Chapter 8
Ohm’s Law
George Ohm experimented with circuits and
came up with a mathematical relationship
relating voltage, current and resistance in a
circuit.
 The relationship is called OHM’S LAW.

V=I·R
I = V/R
V/I
R=
Using Ohm’s Law

A light bulb with a resistance of 2 Ω is
connected to a 1.5 V battery. Calculate the
current that will flow through the circuit.
V = IR
I=V
R
I = 1.5 V
2Ω
I = 0.75 A
Using Ohm’s Law

A light bulb requires 3 amps to produce light. If
the resistance of the bulb is 1.5 ohms, How many
batteries will you need? (Each battery is 1.5 volts)
V = IR
V = (3 A)(1.5 Ω)
V = 4.5 V
Batteries = 4.5 V
1.5 V
Batteries = 3
Graphing Ohm’s Law
Ohm’s Law tells you how much current
flows based on the voltage, if the device
has the same resistance.
 If a device has a variable resistance, it
does not follow Ohm’s Law directly. An
example is a light bulb.

Graphing Ohm’s Law
A current vs. voltage graph
shows us if resistance
changes. Often, these graphs
have both positive and
negative values of current and
voltage. These positive and
negative values are just a way
to refer to the direction of
current in a wire. You can
apply voltage two ways across
a wire. How you apply voltage
determines current direction.
One direction is positive and
the other negative.
Graphing Ohm’s Law
A simple resistor obeys Ohm’s
law—its current vs. voltage graph
is a straight line. Resistance is
the same at all values of voltage
and current.
For a diode, the graph is not a
straight line. A diode only allows
charge to flow in one direction!
This is why current is zero when
voltage is negative. Diodes do not
obey Ohm’s law.
Diodes, like computer chips, are
made from semiconductors.