Physics Resistance and Resistivity Revision Notes

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Resistance and resistivity
Page 117
Chapter 10
Electrical resistance
There are two factors that affect the size of the current through a component:
1. The potential difference or voltage across the lamp ­ the greater the voltage, the
greater the current for a given component.
2. The resistance of the component ­ the greater the resistance, the smaller the current for
a given voltage.
We can measure the potential difference of a component by connecting a voltmeter in parallel
to the circuit across the component. Two electrical components can have the same current
supplied to them however the voltage across the component is different due to the change in
resistance.
Resistance (ohms) = Voltage (V) / Current (A)
Quantity Symbol for quantity Unit Symbol for unit
Current I Ampere A
Voltage V Volt V
Resistance R Ohm Ohm
Defining the ohm
The unit of resistance, the ohm, can be determined from the equation that defines resistance
Resistance = Voltage / Current

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Resistance and temperature
In order for a component to obey Ohm's law, the temperature must remain the same. As
temperature increases the resistance increases and therefore the resistance is not constant and
so it doesn't obey ohms law.
A good example of this is the filament lamp. For low voltages, the filament lamp obeys ohms law
because it is a straight line. However at higher voltages the filament lamp does not obey ohms
law as the graph is a curve, so not a straight line.…read more

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And
Resistance is inversely proportional to cross section area
Therefore
Resistance = length / cross sectional area
Resistivity = (resistance x cross sectional area) / length
Summary
Resistance is defined as the ratio of voltage to current
Another term for voltage is potential difference
The ohm is the resistance of a component when a potential difference of 1 volt is prodced per
ampere
Ohms law can be stated as:
For a metallic conductor at constant temperature, the current in the conductor is directly
proportional to…read more

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