+Electrical power and energy

When a direct current of I amperes is flowing in an electric circuit and

the voltage across the circuit is V volts, then

 Electrical energy      = Power x time  

                                 = VIt Joules  

Although the unit of energy is the joule, when dealing with large amounts

of energy, the unit used is the kilowatt hour (kWh) where 

+Electrical potential and e.m.f.

The unit of electric potential is the volt (V) where one volt is one joule

per coulomb. One volt is defined as the difference in potential between

two points in a conductor which, when carrying a current of one ampere,

dissipates a power of one watt, i.e.

A change in electric potential between two points in an electric circuit is

called a potential difference. The electromotive force (e.m.f.) provided

by a source of energy such as a battery or a generator is measured in volts.

+Power

The unit of power is the watt (W) where one watt is one joule per second.

Power is defined as the rate of doing work or transferring energy. Thus,

where  W is the work done or energy transferred in joules and t is the

time in seconds. Thus

+Resistance and Conductance

The unit of electric resistance is the ohm (Z) where one ohm is one
volt per ampere. It is defined as the resistance between two points in a

conductor when a constant electric potential of one volt applied at the

two points produces a current flow of one ampere in the conductor. Thus,

where V is the potential difference across the two points in volts and I is

the current flowing between the two points in amperes.

        The reciprocal of resistance is called conductance and is measured in

siemens (S). Thus,

where R is the resistance in ohms.

 

+Work

The unit of work or energy is the joule (J) where one joule is one newton

metre. The joule is defined as the work done or energy transferred when

a force of one newton is exerted through a distance of one metre in the

direction of the force. Thus

where  F is the force in newtons and s is the distance in metres moved

by the body in the direction of the force. Energy is the capacity for

doing work.

+Force

The unit of force is the newton (N) where one newton is one kilogram

metre per second squared. The newton is defined as the force which, when

applied to a mass of one kilogram, gives it an acceleration of one metre

per second squared. Thus,
  

where m is the mass in kilograms and a is the acceleration in metres

per second squared. Gravitational force, or weight, is mg, where
g = 9.81 m/s2

+Charge

The unit of charge is the coulomb (C) where one coulomb is one ampere

second. (1 coulomb = 6.24 x 10^18 electrons). The coulomb is defined as

the quantity of electricity which flows past a given point in an electric

circuit when a current of one ampere is maintained for one second. Thus,

+SI Units

The system of units used in engineering and science is the Systeme Internationale

d’Unites (International system of units), usually abbreviated to

SI units, and is based on the metric system. This was introduced in 1960

and is now adopted by the majority of countries as the official system of

measurement.

The basic units in the SI system are listed with their symbols, in

Table 1.1.

Derived SI units use combinations of basic units and there are many of them.
Two examples are:

• 
  
  Velocity—metres per second (m/s2)
• Acceleration—metres per second squared (m/s2)
  
 

SI units may be made larger or smaller by using prefixes which denote

multiplication or division by a particular amount. The six most common

multiples, with their meaning, are listed in Table 1.2.