Which one of the following statements is not correct?
- (a)An electric motor converts electrical energy into mechanical energy.
- (b)An electric generator works on the principle of electromagnetic induction.
- (c)The magnetic field at the centre of a long circular coil carrying current will be parallel straight lines.
- (d)A wire with a green insulation is usually the live wire of an electric supply.
Correct — D, A wire with a green insulation is usually the live wire of an electric supply. Green marks the earth wire, never the live one. In the three-wire domestic supply the colours taught in Indian school science are red for live, black for neutral and green for earth, and the earth wire is the safety conductor — it runs from the third pin of the plug to the metal body of the appliance and on to a plate buried in the ground, so that if a live wire ever touches the casing the fault current flows harmlessly into the earth instead of through whoever is holding the appliance. Calling that conductor 'live' inverts the one association the wiring colour exists to fix, which is why this is the statement that fails.
- (a)An electric motor converts electrical energy into mechanical energy. — A correct statement, so it is not the answer. A motor carries current through a coil in a magnetic field, the coil experiences a force by Fleming's left-hand rule, and it turns — electrical energy in, rotation out.
- (b)An electric generator works on the principle of electromagnetic induction. — Also correct, and therefore not the answer. Moving a conductor in a magnetic field, or changing the flux through a coil, induces a current; that is Faraday's law, and a generator is its direct application.
- (c)The magnetic field at the centre of a long circular coil carrying current will be parallel straight lines. — Correct as school physics states it, so not the answer. Inside a long coil of the kind the option describes the field is uniform and its lines run as parallel straight lines. The phrase 'long circular coil' is the paper's own loose way of naming a solenoid, but the physics it points at is the standard textbook result.
This is a sweep across the magnetic-effects chapter, testing four separate results at once. A motor turns electrical energy into mechanical energy through the force on a current-carrying conductor in a magnetic field. A generator does the reverse through electromagnetic induction. Inside a long current-carrying coil the field lines are parallel and the field is uniform, which is the arrangement used to make electromagnets. And domestic supply uses three wires — live, neutral and earth — distinguished by colour so that anyone working on the circuit can tell them apart without testing.
The way through a mixed 'which is not correct' item is to certify the statements you are sure of and let the remainder stand. Three of these are results that any candidate has met by name; the fourth is a fact about wiring colour, and it is the one stated backwards. It is worth being precise about what green means, because two colour schemes circulate. The scheme printed in Indian school textbooks is red for live, black for neutral and green for earth. Much modern flexible cable instead follows the internationally harmonised set, with brown for live, blue for neutral and green-with-a-yellow-stripe for earth. Green belongs to earth in both, which is exactly the association this option contradicts.
- An electric motor converts electrical energy into mechanical energy; a generator does the reverse using electromagnetic induction.
- The magnetic field inside a long current-carrying coil is uniform, and its field lines are parallel straight lines.
- In the colour scheme used in Indian school science, red is live, black is neutral and green is earth.
- In the internationally harmonised scheme, brown is live, blue is neutral and green-with-yellow is earth — green still means earth.
- The earth wire connects the metal body of an appliance to a plate buried in the ground, so a fault current bypasses the user.
The option under test calls the green wire live. Green means earth in either scheme in use, which is what makes the statement fail.
- Reading 'earth' as a spare or unused wire rather than the safety conductor.
- Mixing the two wiring colour schemes and assuming only one is ever met in practice.
- Reversing motor and generator, so that induction is attached to the motor.
As a mixed statements item covering the whole magnetic-effects chapter, or as a single question on the colour or purpose of one wire in the domestic supply.
The connecting cable of electrical appliances like electric iron, water heater or room heater contains three insulated copper wires of three different colours—red, green and black. Which one of the following is the correct colour code?
- (a) Red—live wire, Green—neutral wire, Black—ground wire
- (b) Red—neutral wire, Green—ground wire, Black—live wire
- (c) Red—live wire, Green—ground wire, Black—neutral wire
- (d) Red—ground wire, Green—live wire, Black—neutral wire
Answer(c) Red—live wire, Green—ground wire, Black—neutral wire
The whole colour code asked as its own question, and the keyed answer settles this one directly — green is the ground wire, so a statement calling it live cannot stand.
The r.m.s. potential difference between the red live wire and black neutral wire in Indian domestic electric supply is
- (a) 160 V
- (b) 220 V
- (c) 300 V
- (d) 410 V
Answer(b) 220 V
The same wiring colours put to work a year later, this time to fix a number. It names red as live and black as neutral in passing, which leaves green for earth — the association the statement in this question gets backwards.
- practice — not a real PYQ
In a domestic three-pin plug, the earth pin is made longer and thicker than the other two so that
- (a)it carries more current than the live pin
- (b)the appliance is earthed before the live contact is made, and disconnected after it is broken
- (c)it can be used as the neutral pin when required
- (d)it reduces the resistance of the circuit
Answer(b) the appliance is earthed before the live contact is made, and disconnected after it is broken — the longer pin enters the socket first and leaves it last, so the casing is never live while unearthed.
- practice — not a real PYQ
Which one of the following statements about an electric generator is correct?
- (a)It converts mechanical energy into electrical energy using electromagnetic induction
- (b)It converts electrical energy into mechanical energy using the force on a current-carrying conductor
- (c)It works only when the magnetic field is constant in time and space
- (d)It requires a source of direct current to operate
Answer(a) It converts mechanical energy into electrical energy using electromagnetic induction — a changing flux through the coil induces the current, which is Faraday's law.