resistance

1. ONACD - Editable curriculum designed for teachers by teachers
Resistance
Ohm’s Law
ONACD –Editable Curriculum designed for teachers by teachers
Georg Ohm
(1789 – 1854)

2. ONACD - Editable curriculum designed for teachers by teachers
Resistance
• The “loss” of energy in the form of heat creates
inefficiencies in light bulbs, household wires
and computer circuits to name a
few.
• However in stovetop heating elements, for
example, the heat is
wanted.
• This “loss” of energy is called Resistance and
is very important to understand when dealing
electricity.

3. ONACD - Editable curriculum designed for teachers by teachers
What is resistance?
For current in a conductor there is heat produced. Resistance is the
heat produced from using voltage to enable current.
Georg Simon Ohm (1789 - 1854) was a high school teacher who made
his own equipment and discovered that amount of current that could
be in a wire was dependent on the diameter of the wire and the
length of the wire.
From this he developed Ohm’s Law, which relates resistance, voltage
and current. In his honour the unit for measuring resistance was
named after him.
1 Ohm = A decrease of 1Volt is required for 1Ampere to flow
The symbol for Ohm is the Greek letter omega: Ω

4. ONACD - Editable curriculum designed for teachers by teachers
Factors of a material that affect resistance include:
• Thickness: Thinner increases resistance
• Length: Longer increases resistance
• Temperature: Hotter increases resistance
• Material: Impurities and defects in molecular structure
Materials can be of act as one of four types:
1. Superconductor: No resistance (below 20 degrees K)
2. Conductor: Low resistance (copper, aluminum …)
3. Semiconductor: Controlled resistance (doped silicon …)
4. Insulator: High resistance (plastic, rubber …)
Factors of Resistance

5. ONACD - Editable curriculum designed for teachers by teachers
Voltage, Current and Resistance
A common analogy to help you understand voltage, current and resistance is to
think of a waterfall that is diverted into a pipe down a hillside.
Voltage
• The height of the source of the water determines how much potential
energy the water has before it enters the pipe. This also determines the
pressure of the water.
• This is like the amount of attraction there is between the charges.
Current
• The amount of water flowing past a point is like the amount of charges
flowing past a point in the circuit.
Resistance
• The diameter of the pipe will significantly restrict or allow the water to flow.
Changing the diameter of the pipe changes the amount of current that can
flow from the pressure.
• If the voltage is constant then resistors change the amount of current
that can flow in the circuit.

6. ONACD - Editable curriculum designed for teachers by teachers
Question
With what you now know about resistance, why is beneficial that
computers are kept cool when they are being used?
The circuits in computers use incredibly thin “wires” which create heat
from their resistance. This heat then increases the resistance even
more. As the resistance increases the current is restricted and the
computer starts to slow down or even ironically “freeze”. People
who “over-clock” their CPU’s often use elaborate systems to cool
their computers. Some even used liquid cooled systems!

7. ONACD - Editable curriculum designed for teachers by teachers
Question
With what you now know about resistance, why is beneficial that
computers are kept cool when they are being used?
The circuits in computers use incredibly thin “wires” which create heat
from their resistance. This heat then increases the resistance even
more. As the resistance increases the current is restricted and the
computer starts to slow down or even ironically “freeze”. People
who “over-clock” their CPU’s often use elaborate systems to cool
their computers. Some even used liquid cooled systems!