Electricity – Ohm’s Law


Electricity is the phenomena associated with the presence and flow of electric charge.

Electric charge

Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter.

Positively charged substances are repelled from other positively charged substances, but attracted to negatively charged substances; negatively charged substances are repelled from negative and attracted to positive

The symbol Q is often used to denote a charge.

Electric current

An electric current is a flow of electric charge through an electrical conductor.

Electric charge flows when there is voltage present across a conductor.

The SI unit for measuring an electric current is the ampere, which is the flow of electric charges through a surface at the rate of one coulomb per second.

Ohms law

I= V / R


I is the current through the conductor in units of amperes

V is the potential difference measured across the conductor in units of volts

R is the resistance of the conductor in units of ohms.

Direct current (DC)

Direct current (DC) is the unidirectional flow of electric charge.

Direct current is produced by sources such as batteries, thermocouples and solar cells.

Alternating current(AC)

In AC the movement of electric charge periodically reverses direction.

In DC the flow of electric charge is only in one direction.

The usual waveform of an AC power circuit is a sine wave.

Resistive heating (Joule heating or ohmic heating)

It is the process by which the passage of an electric current through a conductor releases heat.


Q α I2 R


Q – heat produced

I – current

R- electrical resistance of the wire.

This relationship is known as Joule’s First Law.

Electric power

Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second.

The electric power produced by an electric current I consisting of a charge of Q coulombs every t seconds passing through an electric potential (voltage) difference of V is

P = work done per unit time = Q V / t

= I V


Q is electric charge in coulombs

t is time in seconds

I is electric current in amperes

V is electric potential or voltage in volts

In the case of resistive (Ohmic, or linear) loads, Joule’s law can be combined with Ohm’s law (V = I·R) to produce alternative expressions for the dissipated power:

P = I2 R

= V2 / R

where R is the electrical resistance.


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