Energy resources and energy transfer - Shawon Notes

# a) Units

## 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).

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)

# b) Energy Transfer

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

For energy to be useful, we need to be able to transfer it from place to place and be able to convert it into whatever form we require. Unfortunately when we try to do so, some of the energy is converted to unwanted forms known as “wasted” energy. Eg:

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.

## 4.3 understand that energy is conserved

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

## 4.4 know and use the relationship:

Efficiency = Useful Output Energy/ Total Input Energy

## 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

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:

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

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.

## 4.7 explain the role of convection in everyday phenomena

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.

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

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.

# c) Work and Power

## 4.9 know and use the relationship between work, force and distance moved in the direction of the force:

work=force x distance
W=F x d

## 4.10 understand that work done is equal to energy transferred

Doing work means the energy is either decreased or increased. If a weight of 500N is raised 2m, 1000J of work is done. That means energy is increased by 1000J. Therefore work done is equal to energy transferred.

## 4.11 know and use the relationship:

gravitional potential energy=mass x gravitional acceleration x height
G.P.E=mgh

## 4.12 know and use the relationship:

Kinetic energy = ½ x mass x velocity2
K.E = ½ x m x v2

## 4.13 understand how conservation of energy produces a link between gravitational potential energy, kinetic energy and work

An object of mass, m weights mxg newtons. So the force, F, needed to lift is mg. If we raise the object through a distance h, the work done on the object is mgh. This is also the gain of GPE.

When the object is raised, it falls-it loses GPE but gains KE. At the end of the fall, all the initial GPE is converted into KE. And that’s how energy is conserved.

work done lifting object=gain in GPE=gain in KE of the object just before hitting the ground

## 4.14 describe power as the rate of transfer of energy or the rate of doing work

Power is the rate of transferring energy or doing work. Its measures how fast energy is transferred.

## 4.15 use the relationship between power, work done (energy transferred) and time taken:

power=(work done)/time
P=W/t

# d) Energy resources and Electricity generation

## 4.16 describe the energy transfers involved in generating electricity using:

• Wind: Winds are powered by the Sun's heat energy. Wind is a renewable source of energy. Wind mills have been used o grind corn and power machinery like pumps drain lowland areas. Today, wind turbines drive generators o provide electrical energy. Here, kinetic energy is transformed to electrical energy.
• Water: Water is used to generate energy in three ways: Hydroelectric power, Tidal power & Wave energy. All the ways uses the same role using the movement of water(K.E.) to rotate that generator and produce electricity. In this case kinetic energy is also transformed to electrical energy.
• Geothermal resources: Geothermal energy is heat energy stored deep inside the Earth. The heat in regions of volcanic activity was produced by the decay of radioactive elements. The heated water from the earth’s crust is used to rotate turbines in generator. Here, heat energy is converted to kinetic energy which is converted to electrical energy.
• Solar heating systems: Solar heating panels absorb thermal radiation and use it to heat water. The panels are placed to receive the maximum amount of the Sun’s energy. This produce steam which can be used to drive electricity generators.
• Solar cells: Solar energy directly convert light energy into electrical energy.
• Fossil fuels : Fossil fuels are natural gas, oil and coal. Those are burned which rotates the turbine in the generator to produce electricity.
• Nuclear power: Nuclear fuels like uranium are used in nuclear generator. The heat produced in nuclear reaction is used to produce steam from water which rotates the turbine and produce electricity.

## 4.17 describe the advantages and disadvantages of methods of large- scale electricity production from various renewable and non- renewable resources.

### Wind energy:

Relatively cheap to set up
clean – no waste products
Relatively efficient at converting energy into electricity

Only produce energy when it is windy
Can be used only in certain places
Can be an eyesore
Can produce noise pollution

### Wave energy:

Continuously available
Clean - no waste products
Moderately effecient

Expensive to set up
Only suitable in certain locations

### Tide energy:

Continuously available
Clean – no waste products
Efficient

Damaging to environment
Expensive to set up
Only suitable in certain geographical locations

### Solar energy

Clean-no waste products

Expensive in terms of amount of energy produced
Not very efficient method
Energy supply is not continuously available
Best suited to climates with low amounts of cloud cover

### Geothermal energy

Clean- no waste products
Can provide direct heating as well as heat/steam to drive electricity generators
Moderate start-up costs

Suited only to geographics locations with relatively thing ‘crust’ or high volcanic activity

### Hydroelectricity

Clean – no waste products
Continuously available

Needs large reservoirs, which may displace people or wildlife
Can be built only in hilly areas with plenty of rainfall

### Biomass

The carbon dioxide it releases when it burns has only recently been taken out of the atmosphere by crops

Growing biomass crops instead of food can cause food shortages.

Easy to produce