Resistance
Theory
When it comes to electricity, materials can be placed into one of three categories: insulators, conductors, and
semiconductors.
A perfect insulator would be a device that completely resisted the flow of electricity through
it.
A perfect conductor would be a device that allowed electricity to flow through it with no impedance.
Semiconductors are the middle ground between the two, acting like insulators under certain conditions and
conductors under others.
Ohm’s Law
The earliest studies of electricity showed that most conductors have a similar profile in how they transmit
electricity.
As discovered by Georg Ohm in 1827, the current flowing through a conductor is proportional to
the voltage applied to it.
This relationship, known as Ohm’s Law, can be stated mathematically as
V = IR
(Equ. 1)
where V is the applied voltage, I is the current, and R is the proportionality constant known as the resistance.
Materials that follow this equation are now known as ohmic devices and include most metals found in nature.
The resistance for these types of materials was found to have a very simple form, it is inversely proportional
to the crosssectional area of the material A and linearly dependent on the length of the material L.
L
R
A
(Equ. 2)
where
is the resistivity of the conducting material.
The resistivity depends only upon what the substance is
composed and its temperature (for most materials, the hotter it is, the higher its resistivity).
The equation
shows that the longer a wire is, the more resistance it has.
It also shows that the thicker the wire, the less
resistance it has.
A devices is said to obey Ohm’s law if its current voltage characteristic is described by equation 1. The
purpose of this lab is to investigate whether the two provided devices obey Ohms law. To be able to do that,
we need to take voltage and current measurements.
Use of Meters
We usually use an Ammeter to measure current and a
Voltmeter to measure voltage. When drawing a schematic of
a circuit, we usually label the Ammeter as “A” and the
voltmeter with “V”. An example schematic is shown in Figure
1. Along with the ammeter and the voltmeter, the schematic
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 Summer '10
 Adler
 Physics, Resistance, Semiconductors, Resistor, power supply, Electrical resistance

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