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Chapter 4 Work And Energy Physics 11th Class Punjab Short Questions

Question No. 1 - What sort of energy is in the following? 1. A Moving Car 2. A Compressed Spring

Answer No. 1 - A Moving car has kinetics energy.
A Compressed Spring has elastic potential energy.

Question No. 2 - What are renewable & non-renewable energy sources? Give example of each.

Answer No. 2 - Renewable energy is derived from natural processes that are replenished constantly. Such as wind tides etc.
Non-renewable energy source is a source that does not renew itself. Such as coal, natural gas etc.

Question No. 3 - How electrical energy can be obtained from sunlight by indirect conversion method?

Answer No. 3 - By using semiconductor devices, the solar cell also called photo voltaic cell: sunlight can be directly converted into electricity. These solar cells are made of silicon wafers. Electron in the silicon gain energy from sunlight to create voltage.Voltage can be increased by increasing the number of solar cells.

Question No. 4 - How many most common methods can be used for the conversion of biomass into fuel? Write their names.

Answer No. 4 - There are two methods for conversion of biomass into fuel:
1. Direct Combustion Method
2. Fermentation Method

Question No. 5 - Define conservative and non-conservative fields?

Answer No. 5 - Conservative field: The field in which work done along a closed path is zero is called conservative field. For Example, Earth's gravitational field.

Non-Conservative fields:The field in which work done along a closed path is not zero is called non-conservative field. For Example, field of frictional force.

Question No. 6 - What sort of energy is in the following? (A) Compressed spring (b) Water in high dam

Answer No. 6 - a) A compressed spring has Elastic Potential Energy stored in it.
B) Water in a high dam has Gravitational potential Energy in it.

Question No. 7 - Show that work done in a gravitational Field is independent of the path?

Answer No. 7 -

Question No. 8 - What is geothermal energy? How is it generated ?

Answer No. 8 - The heat energy extracted from inside the Earth in the form of steam or hot water is called geothermal energy.
It can be generated by the following processes:
1. Radioactive Decay
2. Residual Heat of Earth
3. Compression of Material

Question No. 9 - A girl drops a cup from a certain height which breaks into pieces. What energy changes are involved?

Answer No. 9 - When the cup was in the hands of girl, it had gravitational P.E when the cup is dropped, its P.E. Is converted into the K,E On striking the ground , this energy is converted into sound energy, heat, work done in breaking the cup into pieces and kinetic energy of pieces.

Question No. 10 - What is a machine? Discuss the input and output and efficiency of a mechanical machine?

Answer No. 10 - Machine: A machine is a thing or device with the help of which we can do more work than we can do by hands.
Input of Mechanical Machine: The amount of work dine on a machine by a given effort force is known as input of machine.
Input = effort *effort arm
Input = P*d
Output of Mechanical Machine: The amount of work done by a machine on the load (weight) is known as output of the machine
output = load * load arm
output = w*d
Efficiency: The ratio of output and input of a machine is known as efficiency
Efficiency: Output / input
n = w*d/ p*d

Question No. 11 - How sunlight is directly converted into electricity by solar cells?

Answer No. 11 - By using semiconductor devices,the solar cell also called photo voltaic cell, sunlight can be directly converted into electricity. These solar cells are made of silicon wafers. Electron in the silicon gain energy from sunlight to create a voltage. Voltage can be increased by increasing the number of solar cells.

Question No. 12 - What is conservation field ? Give example. (OR) Loop?

Answer No. 12 - Conservation Field: A field is said to be conservation field if the work down in it along a closed path is equal to zero.
Example:
1. Gravitational field
2. Electrostatic Field
3. Magnetic field
Loop: A closed path is that path in which a body reaches to its starting point again after passing through several points.

Question No. 13 - Define conservative field. Give its example.

Answer No. 13 - Conservative Field: The field in which work done along a closed path is zero is called conservative field. For example, Earth's Gravitational field

Question No. 14 - What do you mean by variable force? Give its two examples?

Answer No. 14 - The force which is not constant but varies in magnitude and direction or in both is called a variable force.
1. Force a gravity acting on a rocket moving away from earth.
2. Force exerted by the spring increases by the amount of stretch.

Question No. 15 - Name at least two renewable and two non-renewable energy sources.

