Magnetism

Magnetic field

poles?

North and south

region where other magnets or magnetic materials experience a force - non contact force

Magnetic field lines

draw form north to south show which way a force would act on a north pole if it was put on that point in the field

closer the lines stronger the magnetic field

Where is the magnetic field the strongest?

poles

what happens if 2 poles are put near eachother

exert a force on eachother -attractive or repulsive

like charges repel

opposite attract

How to plot magnetic field lines?

Place bar magnet in the centre of a sheet of paper

position a magnetic compass somewhere around the magnet

observe the direction of the needle and draw a dot at the circumference for the magnet in li e with each end of the needle

repeat steps for several positions around magnet

join arrows to complete the field lines

Permanent vs induced magnet

produce their own magnetic field

turn into a magnet when put into a magnetic field

force between permanent and induced magnet is always attractive

when the magnetic field is took away induce magnets lose magnetism and stop producing field

Electromagnetism

When a current flows through a wire a magnetic field is created around the wire

the field made up of concentric circles perpendicular to the wire

changing the direction of the current changes the direction of the magnetic field

The larger the current through the wire or the closer to the wire you are the stronger the field is

The right hand thumb rule

use our right hand point your thumb in the direction of the current and curl your fingers. the direction of your fingers around the wire is the direction of the field.

Solenoid

coil of wire

increase strength of magnetic field by wrapping wire into a coil - solenoid because field comes around each loop of wire line up with eachother

so lots of field lines point in the same direction that are close to eachother

magnetic field is strong and uniform

can increase strength by adding a block of iron in the centre of the core iron core becomes an induce magnet

if the current is stopped the magnetic field disappears.

electromagnet

A solenoid with an iron core that an be turned off and on

Uses of Electromagnetism

cranes to pick up stuff and drop

circuits to act as switches

How to test for magnetism

permanent magnet will attract or repel another magnet - magnetic materials will be attracted to it

The Earth’s magnetic field

outer core of earth contains iron in a molten state

as earth spins the iron spins and creates the earths magnetic field

the earth behaves as if their is a huge magnetic field at it’s centre

Evidence the earth magnetic field is changing?

direction of magnetic field has reversed

poles have reversed over time

Compass and bar magnet

needle is a small bar magnet so needle and bar magnet will exert a force on eachother

Motor effect

when a current carrying wire is put between magnetic poles the magnetic field around the wire interacts with the magnetic field and causes the magnet and conductor to exert a force on eachother cases wire to move

What needs to happen to increase the full force

the wire has to be at 90 degrees to the magnetic field

if the wire runs parallel it wont experience any force

strength of the force increases with strength of magnetic field and amount of current through conductor

Magnetic flux density

length

using equation what does these mean

how man field lines in region - shows the strength of magnetic field

length of conductor in magnetic field

Fleming left had rule

use left hand point first finger in direction of field

using second finger point in direction of current

thumb point to direction of force(motion)

shows that of either current of magnetic field is reversed then direction of force is reversed

Electric motor

When a current flows through a coil in a magnetic field, the motor effect produces opposite forces on each side of the coil, causing it to rotate.

When the coil reaches the vertical position, the forces become parallel to the magnetic field, so the turning effect is zero.

A split-ring commutator is used to reverse the current every half turn, ensuring the forces on each side keep acting in the same overall direction. This allows the coil to rotate continuously in one direction.

Loud speakers

• Alternating current passes through a coil attached to the base of a paper cone.

• The coil surrounds one pole of the permanent magnet and is surrounded by the other pole.

• The current produces a force on the coil, moving the cone.

• When the current reverses, the force reverses-acts in the opposite direction, so the cone moves in the opposite direction.

• Variations in the current make the cone vibrate, which makes the air around the cone vibrate and creates variations in pressure that produce a sound wave.

• The frequency of the sound wave is the same as the frequency of the alternating current, so the sound produced can be changed.

Generator effect- definition

The induction of a potential difference(and current if complete circuit) in a wire which is moving relative to a magnetic field or experiencing a change in a magnetic field

The Generator effect

• Moving a magnet in a coil of wire or moving a conductor in a magnetic field (cutting magnetic field lines) creates a potential difference.

• If the circuit is complete, this produces a current.

• Shifting the magnet side to side produces a brief blip of current in the circuit.

• If the magnet or conductor moves in the opposite direction, the potential difference and current are reversed.

