One mark questions with answers

Q1. What is the use of steering device in boats?

Ans1. The boat can be made to move in any direction irrespective of the direction of wind blowing at that time.

Q2. Give one importance of Joule's experiment.

Ans2. Joule's experiment proved that there is an exact equivalence between work done and heat produced.

Q3. How many Joule's of mechanical energy must be spent to increase the temperature of 200gms of water by 3oC.

Ans3. Heat energy required (Q) = mass x specific heat of water x temperature change
Q = 0.2 kg x 4200 J/Kg/oC x 3 = 2520 J

Q4. Which stroke of petrol engine provides energy to the engine?

Ans4. The power stroke is the only stroke which provides power to the engine. All other strokes of the engine work by the power derived from this stroke.

Q5. Write two differences between a steam engine and a petrol engine.

Ans5. Petrol engine causes more serious pollution than that produced by a steam engine.
Petrol engine has higher efficiency than that of steam engine.
Efficiency is 40% for petrol engine and 20% for steam engine.

Q6. How much energy will an electric motor of 3.5 horse-power consume in 5 seconds?

Ans6. Energy = 3.5 horse-power x 5 seconds
Energy = 3.5 x 746 J/sec x 5 sec.
= 13055 Joules

Q7. If a coolie carries a load of 40 kg from one platform to the other, how much work does he do against gravity?

Ans7. When the coolie carries the load from one platform to the other, no distance is covered along the vertical direction so, he does no work against gravity.

Q8. Give an example where potential energy is acquired by a body due to change in its shape.

Ans8. Winding up of a clock (energy stored in the spring of the clock).


Two mark questions with answers

Q1. A person weighing 70 kg carries a weight of 40 kg on his head along an inclined plane of height 5m in 5 minutes. Calculate his power.(g = 10 m/s2)

Ans1. work done = mgh, where 'm' is the total mass of the person and the weight. So, m = 70 + 40 = 110 kg, h = 5m.
W.D = mgh = 110 x 10 x 5 Joules = 5500 J
Power = mgh/t = 5500/(5 x 60) = 18.33 W

Q2. A pump is used to lift water from a well to a tank to a height of 10m. Write the transformation of energy from one form to another in the entire process.

Ans2. Chemical energy of the fuel in the pump is converted into mechanical energy of the pump which inturn is used to lift water from well to the tank. Now the mechanical energy is converted into potential energy of water.

Q3. Why does a Saw become warm when it is used to cut a log of wood? State the energy transformation taking place.

Ans3. There is friction between the moving Saw and the wood. So, some work is done to move the Saw through the wood. A part of this work gets converted into heat
Mechanical energy ¾® Heat energy.

Q4. Which sources of energy and simple machines were used by the primitive man?

Ans4. Sources of kinetic energy : (i) Muscular energy.
(ii) Kinetic energy of wind and flowing water
(iii) Energy of the domestic animals.
Simple machine : lever, pulley.

Q5. What is an important limitation of the single sail boat and how is it overcome?

Ans5. A single sail boat can go only in the direction of the wind.To overcome this problem two sails are used in the same boat. These sails are controlled by steering device called Rudder.
These sails are held at some angle with each other. One of them diverts the direction of the wind while the other propels the boat in the direction as required.

Q6. 'Hydroelectricity generated at a dam may be considered to be another form of solar energy.' Explain.

Ans6. Hydroelectricity is generated by using the potential energy of the water stored behind the dams. Water comes from the melting of snow on the mountains or from the rain water. The rain and the snowfall occur due to the condensation of water vapours in the air. These vapours are formed when the water present in seas, oceans and lakes gets evaporated by the sunlight. Thus, the energy of the water stored in a dam infact comes from the sun.

Three mark questions with answers

Q1. With the help of a labelled diagram describe in brief the working of a wind-mill.

Ans1. The blowing wind rotates the blades of the windmill. This rotatory motion can be used,
(i) for doing mechanical work e.g., grinding, pumping of water etc.
(ii) for generating electricity by connecting a dynamo to it.

Q2. Describe the principle of a steam engine.

Ans2. Principle: The heat obtained from burning of the coal is used to boil water and the steam produced is compressed. This compressed steam is introduced into a metal cylinder fitted with a movable piston. A large amount of force is exerted on the piston by the expanding hot steam. The piston is pushed outward. The moving piston drives the wheels. Now the piston falls back into the cylinder. The fresh steam is now brought into the cylinder, which is used to push the piston again. The same process is repeated and the wheels are driven constantly. This is how engine starts working.

Q3. Why is hydro-energy considered an indirect source of solar energy? Explain how this can be converted into electrical energy. State two advantages of it.

