P2P - Magnetism 3

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Bar magnet

Bar magnet

• magnetic forces are strongest at poles of a magnet

• bring two magnets close together = exert a force on each other - NS = attract, SS = repel

• non-contact force

Permanent vs induced magnet

• Permanent produces own magnetic field (bar)

• Induced is an object that becomes a magnet when placed in a magnetic field

• Induced magnetism always causes a force of attraction and stops being a magnet when seperated

v1 - permanent and induced magnets

Four types of magnetic material

• iron, steel (which is an alloy of iron), cobalt and nickel

• all can be made into a permanent or induced magnet

magnetic field and strength

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

• strength of field depends on distance from magnet

How to find direction of a magnetic field

• found using magnetic compass, containing a small bar magnet

• place near = plot magnetic field

1. place compass near N pole. draw cross at N pole of the compass

2. move compass so S pole is on the cross. draw cross at N

3. repeat until reaching S pole of bar magnet = complete magnetic field line

4. connect all dots with a line and show direction of field line with arrow.

   • Direction is always N to S

5. Repeat starting at diff points around the N pole of the bar magnet

   • where line are closer, field is stronger = poles

Magnetic Compass and earths field

• contains a small bar magnet

• hold the compass away from any magnets, the needle always points in the North - South direction

• Tells us that the Earth has its own magnetic field. Due to the earths core

v2 - magnetic fields

Current flowing through wire and compass

• what is produced around the wire and how to prove

• how magnetic field strength and direction changes

• rule to work out magnetic field direction

Magnetic field is produced around the wire:

1. prove by using compass

2. turned off switch = compass needle aligns with earths magnetic field

3. turned on switch = compass needle deflects

• Strength of magnetic field depends on current size

  • larger I = stronger field

  • Magnetic field is strongest closer to the wire

• Change current direction = change magnetic field direction = compass deflects in opposite direction

• right hand grip rule = thumb shows conventional current direction = fingers are field direction

Another way of increasing magnetic field strength

• name of shape

• field similarity

• rule for field direction

• coiling the wire in a circuit = solenoid

• turn on current = strong and uniform field inside solenoid, and field around is similar to bar magnet

• right hand grip rule = use fingers to wrap in current direction, thumb points N, N is field direction

3 ways to increase strength of magnetic field produced by a solenoid

• Increase current size

• Increase number of turns of coil

• Place iron inside solenoid = iron core

Electromagnet and usefulness

• solenoid containing an iron core

• extremely useful as we can change strength of magnetic field by changing current size

• also can turn on or off

v3 - electromagnets

High voltage circuit dangers and set up

Can be dangerous to use switch to turn these on and off (sparking and electrocution) so:

• use a relay to turn these on and off, which contains 2 separate circuits:

  • low voltage circuit containing electromagnet which is safe to switch on/off

  • replace switch of high voltage circuit with 2 metal contacts

  • one contact is connected to a spring that keeps the contacts apart

  • also has iron block next to spring

• low voltage circuit off = no current is flowing through EM and theres no magnetic field

• high-voltage circuit is turned off since contacts aren’t touching

High voltage circuit stages of circuit

1. turn on low voltage circuit = current flows around = magnetic field around EM

2. magnetic field attracts iron block in the high voltage circuit = causes contacts to close and switches on high voltage circuit

3. switch off low voltage circuit = no magnetic field = contacts spring apart and high voltage is off

Appliance using EM

Doorbell:

1. switch is closed when buzzer is pressed = current flows through circuit = magnetic field is produced by EM

2. iron contact is attracted to magnetic field = contact moves to field, clapper hits bell

3. same time, this breaks the circuit = no current = no field = iron contact springs back to original position

4. now circuit is complete again so current flows and process is repeated

v4 - electromagnetic devices

Motor effect

• Wire placed into magnetic field between 2 bar magnets

• Magnetic field around wire interacts with magnetic field between the magnets = wire experiences force up or down depending on field and current direction

• This case upwards = wire moves up

Calculate force from motor effect

• applies to a wire at right angles to the magnetic field

• magnetic flux density measures strength of magnetic field

Flemings left hand rule

1. thumb is motion (direction of force that moves wire)

2. first finger is field

3. second finger is current

Conductor parallel to magnetic field

won’t experience force

v5 - motor effect

Motor effect - electric motors

1. 2 magnets, north (left) to south (right), current flowing to the right.

