1. 22 electrical Quantities
Electrical energy is an important form of energy because many modern machines use it as a source of power and it can be easily converted into other forms of energy.
Generators, batteries and fuel cells can produce electrical energy. Fossil fuels are generally burnt to produce electricity but other alternate means of producing electricity such as solar, wind and hydroelectric are becoming popular. 22
Chapter
electrical Quantities

2. 22.1 Energy
Chapter 22: Electrical Quantities

3. Electromotive force (e.m.f.)
Electrical energy is one of the most useful forms of energy.
Examples of electrical energy sources:
Batteries
Dynamos
stores chemical energy for conversion
converts mechanical energy to electrical energy
Electromotive force (e.m.f.)
The work done by the force acting on charged particles to convert other forms of energy to electrical energy.
The unit for electrical energy is joules (J)
Chapter 22: Electrical Quantities

4. Electrical Energy Conversions
Chapter 22: Electrical Quantities

5. Light Bulbs Large electron flow Electrical Energy
and other equipment that uses resistance filaments…
Large electron flow
Electrical Energy
Increase in collisions between the electrons and atoms of the filament
Atoms gain kinetic energy
Thermal and Light Energy
Increase in temperature and hence thermal energy
The electrical current that passes through the resistance filament is very high
Heating of coils which radiates as light
Chapter 22: Electrical Quantities

6. Potential Difference
Potential difference V is defined as the amount of work done W (or energy E converted) to drive a unit charge Q through any two points in a circuit.
Chapter 22: Electrical Quantities

7. V = = E Q J (Joule) C (Coulomb) V (Volt) W Q Potential Difference
work done or energy converted
V = = E Q
J (Joule)
C (Coulomb)
V (Volt) W Q
p.d.
charge
Chapter 22: Electrical Quantities

8. 22.2 Electrical Power
Chapter 22: Electrical Quantities

9. The SI unit of electrical power is watt (W), where 1 W is 1 J/s.
Electrical power, P, is the rate of electrical energy transformed to other forms in any electrical device.
P = E t
= QV
Rate of change of electrical energy
Electric current, I, is the rate of flow of charge, I = Q t
Hence,
P = IV
The SI unit of electrical power is watt (W), where 1 W is 1 J/s.
Chapter 22: Electrical Quantities

10. Energy used = Power x time
Cost of Electricity
We pay for the electrical energy used or converted in our homes. How much we pay depends on the power output of the electrical appliance and the period of time it is used.
Hence,
Energy used = Power x time
kilowatt
kWh
hour
As the SI unit for energy, joule (watt x second), is too small for the calculation of electricity consumed in a modern home; we use a more convenient unit called the kilowatt-hour (kWh).
Chapter 22: Electrical Quantities

11. Sample Calculation
Below is the information given on a label of an electric iron.
220 V to 240 V – 1200 Watts
A.C. only
Model XYZ
If electricity costs 16 cents per unit, what is the cost of using the iron at maximum power 30 minutes every day in the month of May?
Cost of electricity = energy consumed x cost per unit
= power x time x cost per unit
= 1.2 x 0.5 x 31 x 16
= cents
= $1.54
Chapter 22: Electrical Quantities

12. 22.3 Conserving Energy

13. Increase in demand for electric energy More fossil fuels consumed
Turbine generators are used to produce electricity in power stations. The rotors of the turbine need a source of energy to rotate them.
Fossil fuels
Fuel is burned to heat water.
The heated water produces a high pressure steam that is used to make the turbine blades spin.
The turbine then drives the generator which produces electricity.
Increase in demand for electric energy =
More fossil fuels consumed
Chapter 22: Electrical Quantities

14. The increased rate of consumption of fossil fuels is a cause for concern as there are limited amounts of fossil fuels.
Fossil fuels
Naturally found on earth.
It takes millions of years to form in the ground from dead
vegetation and animals.
It is a form of non-renewable energy source (cannot be replaced).
Therefore, electricity users should conserve electricity and reduce the amount of wastage.
Chapter 22: Electrical Quantities

15. Conserving Electricity
Simple yet effective steps can be taken in homes to reduce the consumption of electricity
Use cold water rather than hot water
Switch off electrical appliances when not in use.
Use fans instead of air-conditioners
Use energy efficient lights
Energy saving devices
Fluorescent lamps vs
Incandescent bulbs
are gas-discharge lamps
that produce visible light
through the production of UV
light that causes the phosphor
in the lamp to fluoresce
produce light by heating up
a filament, that has high
electrical resistivity, until it is
hot
Fluorescent lamps convert electrical energy to light energy more
efficiently than incandescent bulbs.
Chapter 22: Electrical Quantities

16. Energy saving devices Solar water heaters
Water heating consumes a large amount of energy.
Solar water heaters are an alternative way of heating water. Solar panels absorb heat energy from the Sun and transfers the energy to the water.
Solar water heaters also help to reduce the emission of greenhouse gases to the environment.
Chapter 22: Electrical Quantities

17. Hydroelectric energy Solar energy Wind energy
Hydroelectric, solar and wind energy are good alternatives to fossil fuels. They are renewable sources of energy
Hydroelectric energy
Hydroelectric power stations use the stored energy of water to generate electricity.
Solar-powered devices use energy from the Sun to generate electricity.
Windmills use wind power to drive generators that provide electrical energy.
Solar energy
Wind energy
Chapter 22: Electrical Quantities