Electric Generator: AC and DC - UNSOLVED PRACTICE SET

Q7. State the principle of an electric generator. Name the scientist whose discovery forms the basis of this principle.

Q8. List the main parts of an AC generator:
(a) armature coil,
(b) field magnets,
(c) slip rings,
(d) brushes. State the function of each.

Q9. What is the key structural difference between an AC generator and a DC generator? How does this difference affect the output current?

Q10. Why does the induced EMF in a rotating coil generator vary continuously (rather than being constant) as the coil rotates?

Q11. Explain why, in a DC generator, the output current — though unidirectional — is not perfectly steady (it has 'ripples'). How can this be smoothed out?

Q12. Both an electric motor and an electric generator have similar construction (coil, magnets, commutator/slip rings). What is the fundamental difference in how energy flows in each device?

Q13. Describe the construction and working of an AC generator in detail. Your answer must cover:
(a) the main parts (armature coil, field magnets, slip rings, brushes),
(b) how rotating the coil in the magnetic field induces an EMF (using Faraday's law and Fleming's Right Hand Rule conceptually),
(c) why the induced EMF and current alternate in direction as the coil rotates through one full revolution,
(d) the role of slip rings in transferring this alternating current to the external circuit without rectification, and
(e) why the resulting output is called 'alternating current'.

Q14. Compare AC and DC generators in detail.
(a) Describe the key structural difference (slip rings vs split-ring commutator).
(b) Explain how this structural difference leads to AC output in one case and DC (pulsating) output in the other.
(c) Sketch (or describe) the shape of the output (EMF vs time) for each type.
(d) Why is AC preferred for long-distance electrical power transmission in India (e.g., from a hydroelectric dam to a city)?
(e) Where might DC generators still be useful despite this disadvantage?

Q15. India's electricity grid operates on AC at 50 Hz, 220-230 V (domestic).
(a) Explain, using the generator principle, how a hydroelectric dam (like Bhakra Nangal) converts the kinetic energy of falling water into electrical energy.
(b) What role does the rotation speed of the turbine/generator play in determining the frequency of the AC produced?
(c) Why must power stations maintain a precise rotation speed to keep the grid frequency at exactly 50 Hz?
(d) If a generator's rotation speed were to suddenly slow down, what would happen to the frequency of the electricity supplied?
(e) Briefly explain the chain of energy conversions from water in a dam to the light in your home: water's kinetic energy → ? → ? → light energy.

Q16. A simple AC generator's coil rotates at 50 revolutions per second, producing AC at 50 Hz (matching the Indian grid frequency).
(a) How many complete cycles of alternation (positive to negative and back to positive) occur per second?
(b) How many times per second does the current change direction (from positive to negative, or negative to positive)?
(c) If the coil's rotation speed is doubled to 100 revolutions per second, what would the new frequency be?
(d) Why must power plants carefully regulate rotation speed rather than letting it vary?

Q17. A student builds a model AC generator using a coil, two magnets, and slip rings, connected to an LED. When the coil is rotated by hand, the LED flickers (turns on and off rapidly) rather than staying steadily lit.
(a) Explain why the LED flickers — relate this to the EMF varying (and reversing direction) as the coil rotates.
(b) Does the LED light up when the EMF is in one direction but not the other? Why might this be (consider what an LED does with reversed current)?
(c) If a DC generator (with commutator) were used instead, connected to the same LED, would the flickering pattern be different? Explain.
(d) Suggest one modification (involving additional electronic components, conceptually) that could make the LED glow steadily with an AC generator. (You do not need to name specific components — describe the function needed.)

Q18. A small wind turbine in a village in Tamil Nadu has a generator that produces AC electricity when the blades rotate.
(a) On a windy day, the blades rotate faster — what happens to the frequency and magnitude of the generated EMF?
(b) On a calm day with very little wind, the blades rotate slowly or not at all — what happens to the EMF generated?
(c) Why might the village need a battery storage system in addition to the wind turbine?
(d) If the village wants to use this AC electricity to charge DC batteries, what additional device (conceptually, converting AC to DC) would be needed?
(e) Compare this wind-based generation to a large hydroelectric dam in terms of consistency/reliability of power output.