Resistance Of Wire
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METHOD
Set p the equipment as shown on the diagram.
One meter length of wire is fixed to a one meter ruler.
The first crocodile clip is clipped to the wire at the 0cm position on the one meter ruler.
The second crocodile clip is clipped to the 10cm position.
The power supply is turned on. The voltage and current are then read of the ammeter and voltmeter, and recorded.
The power supply is then turned off and the second crocodile clip is moved to the 20cm position.
Then the power supply is turned and you record the readings from the ammeter and voltmeter.
Then you turn the power off and move the clip to the next position, you do that until you get to 90cm then you stop.
After you do that, it is best to repeat the same thing from the beginning at least three time to get fair and accurate results.
BACKGROUND INFORMATIOMS
Current can easily pass through a piece of copper connecting wire. It does not pass trough as easily in a thin nichrome wire of an electric fire element. This wire has much more resistance. Energy has to be spent to force electrons through it. And heat comes off as a result.
All conductors have some resistance but:
Long wires have more resistance than the short wires.
Thin wires have more resistance than thick wires.
Nichrome wire has more resistance than copper wire of the same size.
Resistance is calculated using this equation:
VOLTAGE
RESISTANCE =
CURRENT
The unit of resistance is the ohm
Here is an example:
If there is a voltage of 12 volts across this nichrome, then a current of 4 amperes flows through.
Resistance =
ohms
= 3 ohms
If there is a voltage of 12 volts across this piece of nichrome, then a current of 2 amperes flows through.
So:
Resistance =
ohms
= 6 ohms
The higher the resistance, less current flows for each volt across the wire.
Like electric fires, kettles and hairdryers have heating elements made from coils of thin nichrome wire. The wire gives off heat when a current passes through. But that isnt their job. In some circuits , they are used to reduce the current. In radio or TV circuits they keep currents and voltages at the levels needed to make other parts work properly.
In a variable resistors there is sliding contact which moves along a coil of nichrome wire. By moving the contact you can change the resistance. Variable resistors like this are used as volume control in TVs and radios and also in computer joysticks.
A piece of metal consist of positive metal ions and free electrons. The free electrons are the outermost electrons which break free when the metal atoms form ions. Free electrons move about between the metal ions from being driven apart by repulsion between their positive charges.
The metallic bond explains how metals conduct electricity. It also explains how metals can change their shape without breaking. When a metal is bent the shape changes but the free electrons continue to hold the metal ions together.
Ohms Law
The current flowing through a conductor is proportional to the potential difference across it, provided the physical conditions remain constant.
V is the potential difference (voltage), I the current.
The value V/I is the resistance R of the conductor and is a constant.
Ohmic or linear conductors
Materials that follow Ohms Law are often called ohmic conductors or linear conductors.
A graph of the current and voltage of an ohmic conductor is a straight line through the origin. In the graph above the gradient is equal to 1/R.
Metals and alloys follow Ohms Law. Carbon is a non-metal that also follows Ohms Law. For most other materials the resistance is not a constant and changes with the applied voltage.
Physical Conditions
Note that Ohms Law only applies (i.e. the resistance is a constant) provided that the physical conditions remain constant. Physical conditions that can change the resistance of a conductor include:
temperature
strain (stretching or bending)
strong magnetic field
light
Temperature
In metals resistance increases with temperature. In most practical circuits heating is an unavoidable consequence of passing a current through a conductor. So the resistance of hot components in a circuit will be more than when the circuit is switched off and they are cold.
Strain
Applying a stretching force to a wire makes it longer and thinner. This increases the resistance of the wire
Resistively
The resistance R of a uniform conductor is proportional to the length l and inversely proportional to the cross-sectional area A. The resistivity ρ