Answer No. 15 - The renewable source of energy are:
1. Wind
2. Tides
3. Biomass
4. Sunlight

The non-renewable sources of energy are
1. Coal
2. Natural gas
3. Oil
4. Uranium

Question No. 16 - What is solar constant ? What is its value?

Answer No. 16 - Solar energy at incidence outside the Earth's atmosphere is about 1.4kw m-2 which is referred as solar constant.

Question No. 17 - How can you get energy from the waves?

Answer No. 17 - The tides and winds blow across the surface of ocean due to which strong water waves are produced. Professor Salter invented a devices called Salter's duck.It has two parts: duck float and balance float. The wave energy produces the movement in duck float relative to balance float. The relative motion of duck float can be used to run electricity generators.

Question No. 18 - Define energy ? write the units of energy?

Answer No. 18 - Energy: The capacity of a body to do work is known as energy.
Unit of Energy: Joule, Kilo, Mega joule, kilo watt hour are the units of energy.

Question No. 19 - How can we gain energy from tides?

Answer No. 19 - The tides raise the water in the sea roughly twice a day. If the water at the high tides is trapped in a basin by constructing a dam, then it is possible to use this as a source of energy. There dam is filled at high tide and water is released in a controlled way at low tide to drive the turbines and generate electricity.

Question No. 20 - Define kilowatt hour (kWh).

Answer No. 20 - Kilowatt hour is the work done in one hour by an agency whose power is one kilowatt.
1 kWh = 3.6x10^6

Question No. 21 - An object has one joule of potential energy. Explain what it means.

Answer No. 21 - It means that work has been done on the body by the force of 1 N which has lifted the body through a distance of 1 m. This work has been stored in the body in the form of P.E. Which is 1J.

Question No. 22 - Discuss the work done by a variable force?

Answer No. 22 - Consider a body of mass 'm' allowed to move from point 'A' to 'B' under the action of variable force.

Question No. 23 - A boy uses a catapult to throw a stone which accidentally smashes a greenhouse window.What energy changes are involved?

Answer No. 23 - Initially, the catapult had elastic P.E when the stone is thrown, its P.E is converted into K.E On striking the window, this energy is converted into sound energy, heat energy work done in breaking the window into pieces and kinetic energy of pieces.

Question No. 24 - What is Salter's duck?

Answer No. 24 - Salter's duck is a device which can be used to utilize the water waves energy and to generate electricity. It consists of two parts:
1. Duck float
2. Balance float
The wave energy makes duck float move relative to the balance float. The relative motion of the duck float is then used to run electricity generators.

Question No. 25 - What are the essential conditions for conservative field?

Answer No. 25 - For a field to be conservative, energy should be conserved and work should be independent of the path followed.As in case of work done along a path in the gravitational field of earth.

Question No. 26 - How energy can be obtained from waste products?

Answer No. 26 - Water products like wood waste, crop residue, and municipal solid waste is burnt in a confined container. Heat produced in this way is directly utilized in the boiler to produce steam that can run turbine generator.

Question No. 27 - Differentiate between geyser and aquifer.

Answer No. 27 - Geyser is a hot spring that discharges steam and hot water, intermittently releasing an explosive column into the air.

Aquifer is a layer of rock holding water that allows water to percolate through it with pressure.

Question No. 28 - Discuss the work done in gravitational Field?

Answer No. 28 -

Question No. 29 - Differentiate between conservative and non-conservative force.

Answer No. 29 - Conservative Force: The force by which work done along a closed path is zero is called conservative force. For Example, gravitational force, electric force and magnetic force.

Non-Conservative Force: The force by which work done along a closed path is not zero is called non-conservative force. For example,frictional force, air resistance and normal force.

Question No. 30 - Write down two sources of energy which are renewable.

Answer No. 30 - The renewable sources of energy are:
1. Hydroelectric Energy
2. Wind Energy
3. Tides Energy
4. Biomass Energy
5. Sunlight Energy

Question No. 31 - Define and explain work?

Answer No. 31 - Work: The scalar product of force and displacement is known as work.
Explanation: Consider a force 'F' is acting on a body of mass 'm' and moves it form point 'A' to point 'B' through a displacement 'd'
Work = Force * displacement
W = F*d cosO

Question No. 32 - A person holds a bag of groceries while standing still. A car is stationary with its engine running.How are the two situations similar from the point of view of work?