2. As current flows, when reaching S, wire experiences a force down (right side), and when reaching N, wire goes up (left side) = a moment on the left and right side

3. Loop will rotate in clockwise direction

4. Once loop is at 90 degrees it stops rotating

   • imagine loop goes beyond 90, direction of the current travels in the opposite direction than the previous current, which means left is acting downwards and force on right is upwards

   • these forces push the loop back to the 90 degrees

   • can solve this problem by switching direction of the current when the loop passes 90

5. Do this with a split-ring commutator: a split metal ring connected to conducting brushes. Brushes allow the electric current to pass onto the ring.

6. Current produces a turning force on the motor = forces makes motor rotate clockwise = current is broken for a tiny fraction of a second, but wire keeps turning due to momentum.

7. Current now switches direction, and the force on the left still acts up, and force on the right still down (same as step 2)

• By switching direction of the current, split ring commutator allows motor to keep rotating in same direction

• electrical energy → kinetic energy

Motor effect - electric motors diagrams

v6 - electric motor

Motor effect in loudspeakers and headphones

Moving-coil loudspeaker is found in speakers on a stereo (headphones are similar but smaller):

1. Cone with coil of wire wrapped around one end. Coil of wire is connected to an A.C electrical supply. Permanent magnet which goes inside the coil of wire.

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

3. Magnetic field from coil interacts with magnetic field from permanent magnet = they attract or repel each other

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

5. When current switches direction, the direction of the forces on the cone reverses = cone moves in and out, generating sound waves.

6. By changing freq of AC, we change freq that cone vibrates. Higher freq = higher pitch

7. By increasing current size, we increase amplitude of vibration. Increases volume

v7 - loudspeakers and headphones

Generator effect - normal wire

1. when a conductor (metal wire) cuts flux (moves through magnetic field) upwards, a potential difference is induced across the conductor (metal wire).

2. wire stops moving, then p.d is lost, but if the wire cuts flux downwards, we get a p.d again, but this has a reversed direction = this p.d is called the induced potential

Key points:

• if we have a complete circuit, we induce a current and that switches direction when the direction of movement switches

• process is the same if the field is moved into a conductor, rather than a conductor being moved into a field

Conditions and relation to motor effect:

• whole thing only happens if conductor CUTS FLUX, not if wire is moved back and forth around a field

• kinetic energy → electrical energy = reverse of motor effect

The induced potential difference and current are larger if we…

• use a stronger magnetic field = magnetic flux density(B)

• move the wire more rapidly

• shape the wire into a coil (greater number of turns on coil = longer)

Generator effect - coiled wire

• magnet moving in and out of a coil of wire also makes induced current.

• direction of current changes when direction of movement changes OR if we flip the poles

1. this induced current creates its own magnetic field, which opposes movement of the magnet

2. insert N pole into the coil = that end of the coil also becomes a N pole = this repels the magnet = harder to push magnet in

3. pull N out = that end of the coil becomes S = attracts the magnet, so harder to pull out

• since induced current makes it harder to move the magnet, we are doing work OR transferring energy from the movement of the magnet, into the movement of the current

v8 - generator effect

Alternator stages and graph

Alternator is a coil of wire rotating in a magnetic field:

• Coil is connected to 2 metal rings called commutators, which allow current to pass out of coil.

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

• graph shows how the p.d changes

1. red side of the wire always connects onto ring A, and orange side always connects onto ring B

2. the max p.d is when the coil is horizontal. At this point, the wire is sweeping directly through the magnetic field lines at the fastest possible rate.

