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Electrical Resistance
University High School
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
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
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
Equation for Electrical
Resistance
Electrical Resistance = voltage drop
current
R – Electrical Resistance
 V – Voltage Drop
 I – Current

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.

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

Resistivity Defined
Measure of the capacity of a material to
resist electrical charge
Resistivity
Factors affecting resistance on a wire:

Length


Cross-sectional area


Longer wire, greater resistance
Smaller area, less resistance
Material

Higher resistivity, greater resistance
Calculating Resistivity
R=p*L
A
R – Resistivity
p – Rho (given constant for each material)
L – Length
A – Cross-sectional area
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

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
Calculating Series
Circuits
R total = R1 + R2 + ……
I total = I1 = I2 = ……
V total = V1 + V2 + …..
 V1 =
R1 * I1
 V2 = R2 * I2
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.
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.
Calculating Parallel
Circuits
V total = V1 = V2 = …..
I total = I1 + I2 + …..
 I1 = (V1 /
R1)
 I2 = (V2 / R2)
R total = R1 + R2
R1 * R2
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
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.
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
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