 0 because coil is now moving parallel to the field and not cutting through the magnetic field lines.

• As the coil continues moving around, we get a p.d. again but in the opposite direction because the two sides of the coil are now moving in a different direction.

• Now the red side is moving up and the orange side down, so the coil turns in the anticlockwise direction again, when vertical p.d. falls to 0.

• When horizontal the p.d. has changed directions again.

• This pattern repeats.

How to increase size of AC?

• Increase strength of the magnetic field.

• Increase the number of turns on the coil.

• Increase the area of the coil

• Increase rotation speed of the coil (also increases frequency of AC).

How does a dynamo generate a direct current?

• Get a dynamo (it has a split ring commutator).

• Side of coil moving down is connect to part A.

• Side of coil moving up is connected to part B.

• Because the coil is cutting through magnetic field lines, a current is induced.

• When coil is vertical, the p.d. is 0 so the current is 0.

• Now the coil has moved around so orange moves down and red moves up.

• However, the side moving down is still connected to part A and side moving up is still connected to part B.

• This means the direction od the p.d. and current doesn’t reverse when coil rotates.

• DC

• On graph there are 2 peaks for each full rotation of the coil because each side of the coil passes through the magnetic field twice during each cycle of rotation (once passing down and once passing up).

How is the generator effect used in a moving- coil microphone?

• Inside of moving-coil microphone consists of:

  • a thing sheet of plastic called a diaphragm.

  • coil of wire attached to diaphragm.

  • permanent magnet which the end of the coil of wire sits over.

• When sound waves hit the diaphragm, they cause it to vibrate.

• Now the coil of wire moves in and out through the magnetic field.

• This induces a p.d. across the ends of the wire. (AC)

• The frequency of the changing p.d. is the same as the frequency of the sound waves.

• The changing pattern of p.d. is now passed through an amplifier and then into a moving- coil loudspeaker.

• This massively increases the volume of the sound.

Structure of transformer

• Consists of two coils of wire (primary and secondary).

• These 2 coils are wrapped around an iron core because iron magnetises easily.

• The primary coil is connect to an AC.

• As the current flows through the primary coil, it generates a changing magnetic field.

• This magnetic field is transmitted along the iron core and passes through the secondary coil.

• When the changing magnetic field passes through the secondary coil, it induces a p.d.

  • The iron core increases the strength of the magnetic field.

  • Transformers only work with AC because we need a changing magnetic field,

  • DC produces constant magnetic field.

• Step- up transformers: number of turns in secondary coil> number of turns in primary coil.

  • This means the p.d. induced in the secondary coil will be greater than the p.d. of the primary coil.

• Step-down transformers: number of turns in primary coil> number of turns in secondary coil.

  • This means the p.d. induced in the secondary coil will be lower than the p.d. in the primary coil.

Equation for transformer calculations

Vp/ Vs= Np/ Ns

• V= p.d

• N= number of turns in coil.

What must be conserved in transformers?

Power

• Power of Primary coil= Primary of secondary coil.

• VpIp= VsIs

• only applies if transformer is 100% efficient.

National Grid: How can a large amount of power be transmitted?

• Using either a high p.d. or a high current.

• However, power is wasted in transmission cables as heat.

• Amount of power wasted depends on the square of the current.

• This means to avoid wasting power, a high p.d. should be used.