This document discusses key concepts related to electrical resistance including: - Conductors, insulators, and semiconductors and examples of each. - The equation for electrical resistance and the unit of measurement (ohm). - Factors that affect resistance such as length, cross-sectional area, and material. - Ohm's law and its application to circuits. - Series and parallel circuits, how current and voltage are calculated in each, and examples of each. - Resistors, how their values are determined using color bands, and their purpose in circuits.

1. Electrical Resistance
University High School

2. Conductors
Possess a great ability of conducting
electricity
Contain free electrons that flow easily
through materials when an electric field
is applied
Examples of conductors:
 metals, some liquids, and plasma

3. Insulators
Conduct very small currents when a
strong electric field is applied
Electrons are tightly bound and do not
move freely
Examples of insulators:
 wood, plastic, glass, and rubber

4. Semiconductors
Depending on their form, they can be
either better insulators or conductors.
 In pure form, they are better insulators, but
if an external substance is added, they
become better conductors
Examples of semiconductors:
 Silicon, germanium, gallium, and arsenic

5. Equation for Electrical
Resistance
Electrical Resistance = voltage drop
current
 R – Electrical Resistance
 V – Voltage Drop
 I – Current

6. Unit of Measurement
Unit of measure for electrical resistance
is the ohm.
If:
 Potential difference is equal to 1, and;
 Flow of current is 1, then;
 Resistance is equal to 1.

7. Resistance Example
A small stereo draws a current of 0.80 A
when the power supply produces a
potential difference of 110 V. What is
the resistance of the stereo?
 R = ?
 V = 110 volts
 I = 0.80 amps

8. Resistivity Defined
Measure of the capacity of a material to
resist electrical charge

9. Resistivity
Factors affecting resistance on a wire:
 Length
 Longer wire, greater resistance
 Cross-sectional area
 Smaller area, less resistance
 Material
 Higher resistivity, greater resistance

10. Calculating Resistivity
R = p * L
A
R – Resistivity
p – Rho (given constant for each material)
L – Length
A – Cross-sectional area

11. Ohm’s Law
This law was devised to aid in
simplifying electrical resistance
Is true when the following criteria are
met:
 Resistance is constant
 Resistance is independent of both potential
difference and current

12. Series Circuits
Contain only one path
for current flow.
Charge flows from
power supply into a
switch, and then each
light. Returns to power
supply.
Current is equal in all
parts of the circuit.
Any break will stop
current throughout the
entire circuit

13. Calculating Series
Circuits
R total = R1 + R2 + ……
I total = I1 = I2 = ……
V total = V1 + V2 + …..
V1 = R1 * I1
V2 = R2 * I2

14. Series Circuit Example
There are two lamps in your home office that
are supplied power through a series
connection. The power supply produces 120
volts. One lamp has a resistance of 90 ohms,
and the other a resistance of 70 ohms.
Calculate:
 The current through the circuit.
 The voltage drop across each lamp.

15. Parallel Circuits
Only partial current
flows through each
path
A positive lead and
a negative leads
starts at the power
supply and ends at
the last source.

16. Calculating Parallel
Circuits
V total = V1 = V2 = …..
I total = I1 + I2 + …..
I1 = (V1 / R1)
I2 = (V2 / R2)
R total = R1 + R2
R1 * R2

17. Parallel Circuit Example
You have two lamps in your living room that
are supplied power through a parallel
connection. The power supply produces 120
volts. One lamp has a resistance of 90 ohms,
and the other a resistance of 70 ohms.
Calculate:
 The total current in the circuit.
 The voltage drop across each lamp.
 The current in each lamp

18. Resistors
An electrical device that has a specific
resistance
Added into a circuit in order to provide
additional resistance that is needed in a
circuit.
Value is shown on the outside of the
resistor by a color coding system.

19. Resistor Values
Has four separate colored bands; with
each color representing a given value.
Band 1 – 1st significant digit
Band 2 – 2nd significant digit
Band 3 – multiplier; number of zeros
added
Band 4 – tolerance of resistor

20. Determining Resistor
Values
Band 1 – Green
Band 2 – Red
Band 3 – Black
Band 4 - Gold
Band 1 – Brown
Band 2 – Orange
Band 3 – Blue
Band 4 - Silver