P6 Electromagnetism and Energy Resources
Magnetic effect of current
- Magnetic pattern around straight current carrying wire
- Magnetic pattern around solenoid
- Use right hand rule to figure out (thumb’s up)
- Relay switches use the effect to reduce danger as a circuit can turn on another circuit.
- Increasing current magnitude results in stronger magnetic field proportionally.
- Reversing current direction results in:
- D.C. North south pole reversed
- A.C. constantly changing direction of flow.
Motor effect
- A current-carrying wire experiences a force in the presense of a magnetic field.
- A force will be creted and the wire’s movement will confirm the effect.
- Practical applications can be loudspeakers and electric motors.
- Left hand rule for movement, field, current
- Reversing current reverses direction of force.
- Reversing field direction reverses direction of force.
- Force size is the greatest when wire is perpendicular to field.
- No force if wire is parallel to field.
D.C. motor
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- Converts electrical energy to kinetic energy.
- There is no current in coil when it’s 90 degree vertical, but it keeps moving due to kinetic energy.
- A current carrying coil experiences a turning effect in a magnetic field.
- The right side always causes a downward force.
- Commutators also rotate as it reverses current every 180 degree so the movement continues.
- Brushes are made of carbon and they conduct the current to complete the circuit.
- Increase # of turns in coil results in faster movement as there is a stronger magnetic field.
- Increase current results in faster movement as there is a stronger interaction with the field.
Inducing electromotive force
- Set up magnets and a wire with ammeter, move wire up and down and there will be readings on the ammeter.
- EMF is induced when there is charge in the magnetic field and it’s proportional to charge per second.
- Wire cuts field lines results in small voltage to be induced.
- Moving the wire faster, using stronger magnet, having more coils all increase the EMF.
A.C. Generator
- Transforms kinetic energy to electrical energy.
- Slip rings conduct electricity, and the induced current reverses every half turn.
Transformers
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- A basic soft iron cored transformer is used to transform voltages.
- Step-up transformers increase voltage from primary to secondary.
- Step-down transformers decrease.
- Calculation of voltage transformed: Vp/Vs = Np/Ns where N is number of coils.
- Power in equals power out for 100 efficiency: V1I1 = V2I2
- Calculating efficiency: energy input/useful energy output * 100%
- Efficiency is the ratio of the useful work performed by a machine or in a process to the total energy expended or heat taken in
- Uses A.C. due to constant change of voltage.
- A.C. current changes, and magnetic field changes to induce a voltage in secondary coil.
- Electricity is transferred through long distances over wires. When current flows, wire is warm and this wastes energy. (P=I2R)
- When voltage is high, current is kept small, and there is less heat loss.
- Transformers bridge the gaps between electrical cables and home use, reduces danger.
Energy
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- Sun is the source of energy for all our energy resources except geothermal and nuclear.
- Energy is released by nuclear fusion in the sun.
- Renewable energy is naturally replenished on a human timescale.
- sunlight, wind, rain, tides, waves, and geothermal heat,
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- Non-renewable energy comes from sources that will run out or will not be replenished in our lifetimes.
- Nuclear, coal, fossil fuels
- Non-renewable energy comes from sources that will run out or will not be replenished in our lifetimes.
- Electricity can be obtained from…
| Source | Advantages | Disadvantage |
| Chemical in fuel |
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| Water (Waves, tides, dams) |
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| Geothermal |
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| Nuclear fission |
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| Heat and light from sun |
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IGCSE REVISION 
