PP_ohms%20law.pptx1

1. Published 4 key
laws of
electromagnetism
1865
Invented & developed
the telephone
1970 - 1975
Developed the
alternating current
induction motor
1880s
Much of the modern electrical sciences discovered in 19th C.

2. Ohm’s Law
Georg Simon Ohm
• Discovered in 1825
• Relates 3 key quantities in electrical circuits
• Voltage (V)
• Current (I)
• Resistance (R)
V = I x R
Voltage = Current x Resistance
In scientific units: Volts = Amperes x Ohms
Think of the voltage as the FORCE which is DRIVING the total
electrical flow rate (current), against the resistance encountered
in a portion of an electrical circuit.

3. Georg Simon Ohm
17889 - 1854
Alessandro Volta
1745 - 1827
Andre-Marie Ampere
1775 - 1836
In scientific units: Volts = Amperes x Ohms
Ohm discovered the merger

4. Compare to pushing or cycling a bike up a hill
1) The force is your capacity for work to push or cycle the bike
(or to ‘drive’ it); that is like the Voltage in a circuit.
2) The resistance is like the friction force on the tyres, the stiffness
of the bike components, and the steepness of the hill; all these factors work
together to determine the rate of progress for a given force.
3) The rate of progress (up the hill) – is similar to the “current” in a circuit, which
measures the total passage of electricity in a given time through a particular point.
Voltage = (electrical) Current x (electrical) Resistance
Electromotive
Force
= VOLTAGE

5. Ohm’s Law
V = I x R
Voltage = Current x Resistance
Electromotive
Force
= VOLTAGE
The wire is not realllly on a slope, like the example of the bike up the hill.
It is not gravity creating the resistance to the work done:
it is the material of the wire itself!
Some materials – such as metals and water- are ‘electrical conductors’ which offer
relatively little resistance to electrons passage through the material.
e- = an electron,
the basic physical
unit of a current.
(billions of billions
pass through a
mains circuit
every second).

6. Suppose a wire has twice the resistance
The greater the electrical resistance,
the greater the applied voltage V
needs to be
to drive the same current I
Electromotive
Force
= VOLTAGE
Doubling the resistance of the
circuit wire will mean twice the
electromotive force (voltage)
required to drive the same
current through the circuit.

7. Ohm’s Law in practise
• A wire is a fixed material, so:
• Usually the resistance of a wire is not varying
• So the value of ‘R’ in the equation V = I x R is fixed in practise.
• What is varied is the Voltage, V
• As the Voltage is increased, the current increases
Divide both sides by the
‘constant’ resistance:
V / R = I x R / R = I
Voltage / Resistance = Current
Rearranging the equation to
express the fact that voltage
drives the change in current:
V = I x R
Voltage = Current x Resistance
In scientific units:
Volts = Amperes x Ohms
Volts / Ohms = Amperes

8. • Consider a wire with resistance 0.2 ohms.
• Increases voltage in steps of 50 volts from 50 to 500.
• Current is: Voltage/Resistance
• Current ranges from 50/0.2 = 250 amps(A) in steps of 250A to 500/0.2 = 2500A
Ohm’s Law in practise

9. • Download the accompanying simple spreadsheet
• Adjust the circuit resistance from 0.2 ohms to other values
• Make a note of how the current values on the graph are changing in response.
• What visual quantity on the graph best represents the circuit resistance?
Ohm’s Law in practise

10. • Download the accompanying simple spreadsheet
• Adjust the circuit resistance from 0.2 ohms to other values
[Just change the value in cell A2 from 0.2 to say 0.5, 1.0, 2.0, 10.0 and drag
it down the column to A11] – the graph will automatically update.
• Notice that the higher the resistance, the lower the current resulting.
• This makes sense, because if a wire has higher resistance, then fewer
electrons can flow through it for a given applied electrical force (‘voltage’).
INPUT = VOLTAGE, working AGAINST = RESISTANCE, to give OUTPUT = CURRENT
Ohm’s Law in practise

11. INPUT = VOLTAGE
working
AGAINST = RESISTANCE
to give
OUTPUT = CURRENT
Ohm’s Law in practise