Ans3. Hydro-energy is produced by using the kinetic energy of the flowing water. Water recycles in nature from oceans/earth surface through rainfall and snowfall under the influence of the sun's energy. Thus, the kinetic energy of the flowing water and therefore the hydro-energy is an indirect source of solar energy.
Production of hydro-energy : The flowing water, or the water stored behind the dams is made to fall on the blades of a turbine. This rotates the armature of the generator to produce electricity.
Advantages of hydro-energy :
(i) It is renewable sources of energy.
(ii) It is non-polluting.
About 23% of our country's total energy need is met by hydro-energy.

Q4. In what respects an internal combustion engine is better than an external combustion engine? Write any three reasons in support of your answer.

Ans4. (i) In internal combustion engine fuel is burnt inside the cylinder of the engine but in external combustion engine it is burnt in the fire box of the boiler and it is outside the cylinder.
(ii) Internal combustion engine can be started at once while the other takes long time to start.
(iii) Efficiency of the internal combustion engine is 40% (petrol engine and even more for diesel) but that of the external combustion engine is 20%.

Q5. Write any three limitations of an external combustion engine compared to an internal combustion engine.

Ans5. (i) External combustion engines are bulky.
(ii) External combustion engines have low thermal efficiency( = 20%).
(iii) External combustion engines take a long time before becoming operational.

Q6. State any three advantages of an internal combustion engine has over a wind-mill.

Ans6. (i) The internal combustion engine can be made to work any time of the day.
(ii) The internal combustion engine is more powerful than a windmill.
(iii) The internal combustion engine is smaller than a windmill and it can do work anywhere. The windmill can work only where it is installed.


Five mark questions with answers

Q1. What is meant by hydroelectric power? How is it produced? Give two advantages and two disadvantages of constructing dams for producing hydroelectric power.

Ans1. The electric power produced by using the K.E. of flowing water is called hydroelectric power.
Water flowing in high altitude rivers is stored behind high dams. This water has a lot of potential energy in it. Water is made to flow from the dam through a pipe. This flowing water rotates the water wheel. A shaft is connected to the water wheel. With the help of this shaft, a coil of electric generator is rotated and electricity is produced. In the production process potential energy of water in converted into K.E. of flowing water. This K.E. is converted into electrical energy through some mechanical process as described above. The diagram below shows the entire process of production of electricity.

Advantages:
(i) It does not produce any environmental pollution.
(ii) Water energy as a source of electric energy will never be exhauted.

Disadvantages:
(i) Plants and trees are destroyed, animals and many people face many problems for their shelter.
(ii) Construction of dam on a river disturbs the ecological balance in the downstream area of the river.

Q2. Calculate the total number of units of your family needs in the month of March, if following items are used per day. If the cost of electricity per unit is Rs. 2.50, find the total cost of electricity consumed.
(a) 4 light bulbs of 200 W each for 12 hours.
(b) 10 tube lights of 40 W each for 10 hours.
(c) Fridge of 350 W for 24 hours.
(d) Electric mixer of 750 W for 2 hours.
(e) One TV of 100 W for 8 hours.

Ans2. There are 31 days in the month of March.
(a) Energy consumed by 4 bulbs in 31 days
= 4 x 200 W x 12 hrs x 31 = 297600 W hrs = 297.6 KW hrs.

(b) Energy consumed by 10 tube lights in 31 days
= 10 x 40 W x 10 hrs x 31 = 124000 W hrs = 124 KW hrs.

(c) Energy consumed by fridge in 31 days
= 350 W x 24 hrs x 31 = 260400 W hrs = 260.4 KW hrs.

(d) Energy consumed by Electric mixer in 31 days
= 750 W x 2 hrs x 31 = 46500 W hrs = 46.5 KW hrs

(e) Energy consumed by TV in 31 days
= 100 W x 8 hrs x 31 = 24800 W hrs = 24.8 KW hrs.
The total energy consumed = 753.3 KW hrs = 753.3 Units
The total cost of electricity = Rs. 753.3 x 2.50 = Rs. 1883.25


Q3. What is meant by hydroelectric power? How is it produced? Give two advantages and two disadvantages of constructing dams for producing hydroelectric power.

Ans. (Try yourself).

Q4. With the help of a labelled diagram describe in brief an experiment to show that 4186 J of work is required to raise the temperature of 1 kg of water through 1 oC.

Ans. (Try yourself).

Q5. Calculate the total number of units of your family needs in the month of March, if following items are used per day. If the cost of electricity per unit is Rs. 2.50, find the total cost of electricity consumed.
(a) 4 light bulbs of 200 W each for 12 hours.
(b) 10 tube lights of 40 W each for 10 hours.
(c) Fridge of 350 W for 24 hours.
(d) Electric mixer of 750 W for 2 hours.
(e) One TV of 100 W for 8 hours.

Ans. (Try yourself).

Q6. What do you mean by a Solar cell? From which material they are made? How are they made? Write their uses.

Ans. (Try yourself).