(2)Resistance and Resistivity

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  • Created on: 25-03-14 13:21
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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 the current for a given
voltage.
Meaning of electrical resistance:
To demonstrate that different lamps have different resistances we can connect and torch bulb and car
headlamp in series to a battery. The current is the same in each component, but the voltage across the
torch bulb will be greater than the voltage across the headlamp. The torch bulb has a larger resistance than
the headlamp.
The resistance of any component is defined as:
resistance = voltage
current
Or
R=V
I
Defining the Ohm:
The unit of resistance, the ohm, can be determined from the equation that defines resistance:
resistance = voltage
current
The ohm is equivalent to `1 volt per ampere'. That is:
1 = 1V A-1
Determining Resistance:

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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 both.
The voltmeter is connected across (in Parallel with) the conductor, to measure the voltage across
it.…read more

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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 Connected the other way around (negatively biased),
it allows only a tiny current through and has almost
infinite resistance.…read more

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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 thermistor
decreases with increasing temperature. Those commonly used in schools and colleges may have a
resistance of many thousands of ohms at room temperature, falling to a few tens of ohms at
100°C.…read more

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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 the free
flow of electrons. In colliding with impurity atoms, the electrons lose energy to the vibrating atoms.
Electrons tend to lose energy when they collide with vibrating ions or impurity atoms. They give up energy
to the metal, so it gets hotter.…read more

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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 is because there are
frequent collisions between the conduction electrons and the
vibrating ions of the metal.
For a semiconductor, the picture is different. The resistivity of a
semiconductor decreases with temperature.…read more

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