topic 7 magnetism and electromagnetism

what do all magnets produce?

A magnetic field - a region where other magnets or magnetic materials experience a force

what direction do magnetic field lines always point?

always go from north to south and they show which way a force would act if it was put in that point in the field

What do magnetic field lines show?

- the closer together the lines are the stronger the magnetic field.
- the magnetic field is stronger at the poles of a magnet
-the force between a magnet and magnetic material is alway attractive, no matter the poles

what happens if two poles of a magnet are put together?

if two poles of a magnet are put near eachother they will exert a force on each other which can be either attractive or repulsive. two poles that are the same will repel each other.

How do compasses show the direction of magnetic fields?

inside a compass is a tiny bar magnet. the north pole of this magnet is attracted to the south pole of any other magnet it is near. so the compass points in the direction of the magnetic field its in.

when near a magnet what direction do compasses point in?

north

Why do compasses always point north?

this is because the earth generates its own magnetic field which shows the core of the earth

what are the two types of magnets?

permanent and induced

what does a permanent magnet do?

Produces its own magnetic field

what are induced magnets?

magnetic materials that turn into a magnet when they're put into a magnetic field

what is the force between permanent and induced magnets?

always attractive

what happens when you take away the magnetic field for an induced magnet?

it quickly loses its magnetism and stops producing a magnetic field?

what happens when a current flows through a wire?

a magnetic field is created around the wire. the field is made up of concentric circles perpendicular to the wire with the wire in the centre

how can you prove there is a magnetic field around a charge?

you can place a compass near a wire and as you move the compass it will trace the direction of a magnetic field

when a moving charge creates a magnetic field, when does the strength of this magnetic field change?

changes with change in current and distance from the wire. the larger the current through the wire or the closer to the wire you are the stronger the field is.

What is a solenoid?

a coil of wire

how do solenoids affect the magnetic field?

make it stronger by making a coil

How do solenoids produce a strong magnetic field?

because the field lines around each loop of the wire line up with one another. this results in lots of field lines pointing in the sane direction that are really close to each other

Describe the magnetic field inside a solenoid

Strong and uniform - same strength and direction at every point

what is the magnetic field like outside the coil of a solenoid?

the same as the magnetic field around a bar magnet

how can you increase the field strength of a solenoid even more?

By putting a block of iron in the middle of the coil, this iron core becomes an induced magnet wherever current is flowing.

in a solenoid what happens if you stop the current?

the magnetic field disappears

what is a solenoid with an iron core called?

an electromagnet

why are electromagnets useful?

They can be turned on and off

how are electromagnets used in cranes?

they are used in some cranes to attract and pick up object made from iron or steel e.g in scrap yards.

decribe how electromagnets are used in circuits?

they can act as switches:
when the switch in one circuit is closed it turns on the electromagnet which attracts the iron contact on the rocker
the rocker pivots and closes the contacts completing circuit two and turning on the motor

What is the motor effect?

When a current-carrying wire is put between magnetic poles, the magnetic field around the wire interacts with the magnetic field it has been placed in. This causes the magnet & the conductor to exert a force on each other - this can cause the wire to move

for a current in a magnetic field to experience the full force what angle must it be at?

90 degrees

in the motor effect how does the force always act?

at right angles to the magnetic field of the magnets and the direction of current in the wire

how can you show the direction of the force in the motor effect?

apply a current to a set of rails inside a horseshoe magnet. a bar is placed on the rails, which completes the circuit.

what does the force acting on a conductor in a magnetic field depend on?

