Ohm's Law and Circuit Principles Explained

Application of Health and Safety and Electrical
principles
Principles and
Definition of Ohms
Law

Aims and objectives
 State what resistance is and what affects it.
 Understand Ohm’s law and how to apply it.
Resistance
and
Ohm’s law

The very important equation
V = I x R
is an expression
of Ohm's Law.
Resistance is the opposition
to the current flow

How Electricity Works. Ohm's Law Clearly Explained
Available from: http://www.sengpielaudio.com/calculator-ohmslaw.htm

Electric Circuit
An electric circuit requires three things...
 A source
 A load
 A means of transmission
Current can be simplified as a
‘flow of electrons’

The Source Of Supply
The Source Of Supply provides an electromotive force
(emf) to the circuit.
The emf is the drive or push that is applied to the
circuit to force the current around the circuit.
In this simple circuit we have a Source Of Supply
(the cell), a load (what we want to use), and
transmission wires that link the source to the load.
Symbol is emf
Unit is Volt (V)
+ -

Potential Difference
.
In the simple circuit the emf is the same as
the voltage that we would measure across
the load.
This would not always be the case, because
the load would be the potential difference
or voltage, that is dropped across the load.

Potential Difference
The emf provides the force that drives
the current around the circuit.
The circuit may have several loads
connected to it, then the emf would be
made up of all of the voltages dropped.

Volt Drop
The voltages dropped across each resistor are
different to the emf , but combined they add up to
the same value of the source emf
Although emf and volt drop (potential difference) are
measured in volts the symbol is different.
Symbol:
U or V
Unit:
Volt (V)

∆U = voltage drop
V or U for the dimension 'voltage'
In lots of mathematical and similar
contexts, capital ∆ is used to denote a
change in something
It means a change in voltage — in this
instance, between one point in the
circuit and another.

Ohm’s Law
In 1827, George Simon Ohm discovered that if you
have metal, maintained at
a constant temperature
for a constant length and area,
and then if you then increase the
voltage the current increases in the
same proportion.
He discovered that resistance of
the conductor linked the voltage to the current.
Ohm’ law
triangle
Voltage forms a relationship with current
U U
U IR I R
R I
= = =

Resistance
Resistance measured in Ohms (unit Ω)
When any material has an electrical supply
connected to it, it has a particular
resistance.
This resistance
is called the
resistivity of the material
or its specific resistance

Specific resistance of material
Particular materials have a specific resistance
If the length of the material increases the resistance will increase in
proportion.
If the same material changes in size or area it will change its resistance.
Increasing the area reduces the resistance.
(rho) is the symbol for resistivity
R =  l
A
Where R = resistance ()
 = resistivity (m)
l = length(m)
A = area (m2)

Ohm's Law is the linear proportionality
between current and voltage that occurs for
most conductors of electricity.
A graph of voltage against current is a
straight line.
The gradient is the resistance.
Principles and Definition of Ohms Law

Resistance
Resistance is measured in Ohms (symbol )
Resistance is a measure of how
much the current is slowed down.
The bigger the resistance,
the smaller the current.

Test Circuit for a Component.
Anything in an electric circuit
(e.g., lamp, resistor, motor,
diode) is called a component.
Each component has its own
circuit symbol.

A test circuit is used to
find a range of
voltages
and
currents for a
component.

Components which obey Ohm's
Law are Wires and Resistors.
A component will only
obey Ohm's Law at
constant temperature
(meaning that
the temperature
must not change).

 If the resistance of a component is
constant (stays the same)
 For different values of V and I,
then a plot (graph) of V against I will
be a straight line.
 The gradient (slope) of
the line shows how big the
resistance is.

In reality, an increase in current
through a component will change
its temperature
(the temperature usually goes up)
and so Ohm's Law is only an
approximation but it works quite
well for many components.

The thin wire (filament) inside the light bulb
gets very hot when a current flows through it and it
glows brightly.
This rise in temperature causes an increase in
resistance of the filament, and so the gradient (slope)
of the plot is seen to increase.
Components
which don't obey
Ohm's Law

Calculate the resistance of a component.
(A component can convert electrical energy into heat)
Firstly, we need to measure the current flowing through
the component, and the voltage across the component.
The circuit below shows where to place
an ammeter and a voltmeter.
If the ammeter reads 2 A, and the voltmeter reads 6 V,
then V = I x R
R = V divided by I
= 6 / 2
= 3 Ohms
The same method can be used to calculate
the resistance of any component.

A thermistor is a special type of resistor
which has been deliberately manufactured so
that its resistance decreases
as its temperature rises.
Components
which don't obey
Ohm's Law

Current in a Series Circuit
The current in a series circuit is the same
everywhere. An ammeter placed anywhere
in a series circuit always gives the same reading.
In the circuit above, A1 = A2 = A3 = A4.

If an identical cell (battery) is placed
in series with the original cell
the current doubles because
the total voltage of the circuit doubles.
However, two cells together provide
electricity for only the same
amount of time as one cell before they
both run out.
Current in a Series Circuit

Switches and Lamps in Series Circuits
An open switch in a series circuit will
turn everything off,
because the
circuit will be
disconnected
from the cell.

When lamps are connected in series,
the more lamps in the circuit the dimmer they get,
because the voltage is divided between them.
If one lamp in a series circuit breaks or fails,
all the others will go out with it.
For this reason, lamps are always connected in
Parallel (except Christmas Tree Lights or Fairy Lights,
where the large mains voltage is conveniently
divided between the lamps).
Switches and Lamps in Series Circuits

Current in a Parallel Circuit.
1. The current in
a parallel circuit depends on
the resistance of the branch.
2. The total
current flowing in to
the branches is equal
to the total current flowing
out of the branches.
A1 = A5

The current at A2 flowing
through the 2 Ohm
resistor can be found
using V = I x R
If the supply
voltage is 12 Volts…
I = V divided by R
= 12 / 2
= 6 Amps

You would get the same
answer for the 2 Ohm resistor,
whether or not
the other resistors are
connected in the circuit.
For parallel circuits,
each component behaves
as if it is connected
independently to the cell, and
is unaware of the other
components

If an identical cell (battery)
is placed in parallel with
the original cell
the current stays the same
because
the total voltage of the
circuit is the same.
Two cells together provide
electricity for twice as long

A switch at S1 or S5 will
switch all the
lamps off and on
together
(assume that all the
other switches are
"closed" which means
"on")
Switches and Lamps in a Parallel Circuit

The switch at S2 will only
light the lamp at L1.
This is very
useful because it means
that we can switch
the lamp
on and off without
affecting the
other lamps.

Similarly,
the switch at S3
will only
The switch at S4
will only

The brightness of
the lamp does not
change as
other lamps in parallel
are switched on or off.
For this reason lamps
are always
connected in parallel
(except Fairy Lights).

Ohm's Law and Circuit Principles Explained

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Ohm's Law and Circuit Principles Explained