CBSE Class 10 Physics Work Energy and Power MCQs Set 21

Answer: (a) joule

Explanation:
1. Work = force × distance, so its unit is newton metre, which is called the joule.
2. The erg is the CGS unit, the newton is the unit of force and the watt is the unit of power.

Teacher's Note:
1. 1 joule = 10^7 erg.
2. 1 joule is the work done when a force of 1 N moves its point of application 1 m along the force.

Answer: (b) 2.94 J

Explanation:
1. P.E. = mgh.
2. Height h = 20 cm = 0.2 m.
3. P.E. = 1.5 × 9.8 × 0.2 = 2.94 J.

Teacher's Note:
1. Convert the height to metres before substituting.
2. Using g = 10 m/s² would give 3 J, which is only approximate.

Answer: (b) watt

Explanation:
1. Power = work / time, so the unit is joule per second, which is the watt.
2. Horse power is a British unit and erg per second is a CGS unit.

Teacher's Note:
1. 1 horse power = 746 watts.
2. Watt per second would be a unit of rate of change of power, not of power.

Answer: (c) Mechanical energy can be fully converted into heat energy.

Explanation:
1. Joule's experiment showed that 100 per cent of mechanical work can appear as heat.
2. In an incandescent lamp much of the electrical energy appears as heat.
3. Heat engines can turn only part of the heat into work.

Teacher's Note:
1. Heat to work conversion is limited because some heat must be rejected to a colder body.
2. Work to heat conversion has no such limit.

Answer: (a) becomes 4 times

Explanation:
1. K.E. = ½ m v².
2. With v replaced by 2v, K.E. = ½ m (2v)² = 4 × (½ m v²).
3. So the kinetic energy becomes four times.

Teacher's Note:
1. Kinetic energy depends on the square of the speed.
2. Tripling the speed would make K.E. nine times.

Answer: (d) both a heat engine and an internal combustion engine

Explanation:
1. A heat engine converts heat into mechanical work.
2. In a motor car the fuel burns inside the cylinder, so it is an internal combustion engine, which is a kind of heat engine.

Teacher's Note:
1. A steam engine is a heat engine in which the fuel burns outside the cylinder, so it is an external combustion engine.
2. Petrol and diesel engines are internal combustion engines.

Answer: (b) the same units

Explanation:
1. Work done on a body is stored as energy or transferred as energy.
2. Both are measured in joule in the SI system and in erg in the CGS system.

Teacher's Note:
1. Power, not energy, is measured in watts.
2. Kilowatt hour is also a unit of energy.

Answer: (a) Energy can neither be created nor destroyed, only changed from one form to another.

Explanation:
1. This is the law of conservation of energy.
2. Friction changes mechanical energy into heat, so the energy is not lost.
3. Power stations convert one form of energy into another and create none.

Teacher's Note:
1. The total energy of an isolated system stays constant.
2. Chemical, electrical, heat and light energy are all convertible forms.

Answer: (c) the work done per second

Explanation:
1. Power is the rate of doing work, P = W / t.
2. So it depends on how much work is done in each second.

Teacher's Note:
1. Two persons doing the same work have different powers if they take different times.
2. 1 watt is 1 joule of work per second.

Answer: (b) It converts part of the heat taken from a hot source into mechanical work.

Explanation:
1. A heat engine draws heat from a source at higher temperature and converts part of it into work.
2. The rest is rejected at a lower temperature, so the conversion is never 100 per cent.

Teacher's Note:
1. Efficiency of petrol and steam engines is well below 40 per cent.
2. A refrigerator is a heat engine running in reverse.

Answer: (c) intake, compression, power, exhaust

Explanation:
1. The mixture of air and fuel first enters the cylinder (intake).
2. It is then compressed (compression), ignited at the spark plug so the piston is pushed down (power), and finally the burnt gases leave (exhaust).

Teacher's Note:
1. Power is produced only in the third stroke.
2. A flywheel keeps the engine turning through the other three strokes.

Answer: (d) the mechanical equivalent of heat

Explanation:
1. Joule's experiment found that W / H is a constant, 4.2 J/cal (more exactly 4.18).
2. This constant is the mechanical equivalent of heat, denoted by J.

Teacher's Note:
1. 1 kilocalorie = 4200 J.
2. The specific heat of water is 4200 J/kg K in SI units, which has the same number but a different meaning.

Answer: (c) motion; position or shape

Explanation:
1. A moving body can do work because of its motion, so it has kinetic energy.
2. A raised body or a stretched spring can do work because of position or shape, so it has potential energy.

Teacher's Note:
1. A stone at the top of a tower has only P.E., while just before hitting the ground it has only K.E.
2. The sum stays constant.

Answer: (b) capacity for doing work

Explanation:
1. A body or system has energy if it can do work.
2. Rate of doing work is power and force times distance is work.

Teacher's Note:
1. Energy is a scalar quantity.
2. Work is a way of transferring energy.

Answer: (b) watts

Explanation:
1. Power = work / time, measured in joule per second.
2. One joule per second is called one watt.

Teacher's Note:
1. 1 kW = 1000 W.
2. A person walking uphill develops power of about a few hundred watts.

Answer: (c) internal combustion engine

Explanation:
1. In petrol and diesel engines the fuel is burnt inside the cylinder itself.
2. Hence they are internal combustion engines, whereas a steam engine is an external combustion engine.

Teacher's Note:
1. The hot gases formed by burning expand and push the piston.
2. Only part of the heat becomes useful work.

Answer: (c) heat

Explanation:
1. A heat engine takes in heat and gives out mechanical work.
2. The other forms of energy are not the input of a heat engine.

Teacher's Note:
1. The reverse conversion (mechanical to heat) occurs by friction.
2. In an electric motor the input is electrical energy.