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Resistance and Resistivity
Electrical Resistance:
The size of a current is determined by two factors:

The potential difference of voltage V across the lamp ­ the greater the voltage, the greater the
current for a given lamp.
The resistance R of the lamp ­ the greater the resistance, the smaller…

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To determine the resistance of a component, both the voltage V across it and the current I through it need
to be measured. This can be done using an ammeter and a voltmeter.

The ammeter is connected in series with the conductor, so that there is the same current in…

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This figure shows the I-V characteristic for a light-emitting diode.
Some things to notice about the graph:

There are both positive and negative values of current and
voltage included.
o This is because, when connected one way round
(positively biased), the diode conducts and has a
fairly low resistance.
o…

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Thermistors:
These are components that are designed to have a resistance which changes rapidly with temperature.
Thermistors (`thermal resistors') are made from metal oxides such as those of manganese and nickel.
There are two distinct types of thermistor.

Negative temperature coefficient (NTC) thermistors ­ the resistance of this type of…

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the temperature is raised, the ions vibrate with larger amplitudes. The electrons collide more frequently
with the vibrating ions, and this decreases their mean drift velocity. They lose energy to the vibrating ions.

If the metal contains impurities, some of the atoms will be of different sizes. Again, this disrupts…

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We can rearrange this equation to give an equation for resistivity. The resistivity of a material is defined by
the following word equation:

resistivity = resistance x cross-sectional area
length

RA
= L



Resistivity and Temperature:
Resistivity, like resistance, depends on temperature. For a metal,
resistivity increases with temperature. This…

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