PHYSICS P3 ELECTROMAGNETISM

Magnetic Field: A region where magnetic materials (iron and steel) and also wires carrying currents experience a force acting on them. 

• the arrows on a field line always point FROM THE NORTH POLE of the magnet TO THE SOUTH POLE. 
• when a CURRENT FLOWS through a WIRE, a MAGNETIC FIELD is created AROUND the wire. 
• the field is made up of CONCENTRIC CIRCLES with the wire in the centre.

THE MAGNETIC FIELD ROUND A COIL OF WIRE:  the magnetic field inside a coil of wire (solenoid) is strong and uniform.

TO INCREASE THE STRENGTH OF THE MAGNET: add a magnetically 'soft' iron core through the middle of the coil: THIS IS CALLED AN ELECTROMAGNET. 

A magnetically soft material magnetises and demagnetises very easily, this is why it is USEFUL. 

• An electromagnet must be CONSTANTLY SUPPLIED with current- this is what produces a magnetic field. Example: electromagnets are used in some cranes- the magnet can be switched on and off so when it is turned off it drops what has been caught on. 

MOTOR EFFECT: THE FORCE EXPERIENCED BY A CURRENT-CARRYING WIRE IN A MAGNETIC FIELD.

• the force gets bigger if either the current or the magnetic field is made bigger.
• if the direction of the current or magnetic field is REVERSED, then the DIRECTION of the FORCE IS REVERSED TOO.
• to experience the FULL FORCE, the WIRE has to be 90 DEGREES to the MAGNETIC FIELD. 
• if the wire runs ALONG PARALLEL to the MAGNETIC FIELD it won't experience ANY FORCE AT ALL.

Use Fleming's left hand rule to work out motion, field and current. 

2 FACTORS THAT SPEED IT UP:

• MORE CURRENT
• STRONGER MAGNETIC FIELD

Because the coil is on a spindle and the forces act ONE UP and ONE DOWN, IT ROTATES.

THE SPLIT RING COMMUTATOR IS A WAY OF "SWAPPING THE CONTACTS EVERY HALF TURN TO KEEP THE MOTOR ROTATING IN THE SAME DIRECTION"

HOW CAN THE DIRECTION OF THE MOTOR BE REVERSED:

• Either by swapping the polarity of the direct current (DC) supply
• swapping the magnetic poles over

Electric motors are used in CD players, food mixers, fan heaters, fans and printers.

ELECTROMAGNETIC INDUCTION: THE CREATION OF POTENTIAL DIFFERENCE ACROSS A CONDUCTOR WHICH IS EXPERIENCING A CHANGE IN MAGNETIC FIELD.

MOVING A MAGNET IN A COIL OF WIRE INDUCES A VOLTAGE:

• if you move the magnet in the opposite direction, then the PD  will be REVERSED too.
• if the polarity of the magnet reversed, the PD will be REVERSED too.
• if you keep the magnet moving BACKWARDS AND FORWARDS, you produce a PD that KEEPS SWAPPING DIRECTION. (this is how an AC is produced)

HOW DOES A GENERATOR WORK?

• the magnetic field through the coil changes, this change in the magnetic field induces a PD, which can make a current flow in the wire.
• the direction of the magnetic field through the coil reverses. When this happens, the PD reverses, so the CURRENT flows in the OPPOSITE DIRECTION around the coil of wire.
• IF YOU KEEP CHANGING IT IN THE SAME DIRECTION, an AC current will be made

DYNAMOS USE ELECTROMAGNET INDUCTION

TRANSFORMERS ONLY WORK WITH AC COS A CHANGING FIELD IS NEEDED.

STEP UP TRANSFORMER:

• steps the VOLTAGE up.
• MORE TURNS ON THE SECONDARY COIL
• PD ACROSS THE SECONDARY COIL IS GREATER THAN THE PD ACROSS THE PRIMARY COIL.

STEP DOWN TRANSFORMER:

• steps the VOLTAGE down
• MORE TURNS ON THE PRIMARY COIL

TRANSFORMERS WORK THROUGH ELECTROMAGNETIC INDUCTION:

• primary coil produces a magnetic field which stays within the iron core-nearly all of it passes through to the secondary coil and nothing is lost.
• AC IN THE PRIMARY COIL, the field in the coil is constantly changing directions.
• the changing magnetic field is felt by the secondary coil and this induces a AC PD across the secondary coil.

THE IRON CORE IS PURELY FOR TRANSFERRING THE CHANGING MAGNETIC FIELD FROM THE PC TO THE SC

VP/VS = NP/NS or VS/VP = NS/NP

Transformers are nearly 100% efficient so POWER IN = POWER OUT

POWER SUPPLIED FORMULA: POWER= CURRENT X PD

VPIP=VSIS: electrical power input= electrical power output

SWITCH MODE TRANSFORMERS:

• operates at higher frequencies than traditional transformers
• 50 kHz- 200 kHz
• they are much lighter and smaller, useful for mobile phone chargers
• more efficient
• they use very little power when they're switched on but no load is applied.