1. Quantifying Electricity:
Resistance

2. Resistance
Resistance is a property of a substance to hinder the
movement of electricity and turn it into another form of
energy (like heat).
Electronics often
have resistors to
control how much
electricity is used.

3. Resistance
All loads and connecters have at least a small amount of
resistance.

4. Resistance
There are 4 factors that affect how much a material will
resist the flow of electricity:

5. Think. Pair. Share
How would you change a circuit to try to reduce the flow
of charges?
How could you change a circuit to try to increase the
flow of charges?

6. Resistance in Circuits
1) Type of Material
Some metals are more conductive than
others.
Insulators have very high resistance.

7. Resistance in Circuits
2) Cross-Sectional Area (thickness).
Thick wires have the least resistance.
_____
Thin wires have the most resistance.
_____

8. Resistance in Circuits
3) Length
Long wires have the most resistance.
_____
Short wires have the least resistance.
_____

9. Resistance in Circuits
4) Temperature
Warmer wires have the most resistance.
_____
Cooler wires have the _____ resistance.
least

10. Resistance
An ohmmeter is used to measure
the resistance of a circuit.
Symbol for
ohmmeter
Ohmmeter

11. Resistance
Symbol for Resistance: R
Unit of Measurement: Ohm ( ).

12. Resistance
For example, if we say the resistance in 10 metres
of a certain wire is 100 Ohms, then
R = 100

13. Resistance
The German scientist Georg
Ohm was the first to record
his observation which
relates voltage and current:

14. Resistance
Ohm's Law:
The voltage between two points on a conductor is
directly related to the electric current flowing through
the conductor.
This means the graph of V vs I is a straight line for an
ohmic load.

15. Resistance
If the graph of V vs I does not turn out to be a straight
line, the load is said to be "non-ohmic".

16. Calculating Resistance
Potential Difference = Current x Resistance
Volts = Amperes x Ohms
V = I x R
Name Unit Symbol
Potential Difference Volt V
Current Ampere I
Resistance Ohm

17. Try This.
(Problem Set Resistance Question #1)
What is the potential difference across a tungsten
filament in a 100 W light bulb if the resistance in the
filament is 144 Ω and a current of 0.833 A is flowing
through it?
V=IxR
V = (0.833 A) x (144 Ω)
V = 119.952 V
V = 120 V
.: the potential difference is 120 V