1. 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

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

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

4. 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’

5. 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)
+ -

6. 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.

7. 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.

8. 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)

9. ∆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.

10. 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
= = =

11. 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

12. 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)