OxfordAQA International GCSE Physics 3.6 Generating and Distributing Electricity and Household Use Exam Questions

About OxfordAQA International GCSE Physics 3.6 Generating and Distributing Electricity and Household Use

This section of OxfordAQA International GCSE Physics follows electricity from the power station to the home. The separate Physics course covers the generator effect, alternators and dynamos, the National Grid style transmission system, step-up and step-down transformers, the turns ratio equation and switch-mode transformers. All students study ac and dc, mains safety with earthing, fuses and circuit breakers, the motor effect and Fleming's left-hand rule, and electrical power and energy, including kilowatt-hours and electricity costs. It builds on electromagnetism in 3.5. Examiners commonly set transformer, fuse rating and cost calculations, and ask why electricity is transmitted at high voltage.

What you need to know

• Generator effect (separate Physics) A pd is induced when a conductor moves relative to a magnetic field or is in a changing field. The induced pd increases with faster movement, a stronger field, more turns and a larger coil area. Reversing motion or polarity reverses it. An alternator produces ac and a dynamo produces dc; power station turbines turn coils in magnetic fields.
• Transmission (separate Physics) Step-up transformers increase the voltage at power stations so that, for the same power, the current is lower and less energy is lost by heating cables. Step-down transformers lower the voltage for safe use in homes.
• Transformers (separate Physics) Primary and secondary coils on a soft iron core; alternating current in the primary creates a changing field that induces an alternating pd in the secondary. Vp / Vs = np / ns and, for 100% efficiency, Vp x Ip = Vs x Is. Switch-mode transformers work at high frequency, are light and small and are used in phone chargers.
• ac and dc Cells and batteries give direct current in one direction. Alternating current repeatedly reverses; find its period and frequency from a trace (f = 1 / period) and compare peak ac pd with dc pd.
• Mains safety Earthing carries current safely away from a faulty metal case. A fuse or circuit breaker in the live wire cuts the circuit when the current is too high; a circuit breaker acts faster and can be reset. Double insulated appliances do not need an earth wire.
• Motor effect A current-carrying conductor at an angle to a magnetic field experiences a force, which is bigger with a stronger field, larger current or longer length in the field. Use Fleming's left-hand rule for direction. A coil in a field rotates, which is the basis of a motor.
• Power and energy P = E / t, P = I x V and E = V x Q. Choose a fuse rated just above the normal current. Energy (kWh) = power (kW) x time (h); cost = kWh x price per kWh.

Common mistakes in 3.6 Generating and Distributing Electricity and Household Use

• Saying transformers work with direct current. They need alternating current to produce a changing magnetic field in the core.
• Saying high voltage transmission is used because it is faster. It reduces the current, which reduces heating losses in the cables.
• Choosing a fuse rating below the normal current. Choose the nearest standard rating just above the normal working current.
• Using watts and seconds in kWh calculations. Convert to kilowatts and hours.
• Confusing Fleming's left-hand rule with the right hand. Use the left hand: thumb for force, first finger for field, second finger for current.

Try three 3.6 Generating and Distributing Electricity and Household Use exam-style questions