4 Energy resources and energy transfer a) units b) energy transfer

Question 1

4.1 use the following units: kilogram (kg), joule (J), metre (m), metre/second (m/s), metre/second2 (m/s2), newton (N), second (s), watt (W).

Answer 1

Unit of mass: kilogram(kg)
Unit of energy: joule(J)
Unit of distance: metre(m)
Unit of speed or velocity: metre/second (m/s)
Unit of acceleration: metre/second2 (m/s)
Unit of force: newton (N)
Unit of time: second (S)
Unit of power: watt(W)

Question 2

4.2 describe energy transfers involving the following forms of energy: thermal (heat), light, electrical, sound, kinetic, chemical, nuclear and potential (elastic and gravitational)

Answer 2

Thermal energy: If we rub our hands together, kinetic energy will transform into thermal energy.

Light energy: In a filament lamp, electrical energy is converted to heat energy and light energy.

Electrical energy: In an electric generator, kinetic energy is converted to heat and electrical energy.

Sound energy: Clapping our hands will convert kinetic energy to sound and little amount of heat energy.

Kinetic energy: In a ceiling fan, electrical energy is converted to kinetic energy.

Chemical energy: In a motor car, chemical energy is converted to heat, electrical and kinetic energy.

Potential energy: Keep an object 10m above the ground. It will have gravitional potential energy in it. Remove the support, and the object will fall down. That is, potential energy is converted into kinetic energy.

Question 3

4.3 understand that energy is conserved

Answer 3

Energy is not created or destroyed in any process. It is just converted from one from type to another.

Question 4

4.4 know and use the relationship:

Answer 4

Efficiency = Useful Output Energy/ Total Input Energy

Question 5

4.5 describe a variety of everyday and scientific devices and situations, explaining the fate of the input energy in terms of the above relationship, including their representation by Sankey diagrams

Answer 5

Whenever we are transferring energy, proportion of input energy is wasted. Like a lamp has input energy of 100J. It uses 10J to give light and the other 90J is wasted as heat.

efficiency = 10J / 100J = 0.1

In a Sankey diagram it is presented like this:

http://www.shawonnotes.com/IGCSE_Physics/physics_images/sankey-diagram.gif

Question 6

4.6 describe how energy transfer may take place by conduction, convection and radiation

Answer 6

There are three basic ways energy can transfer from place to place: conduction, convection and radiation.

Conduction: Conduction is the transfer of energy through substance mainly metals, without the substance itself moving. They transfer energy through molecular vibration or free electron diffusion.

Convection: Convection is the transfer of energy by means of fluids (liquids or gases) by the movement of molecules.

Radiation: Radiation is the transfer of energy by means of wave. It doesn’t need any medium to flow through.

Question 7

4.7 explain the role of convection in everyday phenomena

Answer 7

Boiling water uses the role of convection to transfer heat. When fire is started, molecules at the bottom gets heated and expands. It gains kinetic energy and rises upwards and the molecules at the top sinks downwards. Now, the molecules at the bottom gets heated again and rises upwards while the others sink down. This keep in a continuous process and current known as convection current.

Question 8

4.8 explain how insulation is used to reduce energy transfers from buildings and the human body.

Answer 8

Energy-efficient houses reduce energy transfer by using two layered walls and double glazing windows. The wall is made wide layers of different materials. The outer layer is made with bricks; these have quite good insulating and weathering properties. The inner layer is built with thermal bricks with very good insulation properties. The two layers are separated by an excellent thermal insulator in form of cavity or gap. Reflective aluminium foil is used to reduce heat by radiation.

Windows are made of thin glasses. Two layers are used to trap air, and the thickness is given in such a way to reduce both conduction and convection.

We reduce heat loss in human body by wearing woolen cloths and jackets. This trap the hot air and prevents cold air from entering the body.