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OHM’S LAW AND
ELECTRICAL SAFETY
Lesson 11
November 23rd, 2010
A Fascination with Electricity

Georg Simon Ohm was born
in German in the early
1800s. He taught math in a
school in Switzerland. He
found in an interest in
electricity and his studies
led to a deep
understanding of how these
different electrical concepts
were related.
Ohms Law
His discoveries are today known as Ohm’s law.
 A law in science is a generalization based
on collection of observable evidence.
 Ohm’s law established the relationships
between potential difference (V), current (I),
and resistance (R). The symbol for resistance is
called the ohm (Ω) in honour of Georg Ohm’s
work in this field.
Quick Lab

Hook up the following in series:
 Battery
 Connectors
 Switch
 Lamp
 Ammeter

in parallel with the lamp
The lamps are 5 Ohms
 Multiply
the Ohms by the amps to find voltage.
Quick Lab

5 Ohms x 0.3 A = 1.5 V
Resistance
(Ω)
5Ω
Current (I)
0.3 A
Potential
Difference
(V)
Resistance x
Current
ΩxI
?
5 Ohms x 0.3 A
= 1.5 V
Potential Difference, Current, and
Resistance


He realized that the potential
difference (V) in a circuit is equal to
the current (I) multiplied by the
resistance (R).
Ohm’s law states that, as long as
temperature stays the same, V = IR
 the resistance of a conductor remains
constant
 the current is directly proportional
to the potential difference
V
I
R
Known Symbols Unknown Symb Unit
Quantity
Quantity
ol
Current,
IR
Potential
V
V
Resistance
Difference
Potential
VR
Current
I
A
Difference
,
Resistance
Potential
VI
Resistance
R
Ω
Difference
,Current
Equation
V=IR
I=V
R
R=V
I
Example 1

A current of 4.0 A flows through a 40 Ω resistor in a
circuit. What is the voltage?
G Current I = 4.0 A
Resistance R = 40 Ω
S V = IR
= (4.0 A)(40 Ω)
= 160 V
R Voltage V = ?
P Therefore, the voltage
in the circuit is 160 V.
A V = IR
Example 2

A 30 V battery generates a current through a 15 Ω
resistor. How much current does the battery
generate?
G Voltage V = 30 V
Resistance R = 15 Ω
S I = V/R
= (30 V) / (15 Ω)
= 2A
R Current I = ?
P Therefore, a current of
2 A is generated.
A V = IR
I = V/R
Example 3
An electric stove is connected to a 240-V outlet. If
the current flowing through the stove is 20 A, what is
the resistance of the heating element?
G Voltage V = 240 V
S R = V/I
Current I = 20 A
= (240 V) / (20 A)
= 12 Ω

R Resistance R = ?
A V = IR
R = V/I
P Therefore, the resistance of
the heating element is 12
Ω.
Ohm’s Law and Temperature


Ohm’s law works for most circuits. However,
temperature affects resistance. Generally,
resistance is lower when a conductor is cooler. As the
temperature increases, resistance increases.
For example, a filament in an incandescent light
bulb often has 10 times more resistance when the
bulb is warm.
Short Circuits


A path that allows electrons to flow along a
different path than the one intended.
A short circuit is an accidental low-resistance
connection between two points in a circuit, often
causing excess current flow
Short Circuit
This will cause your electrical device to stop
working
 Due to the low resistance, the current increases
and the conducting wires can quickly become
hot and can start a fire
 Short circuits can also kill if a person becomes
the ground

Electrical Safety and Safety
Devices

All electrical appliances present a risk of electric
shock. Some electronic devices, such as computers,
retain electric charge even when they are
unplugged. This is why many electrical devices have
a “Do Not Open” warning printed on them.
Fuses and Circuit Breakers



A fuse is a safety device in an electric circuit that has a
metallic conductor with a low melting point compared to
the circuit’s wires. If the current gets too high, the
metal in the fuse melts and the current flow stops.
This prevents further problems, such as damage to your
electrical components or a possible fire. A blown fuse
must be physically replaced as it can work only once.
The symbol represents a fuse in a circuit diagram.
Fuses and Circuit Breakers

A circuit breaker does the same job as a fuse
except that the wire inside does not melt. Instead,
the wire heats up and bends, which triggers a
spring mechanism that turns off the flow of
electricity. Once the breaker has cooled, it can be
reset.
Three-Prong Plug

The third prong of a three-prong electrical plug
connects the device to the ground wire of the
building. The ground wire sends any unwanted
current flow directly to the ground. Instead of
electricity travelling to the metal body of the device
and shocking a person using the device, the current
is directed to the ground.
Ground Fault Circuit Interrupter

A ground fault circuit interrupter (GFCI) or residual
current device is a device that detects a change in
current and opens the circuit, stopping current flow.
For example, if an appliance gets wet while you
are handling it and some current starts to flow
through the water, the GFCI opens the circuit so
there is less chance of injury to you.
Questions – hand in




Practice Problems (finding voltage)
1. A current of 1.5 A flows through a 30 Ω resistor
that is connected across a battery. What is the
battery’s voltage?
2. If the resistance of a car headlight is 15 Ω and
the current through it is 0.60 A, what is the voltage
across the headlight?
3. The current in a circuit is 0.50 A. The circuit has
two resistors connected in series: one is 110 Ω and
the other is 130 Ω. What is the voltage in the
circuit? (Remember to add the resistances together)




Practice Problems (finding current)
1. A fire truck has a searchlight with a resistance of
60 Ω that is placed across a 24-V battery. What is
the current in this circuit?
2. A bulb with 15 Ω of resistance is in a circuit
powered by a 3-V battery. What is the current in
this circuit?
3. What would the current be in question 2 if you
changed to a 45 Ω bulb?




Practice Problems (finding resistance)
1. A current of 0.75 passes through a flashlight bulb
that is connected to a 3.0-V battery. What is the
bulb’s resistance?
2. A current of 625 mA runs through a bulb that is
connected to a 120-V power supply. What is the
resistance of the bulb?
3. A table lamp draws a current of 200 mA when it
is connected to a 120-V source. What is the
resistance for the table lamp?