Much of the modern electrical sciences discovered in 19th C.
Published 4 key
laws of
electromagnetism
1865
Invented & developed
the telephone
1970 - 1975
Developed the
alternating current
induction motor
1880s
Ohm’s
Law
• Discovered in 1825
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Georg Simon Ohm
Relates 3 key quantities in electrical circuits
Voltage (V)
Current (I)
Resistance (R)
V=IxR
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.
In scientific units: Volts = Amperes x Ohms
Alessandro Volta
1745 - 1827
Andre-Marie Ampere
1775 - 1836
Georg Simon Ohm
17889 - 1854
Ohm discovered the merger
Voltage = (electrical) Current x (electrical) Resistance
Compare to pushing or cycling a bike up a hill
Electromotive
Force
= VOLTAGE
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.
Ohm’s Law
e- = an electron,
the basic physical
unit of a current.
(billions of billions
pass through a
mains circuit
every second).
V=IxR
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.
Suppose a wire has twice the resistance
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.
The greater the electrical resistance,
the greater the applied voltage V
needs to be
to drive the same current I
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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
V=IxR
Voltage = Current x Resistance
In scientific units:
Volts = Amperes x Ohms
Volts / Ohms = Amperes
Rearranging the equation to
express the fact that voltage
drives the change in current:
Divide both sides by the
‘constant’ resistance:
V/R=IxR/R=I
Voltage / Resistance = Current
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Ohm’s Law in practise
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
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Download the accompanying simple spreadsheet
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Adjust the circuit resistance from 0.2 ohms to other values
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Make a note of how the current values on the graph are changing in response.
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What visual quantity on the graph best represents the circuit resistance?
Ohm’s Law in practise
•
Download the accompanying simple spreadsheet
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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.
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Notice that the higher the resistance, the lower the current resulting.
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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
INPUT = VOLTAGE
working
AGAINST = RESISTANCE
to give
OUTPUT = CURRENT