Ohms Law 2365.202.3

Current

• The uniform flow of electrons through a conductor is

referred to as electric current.

• The unit of electric current is the ampere (A).

• In formulae the symbol for electric current is I.

Ohm’s Law

Electromotive force (EMF)

• The EMF provides a difference in potential between two

open terminals of an electrical circuit. When the circuit is
complete, this potential difference causes the electrons to
flow in a uniform direction around the circuit and produce a
flow of current.

• The unit of EMF is the volt (V).

• In formulae the symbol for EMF is V.

Ohm’s Law

Resistance

• Every circuit presents some opposition to the flow of current

in the electric circuit, which has to be overcome by the
electrical pressure applied. This opposition is called
resistance.

• The unit of resistance is ohm (Ω).

• In formulae the symbol for resistance is R.

Ohm’s Law

The current flowing in any circuit is directly proportional to the
applied voltage and inversely proportional to the resistance of
the circuit, provided that the temperature of the circuit remains
constant.

V = I × R

I = V

R

R = V

I

Ohm’s Law

=10

20

I= V
R

An EMF of 10 volts is applied to a resistance of 20Ω. Calculate the
current that will flow.

Put some figures in:

Always draw a small circuit diagram, as
this will help you to visualise the situation.

We need to find the formula for I so we
can use the Ohm’s Law triangle.

Covering I gives us:

Calculate through to give: = 0.5A

Example 1

= 2 × 40

V = I × R

Calculate the applied EMF when 2 amperes flows through a resistance
of 40Ω.

Put some figures in:

Always draw a small circuit diagram, as
this will help you to visualise the situation.

We need to find the formula for V so
we can use the Ohm’s law triangle.

Covering V gives us:

Calculate through to give: = 80 volts

Example 2

= 50/5

R = V

I

When an EMF of 50 volts is applied to a circuit, a current of 5 amperes
flows. Calculate the resistance of the circuit.

Put some figures in:

Always draw a small circuit diagram, as
this will help you to visualise the situation.

We need to find the formula for R so
we can use the Ohm’s law triangle.

Covering R gives us:

Calculate through to give: = 10 ohms

Example 3

The end

Unit 202: Principles of electrical science