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What is Resistance?

Resistance is a measure of how much a component

decreases the current (see resistor).

The bigger the resistance, the smaller the current.

Resistance
is measured in ohms (symbol Ω).

See how to calculate
the resistance of a component.

What is Ohm's Law?

The very important equation

voltage = current x resistance V = I x R

is an expression of ohm's law.

If the
resistance of a component is
constant (constant

means it stays
the same) then
a plot (graph) of

current against voltage
will be a straight line. The
gradient

(slope) of the line
shows how big the resistance
is.

A test circuit is used to find
how the current through a

component changes as the voltage
changes.

Below is a plot for two components that obey
ohm's
law.

The component with the blue
line shows a smaller

current flowing and therefore has a bigger
resistance.

Wires and resistors are examples of components that

obey
ohm's
law. To be precise, a component

will only obey ohm's law at constant temperature.

In reality, an increase
in current through

a component will
increase its temperature
and

so ohm's law is only an
approximation

but it works quite well for many
components.

See the theory
of electrical resistance.

Diodes,
lamps and thermistors
are

examples of components that do not obey ohm's
law.

Links Electricity Resistance Revision Questions

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