P4

Created by

Aimee

Cards (33)

Magnet
Any material or object that produces a magnetic field
Two poles ( N and S)
Magnetic material- Iron, nickel, cobalt
Magnetic field
shows size and direction
point from north to south
closer the lines, stronger the field
What is magnetic flux density
Strength of magnetic field
Measured in Teslas
Uniform field
Value of field strength is same everywhere
North and South poles of two permanent magnets near each other
Compass to see magnetic field
put compass on piece of paper next to magnet
Compass needle will point in direction of field lines
Mark directions
Repeat and join up
Why is using a compass better than iron fillings
Always points north - evidence that Earth has a magnetic north and south so have a magnetic core
Drawings of field will still be there after magnet is removed
Iron fillings are messy- hard to take off magnets
Magnets affect magnetic materials and magnets
Like repel, opposites attract
When magnet is brought to magnetic material, the material acts as a magnet
Magnetism induced by original magnet
Closer- the stronger the induced magnet
Permanent magnet
never loses magnetism
Induced other objects
Induced magnet
lose their magnetism once magnet has been removed
Moving charges (Current) create a magnetic effect
larger the current, stronger the magnetic attraction
Magnetic field around a straight wire
> Made up of concentric circles with the wire in the centre
> Use a compass to show magnetic field
> Further away from wire, the weaker the magnetic field
> graph is non-linear as strength decreases quickly and slows down with the distance
Magnetic field around flat circular coil
> Similar to a bar magnet
> Concentric elipses (stretched out circles) around the coil
Flemmings left hand rule
Thumb- movement
Forefinger- field
Second- current
How to increase the size of the force on the wire
> Increase strength of magnetic field
> increase size of current
> Increase length of wire
Magnetic field around a solenoid 
> Magnetic field inside a current-carrying solenoid is strong and uniform
> Outside is like a bar magnet
Ends of solenoid acts as poles
> Electromagnetic
How to increase magnetic effect at end of solenoid
Current in wire increase
Number of turns increase
Length us decreased (number of turns stay the same)
soft Iron core is added
Current in magnetic field
> Experiences a force
> Wire between two poles, the two magnetic fields create a force on the wire
> Wire has to be at 90° to magnetic field
> Force acts in same direction- changing direction of current or the ,acetic field will change direction of force
Calculate force  
Force on current carrying wire (N) =
Magnetic field strength (T) x Current in wire (A)
x length of wire (m)
Motors 
> d.c motor - current carrying coil between two opposite poles of a magnet
> each side experience force in opposite direction- due to current flowing in different direction and coil is perpendicular to magnetic field `
> Coil is on spindle and forces act in opposite directions It rotates
> Split-commutator swaps current at every half turn
How to reverse motor
reversing the current
swapping magnet poles (reversing magnetic field)
Split ring commutator
changes current direction every half term as the loop rotates
force on left hand of the loop is up
force on right side of the loop is down
So it rotates in same direction
How to increase current
> Move wire/magnet faste
> Use a stronger magnet
> More turns on the coil means greater voltage
Generators
opposite of motors
Use relative motion of a conductor and magnetic field to induce a p.d and a current
If direction of rotation is reversed, the direction of the induced p.d are reversed too
current induced in alternator or dynamo will be greater if there are more turns on the coil
Magnetic flux density increases if speed of rotation increases
Alternators and dynamos are types of generators
Alternators
generate alternating currents
Some alternators rotate a magnet in a coil of wire
P.d is induced as magnet spins across ends of coils
p.d changes direction every half turn because direction of field changes as magnet rotates
Produces an alternating current if circuit is complete
Slip rings with brushes connected to the rest of the circuit so the contacts doesn't swap every half term to produce a.c
Dynamos
generate d.c
rotates coil in a magnetic field
Output p.d and current change direction every half rotation of the coil to produce a.c
split ring commutator swaps connection every half turn to keep the current flowing in same direction so changes a.c to d.c
Loud speaker 
> Converts sound waves into electrical signals
> motor effect to convert electrical signals into vibrations with generate sound waves
> Coil and cone slide back and fourth along magnet
Microphone 
> Detects vibrations caused by sound waves and use generator effect to convert into electrical signals
> Involves a.c
How loud speakers work
a.c flows through the wire and creates a magnetic field in the coil
Magnetic field of the coil interacts with magnetic field of permanent magnet
Motor effect causes it to move back and fourth along permanent magnet
Causes cone to change a shape
As the current alternates it causes vibrations
Causes pressure variations in the air= sound waves
How do microphones work
sound waves hit diaphragm
Causes diaphragm and coil of wire to move
Generates a current (electromagnetic induction)
Frequency of p.d is frequency of sound waves
Electromagnetic induction 
> moving a wire (conductor) through a magnetic field
> Creates a p.d in the wire
> p.d will increase current measure with ammeter
Transformers 
> Change size of p.d of an a.c to reduce heating effect
> two coils joined by an iron core
> iron core is a magnetic material that becomes magnetised
> Coil produces alternating magnetic field so magnetism in core alternates
Changing magnetic field induced p.d on second coil
Step down
decreases voltage
More turns in primary than secondary
Step up
increases voltage
More coils in secondary than primary