RESISTANCE AND OHM’S
LAW
A closer look at insulators and
conductors
• Conductors: electrons loosely bound to
nuclei
– Electrons flow easily when voltage is applied
– Eg: most metals
• Insulators: electrons tightly bound to
nucleus
– Electrons do not flow easily
– Eg: glass and rubber
Resistance
• A measure of how difficult it is for current
to flow through a material
• (Resists or slows down charges, which
reduces current)
• Depends on material, thickness, length and
temperature
• The lower the resistance, the better the
conductor
Resistors convert electrical energy to heat
or light
Examples of Resistors
• Light bulb filament (thin wire that glows)
• Heating elements on stove or in oven
• Heating wires inside a toaster
Rheostats
• Variable resistors
• Used to adjust the amount of current
flowing
• Examples:
•
•
temperature control on stove
dimmer switch
volume control knob
Calculating Resistance
• Resistance can be calculated from voltage
and current
• Mathematical relationship is
called Ohm’s Law
• (after Georg Simon Ohm)
OHM’S LAW
Resistance

Voltage (in Volts)
Current (in Amps)
• Resistance is measured in Ohms (W)
NOTE: Must use these base units, not milliAmps,
kiloVolts etc.
3 Forms of Ohm’s Law

Resistance
Voltage
Current
Current

Voltage
Resistance
Voltage
 Current  Resistance
Memory Aids for Ohm’s Law
V
Finding Resistance
• - cover up R
• (V over I) - divide
V
• If V = 12 Volts
• And I = 2 Amps
R 
V
I

12 V
2 A
 6W
Finding Current
• - cover the I
• (V over R) divide
• If V = 24 V and R = 8 W
I 
V
R

24 V
8W
 3 A
V
Finding Voltage
• - cover the V
• (I beside R) multiply
• If I = 500 mA = 0.5 A
• and R = 20 W
V  I  R
 0.5 A  20 W
 10 V
Examples
• A small portable stove operates from a
120V outlet. When it is in use, 6A flow
through it. What is the resistance of the
heating element on the stove?
R = V/I
= 120V/6A
= 20 Ω
• The filament of a certain light bulb has a
resistance of 96 Ω. To operate at its proper
brightness, it needs a current of 1.25A to
pass through it. What voltage must be
applied to this light bulb to make it work?
V=IR
= 1.25 A x 96 Ω
= 120V
• If a kettle is operated from a 120V outlet
and its heating element has a resistance of
10 Ω, what current is in the heating
element?
I =V/R
= 120V / 10 Ω
= 12 A
Ohm’s Law Worksheet
• For each question:
•
Show formula
•
Show work
•
Include correct units
(example)
V=IxR
=2Ax6W
= 12 V