Work and Energy Worksheet-3

Work and Energy Worksheet-3

An electric bulb consumes 7.2 kJ of electrical energy in 2 minutes. What is the power of the electric bulb ?

A. 60 watts B. 3600 watts

C. 600 watts D. 60 kilowatts

A radio set of 60 watts runs for 50 hours. How many 'units' (kWh) of electrical energy are consumed ?

A. 3 B. 30 C. 300 D. 3000

A family uses 250 units of electrical energy during a month. Calculate this electrical energy in joules.

A. 9 × 10⁵ joules B. 9 × 10⁶ joules

C. 9 × 10⁸ joules D. 9 × 10⁹ joules

When an object falls freely towards the earth, then its total energy :

A. increases

B. decreases

C. remains constant

D. first increases and then decreases

Which one of the following is not the unit of energy ?

A. joule B. newton- metre

C. kilowatt D. kilowatt-hour

Which of the following energy change involves frictional force ?

A. chemical energy to heat energy

B. kinetic energy to heat energy

C. potential energy to sound energy

D. chemical energy to kinetic energy

Which one of the following statements about power stations is not true ?

A. hydroelectric power stations use water to drive turbines

B. in a power station, turbines drive generators

C. electricity from thermal power stations differs from that produced in hydroelectric power stations

D. in hydroelectric power stations and thermal power stations, alternators produce electricity

An electric motor raises a load of 0.2 kg, at a constant speed, through a vertical distance of 3.0 m in 2 s. If the acceleration of free fall is 10 m/s², the power in W developed by the motor in raising the load is :

A. 0.3 B. 1.2 C. 3.0 D. 6.0

An object is falling freely from a height x. After it has fallen a height (x/2) it will possess :

A. only potential energy

B. only kinetic energy

C. half potential and half kinetic energy

D. less potential and more kinetic energy

The commercial unit of energy is :

A. watt B. watt-hour

C. kilowatt-hour D. kilowatt

How much energy does a 100 W electric bulb transfer in 1 minute ?

A. 100 J B. 600 J C. 3600 J D. 6000 J

The device which converts mechanical energy into energy which runs our microwave oven is :

A. electric motor B. alternator

C. turbine D. electric heater

A microphone converts :

A. electrical energy into sound energy in ordinary telephone

B. microwave energy into sound energy in a mobile phone

C. sound energy into mechanical energy in a stereo system

D. sound energy into electrical energy in public address system

Answer:

Explanation: Power =

Energy consumed = 7.2 kJ

= 7.2 × 1000 J = 7200 J

Time taken = 2 minutes

= 2 × 60 seconds = 120 s

Power= (7200 J/ 120s)

= 60 J/s

= 60W

Thus, the power of this electric bulb is 60 watts.

Explanation: Power, P = 60 watts

= (60/1000) kilowatts

= 0.06 kW

Time t= 50 h

Now, putting P = 0.06 kW and t = 50 h in the formula :

Power =

0.06 kW =

Energy consumed = 0.06 kW × 50 h = 3kWh

Thus, the electrical energy consumed is 3 kWh or 3 units.

Explanation: 250 units of electrical energy means 250 kWh of electrical energy.

1 kWh of energy = 3.6 × 10⁶ J

250 kWh of energy = 3.6 × 10⁶ × 250 J

= 9 × 10⁸ J

Thus 250 units of electrical energy is equal to 9× 10⁸ joules.

Explanation: Energy can never be created nor destroyed hence the total energy will remain constant.