AQA GCSE Physics 4.7.3 Induced Potential, Transformers and the National Grid Exam Questions

About AQA GCSE Physics 4.7.3 Induced Potential, Transformers and the National Grid

This is a physics only and Higher tier only section of AQA GCSE Physics, examined on Paper 2. It covers the generator effect: a potential difference is induced across a conductor when it moves relative to a magnetic field or when the field changes. You need to explain how alternators, dynamos and moving-coil microphones work, and how transformers change the size of an alternating p.d., using the transformer equations. The national grid application links back to 4.2.4. Examiners test explanations with key words such as 'cuts field lines' and transformer calculations.

What you need to know

• Generator effect If an electrical conductor moves relative to a magnetic field, or there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, a current is induced.
• Direction and size An induced current generates a magnetic field that opposes the original change. The size of the induced p.d. increases with faster movement, a stronger field and more turns on the coil.
• Alternators and dynamos The generator effect is used in an alternator to generate ac (using slip rings) and in a dynamo to generate dc (using a split-ring commutator). Draw and interpret graphs of p.d. against time for each.
• Microphones Microphones use the generator effect to convert the pressure variations in sound waves into variations in current: sound makes a diaphragm and coil move in a magnetic field, inducing a p.d.
• Transformers A basic transformer has a primary coil and a secondary coil wound on an iron core. Iron is used because it is easily magnetised. An alternating current in the primary produces a changing magnetic field in the core, which induces an alternating p.d. in the secondary coil.
• Transformer equations V_p / V_s = n_p / n_s. Step-up transformers have more turns on the secondary (n_s greater than n_p); step-down have fewer. For a 100% efficient transformer, V_s x I_s = V_p x I_p. Both are on the equation sheet.
• National grid use Transformers allow the national grid to transmit at high p.d. and low current, reducing energy loss by heating, then step the p.d. down for safe use in homes.

Common mistakes in 4.7.3 Induced Potential, Transformers and the National Grid

• Saying a transformer works with direct current. It needs an alternating current to produce a changing magnetic field in the core.
• Saying current flows through the iron core from one coil to the other. The coils are linked by the changing magnetic field, not by current through the core.
• Mixing up dynamo and alternator outputs. An alternator gives ac; a dynamo gives dc.
• Flipping the turns ratio, for example using n_s / n_p with V_p / V_s.
• Saying a stronger magnet is the only way to increase induced p.d. Speed of movement and number of turns also matter.

Try three 4.7.3 Induced Potential, Transformers and the National Grid exam-style questions