• If the polarity of the magnet is reversed, the potential difference and current are also reversed.

Explain how an alternating current (AC) generator produces an alternating current. - GENERATOR EFFECT

• Turning a magnet in a coil changes the magnetic field through the coil.

• This change in magnetic field induces a potential difference, which can cause current to flow in the wire.

• After half a turn, the direction of the magnetic field through the coil reverses.

• This makes the potential difference reverse, so the current flows in the opposite direction around the coil.

• If the magnet keeps turning in the same direction, the potential difference keeps reversing every half turn.

• This produces an alternating current (AC).

“Why does the induced current oppose the change that produces it?” / “Explain Lenz’s Law.”

• A change in magnetic field can induce a current.

• When a current flows through a wire, it creates a magnetic field around the wire.

• Therefore there are two magnetic fields: the original field and the field from the induced current.

• The magnetic field created by the induced current acts against the change that produced it (e.g., movement of a wire or change in the field).

• This means the induced current always opposes the change that caused it.

How to change the size of the induced potential difference?

• Change the rate of the magnetic field to affect the induced current.

• Increase the speed of movement → more magnetic field lines are cut in a given time → larger induced current.

• Increase the strength of the magnetic field → more field lines to be cut → larger induced current.

“Explain how an alternator produces an alternating potential difference.”

• Alternators rotate a coil in a magnetic field.

• This cuts m.f lines

• This induces p.d

• As the coil spins, a current is induced in the coil.

• The current changes direction every half turn.

• Unlike a DC motor (which uses a split-ring commutator), AC generators use slip rings and brushes, so the connections don’t swap every half turn.

• This produces an alternating potential difference (AC voltage).

Dynamos

• Like alternators, but have a split-ring commutator instead of slip rings.

• This swaps the connections(COMMUTATOR WITCHES CONTACTS) every half turn to keep the current flowing in the same direction

GENERATE DIRECT CURRENT

SO DIRECTION OF CURRENT DOES NOT CREVERSE EVERY HALF ROTATION

How to use oscilloscope to see generated pd?

• Shows how p.d. in a coil changes over time.

• AC graph goes up and down, crossing the axis; DC stays above the axis.

• Line height shows p.d. at that time.

• Higher frequency of revolutions increases p.d. and produces more peaks.

Microphones

• Sound waves hit a flexible diaphragm causing it to vibrate attached to a coil of wire wrapped around a magnet.

• This causes the coil to move repeatedly/ VIBRATE in the magnetic field, which generates an alternating current.

• The movement of the coil depends on the properties of the sound — louder sounds make the diaphragm move further.

• This is how a microphone converts pressure variations of a sound wave into variations in current in an electrical circuit.

Transformers

AP=APD

• A transformer changes the size of potential difference using two coils and an iron core n an alternating circuit.

• AP in the primary coil magnetises and demagnetises the iron core, producing a changing magnetic field.

• This induces AP in the secondary coil.

• If the secondary coil is in a complete circuit, a current is induced.

• Primary:secondary p.d. ratio = primary:secondary turns ratio.

Step up and Step down transformers

• Step-up transformer → p.d. increases; more turns on the secondary coil than the primary coil.

• Step-down transformer → p.d. decreases; more turns on the primary coil than the secondary coil.

Explain why a current was detected in figure 1 when wire was moved

Moving wire cuts through the magnetic field/cuts mf lines

Induces a p.d

The wire is part of a complete circuit

Explain why sound waves have an effect on electrical circuit in microphone

Pressure vibrations in air cause the diaphragm to vibrate

Diaphragm causes the coil to vibrate

Coil repeatedly changes direction inducing an alternating current

Explain how a dynamo induces direct current

Coil moves through m.f/ cuts m.f lines

P.d is induced

Wire is part of a complete circuit

Each half revolution the commutator switches contacts

so direction of current does nto reverse every half-rotation

Why is dynamo easier to turn when lamp is removed

no induced current

no magnetic field to oppose movement of coil

Explain why there is an alternating current in the output when transformer conencted to a circuit

(the alternating current causes) a changing magnetic field around the primary (coil)

creates magnetic field that changes direction in the core (allow creates a changing magnetic field in the core)

this induces an alternating potential difference across the secondary (coil (causing an alternating current)

Why did you assume calculating flux density

The force is at right angles to a magnetic field

(equation)