Electromagnetism

A transformer consists of an iron core with two coils 

• A varying current flows through the primary coil
• The current creates a varying magnetic field through and around the primary coil

Note: Because the iron core is a good magnetic material, nearly all of the magnetic field lines pass through the iron core and not the surrounding air.

• The varying magnetic field created through the primary coil passes through the iron core and through the secondary coil (due to the varying current in the primary coil) This magnetic field links the two coils magnetically. This induces a varying output coltage across the ends of the secondary coil

A transformer only works if there is a varying voltage across the primary coil, often an AC voltage.

• No transformer can be 100% efficient 
• Heat energy is created when the current flows through the coils which is waste energy
• An ideal tranformer is 100% efficient 

(Vp/Vs) = (np/ns)

e.g an ideal tranformer has 600 turns in the primary coil and 200 turns in the secondary coil. The input voltage across the primary coil is 12V. Calculate the output voltage across the secondary coil.

(12V/Vs) = (600/200)

(12V/Vs) = 3 

Vs = 12V/3

Vs = 4V (a.c)

In the experiment, connect a 1.5V cell across the primary coil of a transfromer and then disconnect the 1.5V cell. A microoammeter is connected across the secondary coil of the transformer.

Observation: When a cell is connected, the microammeter needle flicked to one side of 0 and then returned back to 0, when you disconnect, the needle flicked to the other side and returned to 0.

Steel

• "Hard" magnetic material
• Harder to magnetise than iron 
• Once magnetised, it is harder to remove its magnetism 

Iron

• "Soft" magnetic material
• Easier to magnetise than steel 
• Looses magnetism more easily than steel

Domain Theory

• Iron and steel can be thought to contain tiny magnetic regions called domains 
• When unmagnetised, the point in random directions and their magnetic fields cancel eachother out 
• When magnetised, the domains become more aligned and their magnetic effects reinforce eachother
• The domains in steel keep most of their alignment whereas the domains in iron lose it.

• Grip conducter in your right hand 
• Point your thumb along the direction of the current
• Your finger will show you the direction of the circular magnetic field

It is a coil of wire

When a current flows through a solenoid, a magnetic field is created.

The magnetic field through the centre of a current carying solenoid is straight

• We can use a solenoid to magnetise prevoiusly unmagnetised iron rod or steel 
• This is because the straight magnetic field going through the solenoid will line up the domains that were previously unaligned , making iron or steel behave as a magnet 

Results

When the switch was closed:

• Iron picked up alot of iron nails
• Steel picked up fewer nails 

When the switch was open:

• Iron dropped almost all of the nails
• Steel kept most of the nails

Conclusion: An electromagnet whose magnetism can be turned on and off consists of an iron rod surrounded by a solenoid 

The strength of an electromagnet consisting of a solenoid wrapped around an iron rod depends on the current flowing through the solenoid and the number of turns the wire has.

Number of Turns

The greater the number of turns of wire in the coil, the stronger the electromagnet 

The Current 

The greater the supply voltage (current), the stronger the electromagnet 

Conclusion: To increase the strength of an electromagnet consisting of a solenoid wrapped around an iron rod:

1 - INCREASE THE NUMBER OF TURNS IN THE WIRE

2 - INCREASE THE CURRENT (by increasing the p.d across the solenoid)

• An electric bell consists of an electromagnet that repeatedly switches on and off, moving the hammer backwards and forwards many times per second.
• When the bell switch is pressed on, an electric current flows through the electromagnet and the elctromagnet is turned on
• The electromagnet now attracts the iron armature and the hammer is pulled across making it hit the gong
• This movement makes the contacts open which stops the electric current so the electromagnet is turned off
• This lets the hammer go back and so the contacts are now together 
• The current now starts to flow again and the whole process repeats itself as long as the switch is being pushed

Explain why iron and not steel is used to make the core of the electromagnet

• If we used steel, it would not have released the soft iron armature when the contacts were seperated
• This means the bell would not continue to ring 
• Iron is used because it is a soft magnetic material 

• A relay links two seperate circuits 
• If the switch is closed, a current flows in the first circuit and an electromagnet attracts the iron armature
• The L-shaped armature rotates anticlockwise and forces the contacts to join 
• This closes the gap in the second circuit to make something happen

Operating a lift

• When the switch in circuit 1 is closed, a small current flows, closing the gap in circuit 2 
• This allows a large supply voltage to flow and makes the lift motor work
• It is safer than having a circuit with a large voltage supply because it could electricute the person who pushes the button 

• A current creates a magnetic field
• If you hold a bar magnet close to the wire, the wire moves
• When the power pack is turned on, a current flows through the wire and the wire moves 
• This is because the magnetic field due to the bar magnet interacts with the magnetic field due to the current of the wire 

LEFT HAND MOTOR RULE

First Finger: Field (direction of magnet from N to S)

SeCond Finger: Current (direction of conventional current)

ThuMb: Motion (direction of force acting on the conductor)

• When you increase the current, the coil spins faster
• When you reverse magnets, the coil spins in the opposite direction
• When you reverse connections, the coil rotates in the opposite direction

To increase speed:

• Increase current
• Increase number of turns
• Use a stronger magnet
• Wrap the coil around an iron core (to increase strength of the magnetic field)
• Use curved poles

To reverse the direction of the coil:

• Reverse the magnets
• Reverse the connections

Observations

• Magnet moved into coil: The needle sticks to one side of 0A
• Magnet moved out of coil: The needle sticks to the other side of 0A
• Magnet moved more quickly: Needle sticks further to one side of 0A
• Stronger magnet used: Th needle flicks further to one side of 0A
• More turns of wire: The needle flicks further to one side of 0A
• Magnet at rest: Needle stayed at 0A
• Opposite pole of magnet used: Needle flicks in the opposite direction

Conclusion: To increase the induced current:

• Move magnet faster
• Use stronger magnet
• Increase number of turns

FARADAYS LAW OF ELECTROMAGNETIC INDUCTION

When there is a relative movement between a conductor and a magnetic field, the size of the induced voltage across the conductor is directly proportional to the rate at which magnetic field lines are being cut

Meaning, the faster you cut field lines, the larger the voltage

• When the coil is rotated clockwise, side X is being moved downwards
• Side Y is being moved upwards
• This means they are both cutting across field lines in opposite directions
• The slip rings rotate as the coil rotates 
• Half a turn later, their roles are reversed, making the current flow in an opposite direction 

To increase the maximum output voltage of a generator:

• Increase the number of turns
• Rotate the coil faster
• Stronge rmagnets

• A power station uses fossil fuels or nuclear fuels
• In a gas power station, gas is burnt which allows chemical energy to be converted into heat energy
• The heat energy is used to turn water into steam at a high temperature and pressure
• The steam is forced through a turbine, consisting of a cylinder with angled blades 
• As the steam is forced through, the turbine is forced to rotate
• The turbine is attached to the generator, which has magnets that rotate around a fixed coil
• This induces an a.c voltage acorss the ouput terminals of the generator
• Once passed through the turbine, the water cools and is recylced
• Only a small amount is actually converted into useful electrical energy 
• The genertor is then stepped up to a high value by a step-up transformer at the start of the National Grid

P=V/I

P=I2R

• The output voltage is steppd up to reduce the current that flows along cables
• By reducing the current, the amount of heat energy wasted is reduced
• By reducing the current I, we greatly reduce I2 R.
• This means little as possible is wasted in the form of heat energy 

Why wouldnt you just make the resistence smaller instead of stepping-up?

• We would have to increase the cross-sectional area
• This would make the wires thicker
• This would make them heavier
• This would mean more pylons
• This means visual pollution
• + Extra cost

The Fuse

• A fuse is a piece of wire that melts when the current going through it reaches a certain value
• A 3A fuse will melt if the current through it reaches 3A

The Earth Wire

• The earth wire connects the frame of the appliance to the pin of the plug 
• If a fault develops and the live wire breaks and comes into contact with the metal frame, it could electricute you 

Working Together

• If there is a fault and the live wire breaks

e.g an electric kettle is rated 2.3kW/230V

• Explain what this means

This means that after the kettle is plugged into the mains supply and turned on, it has a power of 2.3kW. This means that it would convert 2300J of electrical energy into 2300J of heat energy each second.

• Calculate the current after the kettle is plugged into the mains supply

P=VXI
I = P/V
I = (2300W/230V) = 10A

• Calculate the resistence of the kettle element when it is on

R = V/I

R = 230V/10A = 23 Ohms

• State (with reason) which is the most suitable fuse 

When we use a 1kW appliance for 1 hour, the amount of electrical energy converted is:

Electrical energy converted (J) = P x t

Electrical energy converted = P x t = 1000W x 3600s = 3600000J

6Electrical energy converted = 1kW x 1h = 1kWh = 1 kilowatt-hour = 36000000J

1 kWh = 1 kilowatt-hour = 1 "unit"

• A circuit breaker involves the use of an electromagnet
• It cuts off the current if it rises to a certain value 
• It does not need to be replaced
• When the correct current flows, the iron cored electromagnet has a certain strength and attracts the iron catch by a certain amount
• Due to the first spring, the iron catch is not attracted strongly enough to allow the second spring to pull the contacts so the current continues to flow
• If the current rises to a certain value, the electromagnet becomes strong enough to pull the iron catch enough to allow the second spring to pull the contacts apart, which pushes the reset button upwards
• The current will now be cut off, as the circuit has been broken
• By pushing the rest button, you are forcing the the contacts together again
• The current then flows again

Advantages

• It can be reset and doesnt have to be replaced
• It cuts off the current after a shorter time than a fuse

• They are much less efficient in converting electrical energy into useful light energy
• They break when the bulb is turned on 
• This is because FLB have a low resistence when it is off and cold.
• When the bulb is turned on, there is suddenly a large mains voltage across the low resistence filament
• This makes the current surge to a high value
• Because current = voltage/resistence 
• As the current surges, the filament rapidly gets hotter and the resistence rises, which reduces the current to its final value 

How are lightbulbs connected in a typical house?

• You want light bulbs to be independant 
• The bulbs are now connected in parallel so they each have the full supply voltage across them
• If one is removed, the others are not effective

• It consists of a piece of metal that is heated by a piece of wire that has a current 
• The heat energy gives electrons in the metal enough energy to escaoe the surface of the metal. 
• This is called THERMIONIC EMISSION
• These electrons are then accelerated through a potential difference in a vacum
• There is a loss of electrostatic PE that is converted into a gain of KE.
• The electron beam can then be deflected upwards or downwards by a potential differnce applied across the Y-plates
• If the top plate is made positive , the beam of electrons will be deflected upwards 
• If the top plate is made negative, the beam of electrons will be deflected downwards

Working out maximum p.d from a graph:

The amount of squares the curve goes across x V

The time for one complete cycle:

V x ms 

Supply frequency:

1/(ms/1000) = Hz