   • red side moving down and orange side moving up

3. coil is vertical = p.d is zero, since coil is moving parallel to the field. At this point, the coil is not cutting through the magnetic field lines.

4. coil continues around = get a p.d again but with reversed direction, since the 2 sides of the coil are now moving in a different direction to before

   • red side moving up and orange side moving down

5. again coil is vertical = p.d is zero, since coil is moving parallel to the field and not cutting flux

• since two sides of the coil are attached to two different rings, and alternator produces an alternating p.d and alternating current

How to increase size of current, freq and p.d with alternator

• increase strength of magnetic field = increase size of AC

• increase size of AC if we increase turns on the coil or area of the coil

• increase rotation speed of coil = increase both the size and freq of AC

Dynamo stages and graph

• produces direct current

• has a split-ring commutator with 2 sides separated by a gap (A,B)

1. Side of coil moving down is connected to A of split-ring commutator. side of the coil moving up is connected to B. Start with red on left (down) orange on right (up).

2. Since coil is cutting through magnetic field lines, a p.d and current are induced.

3. Coil is vertical = moving parallel to magnetic field so p.d is 0.

4. Now coil has moved around, orange on left (down) red on right (up). But side of coil moving down is still connected to A of split-ring commutator, and side moving up is still connected to B.

5. Therefore direction of p.d and current don’t reverse when coil rotates = Direct current

• graph shows how p.d changes

• we get 2 peaks for each full rotation of coil - since each side of coil cuts flux twice during each cycle of rotation. Once passing down, once up.

v9 - Alternator and dynamo

Moving-coil microphone

Very similar to moving-coil loudspeaker:

• coil of wire attached to a thin sheet of plastic, called a diaphragm, end of coil of wire sits over a permanent magnet

1. when soundwaves hit the diaphragm, they cause it to vibrate

2. coil of wire moves in and out through the magnetic field = induces a p.d across the ends of the wire

3. p.d switches direction as coil moves back and forwards through magnetic field. Frequency of the changing p.d is same as freq of the sound waves.

4. Changing pattern of p.d is passed through an amplifier and then into a moving-coil loudspeaker = massively increases volume

v10 - the microphone

Transformer - 100% efficient

• 2 coils of wire wrapped around iron (easily magnetised) core- primary and secondary are completely separate

1. primary coil is connected to AC. As current flows through primary, it generates a changing direction magnetic field.

2. This magnetic field is transmitted along iron core and passes through secondary coil = coil induces a p.d

• Primary and secondary has same number of coil turns = p.d is the same in each. Only true if transformer is 100% efficient (no energy wasted in the transformer). In practise, transformers aren’t 100% efficient

Transformer key points

• iron core increases the strength of the magnetic field

• transformers only work with AC since we need a changing magnetic field to induce a p.d

• DC produces a constant magnetic field = doesn’t work in transformers

Transformer - unequal coils

Step-up transformer:

• more turns in secondary coil than primary coil = p.d induced in secondary coil will be greater than p.d in primary coil

• this is called a step-up transformer, since it steps-up the p.d

• if turns double in secondary, the p.d doubles in the secondary

Step-down transformer:

• exact opposite of step-up

v11 - transformers

Finding p.d of transformer coils - eq

Conservation in transformers and eq

• power must be conserved

• power of primary coil = power of secondary coil

• only applies if the transformer is 100% efficient (no energy wasted). not true in practise, only calcs

National grid - issues and solution

• Electrical power is transmitted from power stations to homes through high-voltage cables

  • power = current x p.d

• To transmit a large amount of power, either use a large current or a large voltage

• Power is wasted in the transmission cables as heat and this amount depends on square of the current (I²)

• Large current in transmission cables = huge amount of power wasted as heat

• So instead use a large p.d

National grid stages

1. Electricity from power station is passed through a step-up transformer. Increases p.d to 400kV.

2. Electrical power is transmitted down high-voltage cables.

3. Passed through a step-down transformer. Decreases p.d to 230V for homes

• Transformers massively reduce power wastage in National Grid

v12 - transformer calculations