Page 63 - Physics_Form_2

Current electricity
Temperature
As temperature rises, the atoms in the
conductor vibrate more, leading to
increased collisions between electrons
and atoms, which raises resistance. In
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some materials, the resistance varies
almost linearly with temperature. For
instance, the resistance of copper (Cu)
varies approximately linearly with
temperature. For some other materials,
resistance does not vary linearly with
temperature. However, the effect of
temperature on the resistance of some
alloys, such as constantan and manganin,
is minimal.
Length of a conductor
When the length of a conductor is
increased, while other factors are kept
constant, the resistance of the conductor
also increases. This is because electrons
and atoms collide more in a long
conductor than in a short conductor. This
means that the resistance (R) of the wire
is proportional to the length (l) of the
wire. That is,
R  l
Cross-sectional area
A conductor with a larger cross-sectional
area has more charge carriers to carry the
electrical current than of a smaller cross
area. This means the resistance R of a
conductor is inversely proportional to the
cross-sectional area A of the conductor,
that is,
1 voltmeter, rheostat,
R 
A connecting
Physics Form 2 Final.indd 57
Nature of material
The resistance of a conductor also depends
on the material used to make the conductor.
For example, a conductor made from
steel will have higher resistance than one
made of copper of identical dimensions
at the same temperature. For example,
steel has a higher resistivity than copper.
A material’s property that resists current
flow is known as the material’s resistivity,
1
l
denoted by ρ. Since, R  and R  A ;
then,
l
R  A
Hence,
l
R = A
where r is the constant of proportionality.
This constant of proportionality is the
resistivity of the material, given as:
ρ = RA
l
Resistivity is, therefore, the measure of
the ability of a material to oppose the
flow of an electric current. The SI unit
of resistivity is the ohm-metre (Ωm).
Activity 2.6 will aid in developing
practical knowledge on determination of
resistivity of wires.
Activity 2.6
Aim: To determine the resistivity
of constantan wire
Materials: battery, ammeter, switch,
wires,
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