SP13: Electromagnetic Induction Flashcards
SP13a
1) Describe how to produce an electric current by induction on a small scale.
2) Describe how electromagnetic induction is used in alternators and dynamos.
1) A magnet and a coil of wire can be used to produce an electric current. A voltage is produced when a magnet moves into a coil of wire. This process is called electromagnetic induction. The direction of the induced voltage is reversed when the magnet is moved out of the coil again.
2) A dynamo is a type of dc generator. In a dynamo, a split ring commutator changes the coil connections every half turn. As the induced potential difference is about to change direction, the connections are reversed. This means that the current to the external circuit always flows in the same direction.
An alternator is a type of ac generator. As one side of the coil moves up through the magnetic field, a potential difference is induced in one direction. As the rotation continues and that side of the coil moves down, the induced potential difference reverses direction. This means that the alternator produces a current that is constantly changing. This is alternating current or ac.
SP13a
1) Describe how different factors affect the size of an induced current/ p.d.
2) Explain how different factors affect the direction of an induced current/ p.d.
1) The size of the induced current/p.d. is determined by:
1. The speed at which the wire, coil or magnet is moved
2. The number of turns on the coils of wire
3. The size of the coils
4. The strength of the magnetic field
2) The direction of the induced current/ p.d. is determined by:
1. The speed at which the wire, coil or magnet is moved:
- Increasing the speed will increase the rate at which the magnetic field lines are cut.
- This will increase the induced potential difference.
2. The number of turns on the coils in the wire:
- Increasing the number of turns on the coils in the wire will increase the potential difference induced
- This is because each coil will cut through the magnetic field lines and the total potential difference induced will be the result of all of the coils cutting the magnetic field lines
3. The size of the coils: increasing the area of the coils will increase the potential difference induced. This is because there will be more wire to cut through the magnetic field lines
4. The strength of the magnetic field: increasing the strength of the magnetic field will increase the potential difference induced
5. The orientation of the poles of the magnet: reversing the direction in which the wire, coil or magnet is moved.
SP13a
1) Describe how the magnetic field produced by an induced potential difference opposes the original change.
2) Explain how microphones work in terms of changing pressure variations into variations in electric current.
3) Explain how loudspeakers change variations in current to variations in air pressure.
1) The induced voltage produces an induced current if the conductor is connected in a complete circuit. As with all currents, the induced current creates a magnetic field around itself. This magnetic field opposes the original change. If a magnet is moved into a coil of wire, the induced magnetic field tends to repel the magnet back out of the coil. This effect occurs whether a magnet is moved into a coil, or a coil is moved around a magnet.
2) Sound waves are pressure variations in air, so when they hit the diaphragm it will vibrate too, which means it moves back and forth between the magnetic poles. This induces an alternating potential difference and as the coil is part of a circuit, an alternating current is induced in the circuit, creating electrical signals.
3) The motor effect is used inside headphones, which contain small loudspeakers. In these devices, variations in an electric current cause variations in the magnetic field produced by an electromagnet. This causes a cone to move, which creates pressure variations in the air and forms sound waves.
SP13b
1) Explain how a transformer works.
2) What type of current does a transformer work with?
3) Use the turns ratio equation for transformers (this is given in the exam).
1) A transformer is made using two coils of insulated wire wound onto an iron core. There is no electrical connection between the two coils of wire. A transformer works by using magnetic induction. When an AC voltage is applied to one coil of wire, it creates a magnetic field that induces a voltage in a second coil of wire. The voltage in the second coil is proportional to the number of turns in the coil.
2) Transformers only work with alternating current, when the direction of the potential difference (and so also the current) changes many times each second. Transformers can change the voltage of an alternating current. The alternating current in the primary coil creates a continuously changing magnetic field, and the iron core of the transformer carries this magnetic field to the secondary coil.
3) Potential difference across primary coil / potential difference across secondary coil = number or turns in primary coil / number of turns in secondary coil
SP13b
1) Describe how the national grid transmits electricity around the country.
2) Explain why step-up and step-down transformers are used in the national grid.
1) In the National Grid, a step-up transformer is used to increase the voltage and reduce the current. The voltage is increased from about 25,000 Volts (V) to 400,000 V causing the current to decrease. Less current means less energy is lost through heating the wire. To keep people safe from these high voltage wires, pylons are used to support transmission lines above the ground. Before reaching the end user, a step-down transformer, reduces the voltage from the transmission voltage to the safer voltage of 230 V for home use. As an electric current flows through the thick cables held up by the pylons, they will get hotter and dissipate energy to the surroundings. To ensure that the minimum amount of power is lost from the cables: the cables are thick so that their resistance is low, and high voltages are used to reduce the current through the transmission lines. A low resistance and a low current mean that the transmission wires will not heat up as much. As a result, most of the power is delivered to the consumer, and not lost through the wires.
2) Electricity is generated in power stations and transported across the UK via the National Grid. To move power around the National Grid:
Before electrical power leaves a power station – it is transferred at high voltages by using ‘step-up’ transformers to increase the voltage from 25,000 V (25kV) to around 400,000 V (400kV). As high currents waste more energy than low currents, electrical power is transported around the grid at a high voltage and a low current.
Before electrical power enters homes and factories – the voltages are decreased by ‘step-down’ transformers to 230 V. These are used when voltages need to be lowered for use in homes and factories. A step-down transformer has fewer turns of wire on the secondary coil than on the primary coil.
SP13c
1) Recall the law of conservation of energy.
2) How is electrical power calculated?
3) Recall and use equations relating current, voltage, power and resistance.
1) Energy can be transferred usefully, stored or dissipated, but cannot be created or destroyed.
2) power (W) = energy transferred (J) / time taken (s)
3) Electrical power (W) = Current (A) x potential difference (V)
Electrical power (W) = current squared (A squared) x resistance (ohms)
SP13c
Use equations to explain the advantages of power transmission in high-voltage cables.
Power is the rate of doing work or transferring energy.
Power (W) = energy transferred (J) / time taken (s)
Electrical power (W) = Current (A) x potential difference (V)
Therefore, for a large output power you need either a large current or a large potential difference.
A high current causes energy to be wasted as it heats the cables. The power lost to resistive heating can be found using the equation:
Electrical power (W) = current squared (A squared) x resistance (Ω)
Using a step-up transformer increases the potential difference in the output electricity. As transformers are almost 100% efficient:
Potential difference across primary coil (V) x current in primary coil (A) = potential difference across secondary coil (V) x current in secondary coil (A)
So increasing the output potential difference reduces the power lost, which makes the national grid more efficient.
