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6.2 Ohm’s Law
Resistance
George Simon Ohm (1787 – 1854) found that for metal conductors at a given
temperature, the current was directly proportional to the voltage across the conductor.
IαV
or
V
I
= constant
This constant is called the resistance, R, and is measures in ohms (1 Ω = 1 V/A)
V
I
= R
V = IR
V
R
= I
p. 230
6.2 Ohm’s Law
Resistors
Every conductors offers some resistance to current flow. A resistor is a device
specifically created to control the current in a circuit.
Two main types of resistors:
In a resistor electrical energy is transferred to thermal energy. There are some
materials when cooled to super low temperatures that offer no resistance. These
materials are called superconductors and are important parts of the magnets in
controlling the beam of protons entering the large HADRON collider at Cern,
Switzerland.
p. 231
6.2 Ohm’s Law
Resistor Code
Resistors either are labelled with their value or use a
colour code to identify the resistance value.
Entire Resistor Code:
Colour
Band
Meaning
1st
First digit value
2nd
Second digit value
3rd
Power-of-ten multiplier
4th
Manufacturer’s tolerance
For example:
Ohm’s Law does not apply to all conductors. For example light bulbs,
conducting solutions and gas-discharge tubes are examples of non-ohmic
resistors.
33 x 102 + 10% Ω
p. 232
6.2 Ohm’s Law
Joule’s Law
James Prescott Joule (1818 – 1889) found out that amount of energy released in a
unit of time (power) by a resistor is proportional to the square of the current.
P α I2
or
P = constant x I2
The constant is the same as the resistance in the circuit.
P = R x I 2 = I2 R
By bringing in Ohm’s law: V = IR
P = I2R = VI =
V2
R
p. 233 - 234
6.2 Ohm’s Law
EMF, Terminal Voltage, and Internal Resistance:
When little current is flowing through a cell the EMF and the
terminal voltage of the cell are nearly equal.
However, when significant current is flowing through the circuit and
the cell the terminal voltage will be less than the EMF.
Vab < Ɛ
When a cell is connected to an external resistor the current drawn by the complete circuit
flows through the cell and there would be some loss of potential energy per coulomb or
voltage by the internal resistance, r, of the cell.
Vab = Ɛ - Vr
For a cell in use:
Vab = Ɛ - Ir
and
Vr = Ir
For a cell being charged:
Vab = Ɛ + Ir
p. 235
6.2 Ohm’s Law
Key Questions
In this section, you should understand how to solve the following key questions.
Page 231 – Practice Problems 6.2.1 #2
Page 233 – Practice Problems 6.2.2 #3
Page 234 – Quick Check #2
Page 239 – 240 – Review 6.2 #2,4,7,8,13 & 16