Answer No. 32 - In both the above two cases, since there is no displacement, therefore the work done will be zero. Hence in this respect, the two situation are similar.

Question No. 33 - Discuss the work-energy principal?

Answer No. 33 - Consider a body of mass 'm' placed on the ground. Now if some force 'F' is applied on the body, then the body will start motion with initial velocity V1 in the direction of applied force. If we neglect the force of friction then the body will continue its motion.

Question No. 34 - What are non-conventional energy sources? Describe briefly.

Answer No. 34 - The energy sources which are not very common theses days are called non- conventional energy sources. Some of the non-conventional energy sources are :
Energy from tides
Energy from waves
Energy from biomass
Energy from waste products
Solar Energy
Geothermal Energy

Question No. 35 - Define power and instantaneous power?

Answer No. 35 - Power: The power of doing work is known as power. It is represented by 'P' and is given by
Power = work/time
P = w/t
Instantaneous Power: The rate of doing work in the limit when 't' approaches to zero is known as instantaneous power.

Question No. 36 - What is "Aquifer"?

Answer No. 36 - Aquifer is a layer of rock holding water that allows water to percolate through it with pressure.

Question No. 37 - Show that 1 kWh = 3.6x10^6J

Answer No. 37 - As
1 kWh=1000 Wx 3600 s
1 kWh =3600000 j
1 kWh = 3.6x10^6 j

Question No. 38 - Define Kinetic energy?

Answer No. 38 - Kinetic Energy (K.E): The energy in the body due to its motion is known as kinetic energy.

Question No. 39 - Name some non- conventional energy sources.

Answer No. 39 - Some of the non-conventional energy sources are:
Energy from tides
Energy from waves
Energy from biomass
Energy from waste products
Solar Energy
Geothermal Energy

Question No. 40 - Explain briefly how the energy is obtained from the fermentation of biomass.

Answer No. 40 - Bio fuel such as ethanol is a replacement of gasoline. It is obtained by fermentation of biomass using enzymes and by decomposition through bacterial action in the absence of air.
The rotting of biomass in a closed tank called a digested produces bio gas which can be pipped out to use for cooking and heating.

Question No. 41 - How energy is obtained from direct combustion and fermentation.

Answer No. 41 - Direct Combustion: It is applied to get energy from waste product commonly known as solid waste and confined it into chamber and ignite it.The heat produced is then used in a boiler to run the turbine of generator.

Fermentation: Bio fuel such as ethanol is a replacement of gasoline which is obtained by fermentation of biomass using. Enzymes and by decomposition through bacterial action in the absence of air . The rotting of biomass in a closed tank called digested produces bio gas which can be piped out to use for cooking and heating.

Question No. 42 - Describes four uses of solar cells.

Answer No. 42 - Solar cells are used.
1. To power satellites
2. In remote ground based weather stations
3. In rain forest communication systems
4. In solar calculators

Question No. 43 - Write some methods to obtain solar energy.

Answer No. 43 - Solar energy can be used directly to heat water using large solar reflectors (mirrors) and thermal absorbers.
Semi-conductor devices called solar cells are used to convert sunlight into electricity. Solar cells are thin wafers made from silicon,
For cloudy days or nights, electric energy can be saved in Nickel cadmium batteries by connecting them to solar panels.

Question No. 44 - How heat is generated within the Earth?

Answer No. 44 - Heat within the earth is generated by the following process.
1. Radioactive decay
2. Residual heat of the earth (usually within 10km of
earth's surface).
3. Compression of material deep inside the earth.

Question No. 45 - When a rocket re enters the atmosphere, its nose con becomes very hot.Where does this heat energy come from?

Answer No. 45 - The atmosphere of earth contains a large number of dust particles,gases and water vapors. So when a rocket enters into the atmosphere and passes through these particles, due to the force of friction, the kinetic energy of the rocket is lost in the form of heat. That's why its nose cone becomes very hot.

Question No. 46 - Explain briefly about residual heat of the earth.

Answer No. 46 - Hot igneous rocks within 10km of earth's surface are present in molten form They conduct heat energy from interior part of the earth. The temperature of these rocks is 200° C or more.

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