Resistance of a Wire
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Task
To investigate how the resistance of a wire is affected by the length of the wire.
Theory
What is resistance?
Resistance is a force which opposes the flow of an electric current around a circuit so that energy is required to push the charged particles around the circuit. The circuit itself can resist the flow of particles if the wires are either very thin or very long.
e.g. The filament across an electric light bulb.
How is it measured?
The resistance of a length of wire is calculated by measuring the current present in the circuit (in series) and the voltage across the wire (in parallel). These measurements are then applied to this formula:
Gradient = resistance
V = I x R
The other ways of writing Ohms Law are:
R = V/I or I = V/R
Ohms Law
It is also relevant to know of Ohms Law, which states that the current through a metallic conductor (e.g. wire) at a constant temperature is proportional to the potential difference (voltage).
Predictions
The longer the wire, the higher the resistance. This is because the longer the wire, the more times the free electrons will collide with other metal ions, the particles making up the metal. Therefore, more energy is going to be lost in these collisions (as heat).
Furthermore, doubling the length of the wire will result in double the resistance. This is because by doubling the length of the wire one is also doubling the collisions that will occur, thus doubling the amount of energy lost in these collisions.
Variables
Independent variable – length of wire. Ill do six different readings, varying the length in each reading.
Dependent variable – what youll measure. Ill measure the resistance in each reading.
Controlled variable – thickness of wire. Type of wire and temperature of wire.
Length of wire – Increasing length of wire, increases resistance
Material of wire – A nichrome wire has more resistance than a copper wire of the same dimensions.
Temperature of wire – In metal conductors, resistance increases with temperature. For semi-conductors, resistance decreases with temperature.
Apparatus
Voltmeter
Ammeter
Battery cells
Method + Circuit Diagram
The following circuit was constructed to perform the investigation:
1. Collect apparatus
2. One metre length “nichrome” wire is fixed to a metre rule.
3. The first crocodile clip is clipped to the wire at the 0cm position on the metre rule.
4. The second crocodile clip is clipped to the relevant position depending on the required length of wire.
5. The voltage and current are then read off the ammeter and voltmeter, and recorded.
6. Leave the wire to cool down then move the second crocodile clip to the next position.
7. The above steps are completed for each length