1. the magnetic flux density - how many field lines there are in a region. this shows the strength of the magnetic field
2. the size of the current through the conductor
3. the length of the conductor thats in the magnetic field

when the current is at 90 degrees to the magnetic field its in how can you calculate the force

force \= magnetic flux density x length x current

how can you find which way the force is acting?

using flemmings left-hand rule:
first finger \= direction of the field.
second finger \= direction of current
thumb \= direction of the force

what does flemming's left hand rule tell you?

if either the current or magnetic field is reversed then the direction of the force will also be reversed

describe how loudspeakers use electromagnets

1. an alternating current is sent through s coil of wire attached to the base of a paper cone
2. the coil surrounds one pole of a permanent magnet and is surrounded by the other pole, so the current causes a force on the coil which causes the cone to move
3. when the current reverses the force acts in the opposite direction which causes the cone to move in the opposite direction too.
4. so variations in the current makes the cone vibrate which makes the air around the cone vibrate and creates the variations in pressure which causes a sound wave
5. the frequency of the sound wave is the same frequency of the ac so by controlling the frequency of the ac you can alter the sound wave produced

what is the generator effect?

The induction of a potential difference (& current if there's a complete circuit) in a wire is moving relative to a magnetic field or experiencing a change in a magnetic field

what does the generator effect create?

a potential difference in a conductor and a current if the conductor is part of a complete circuit

how can you create a generator effect?

by moving a magnet in a coil of wire or moving a conductor in a magnetic field

what happens in a circuit when you move a magnet side to side? (generator effect)

it creates a little blip of current if the conductor is part of a complete circuit

in the generator effect what happens if you move the magnet or conductor in the opposite direction?

the potential difference/current will be reversed. likewise if the polarity of the magnet is reversed then the potential difference/ current will be reversed

what happens if you keep moving the magnet and coil back and forth?

you produce a potential difference that keeps swapping direction - an alternating current

describe how you can create an alternating current?

you can create alternating current by turning a magnet end to end in a coil:
- as you turn the magnet the magnetic field through the coil changes. this change in the magnetic field induces a potential difference, which can make a current flow in the wire
- when you've turned the magnet half a turn the direction of the magnetic field through the coil reverses. when this happens the potential difference reverses so the current flows in the opposite direction around the coil of wire
- if you keep turning the magnet in the same direction then the potential difference will keep on reversing every half turn and you'll get an alternating current

how does a magnetic field produced always act?

against the change that made it so induced current always opposes the change that made it

how do you change the size of the induced pd?

you have to change the rate that the magnetic field is dhanging

how can you increase induced pd?

1. increasing the speed of the movement - cutting more magnetic field lines in a given time
2. increasing the strength of the magnetic field

What do alternators generate?

alternating current

what are alternators construction similar to?

a motor

What do generators do?

Rotate the coil in a magnetic field

describe how alternators produce an alternating potential difference?

as the coil or magnet spins, a current is induced in the coil. this current changes direction every half turn
instead of a split ring commutator ac generators have slip rings and brushes so the contacts don't swap every half turn.

How do dynamos generate a direct current?

work the same as alternator BUT have split-ring commutator instead of slip rings. this swaps the connection every half turn to keep the current flowing in one direction

What do oscilloscopes show?

how the pd generated in the coil changes over time

on an oscilloscope what is the height at any given point?

the generated potential difference at that time

what does increasing the frequency of revolutions do?

increases the overall pd but also increases the number of peaks

describe how microphones work?

- sound waves hit a flexible diaphragm that is attached to a coil of wire wrapped around a magnet
- this causes the wire to move in the magnetic field generating a current

what does the generated current of a microphone depend on?

the movement of the coil depends on the properties of the sound wave

what do transformers do?

Change the size of the voltage of an alternating current

how are transformers structured?

they all have two coils of wire, the primary and secondary, joined with an iron core

how can a current be induced?

when an alternating pd is applied across the primary coil, the iron core magnetises and demagnetises quickly. this changing magnetic field induces an alternating pd in the secondary coil. if the second coil is part of a complete circuit

what is the ratio between the number of coils on the primary and secondary coils equal to?

the ratio between primary and secondary potential differences

What do step up transformers do?

Step the potential difference up
They have more turns on the secondary coil than the primary coil

What do step down transformers do?

They decrease the voltage and increase the current.
there are more coils in the primary than the secondary.