Required Practical - Investigating Resistance

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Investigating Factors that Affect Resistance in Circuits

The aim of this experiment is to explore how the resistance of a wire changes when its length is altered.

Apparatus

Power supply (low voltage, e.g. 1.5 - 3 V)
Ammeter and voltmeter
Resistance wire (e.g. nichrome)
Metre ruler
Crocodile clips and connecting leads
Switch

Method

Build the circuit using a power supply, ammeter, voltmeter, and the resistance wire.
Connect two crocodile clips to the thin resistance wire, 100 cm apart.
Switch the circuit on and record both current and potential difference.
Move one clip to reduce the length of wire between clips by 10 cm each time.
For each length, record the new voltage and current readings.
Use $R=\frac{V}{I}$ to calculate resistance for each length.

Results & Analysis

The resistance increases with wire length.
The relationship should be directly proportional (a straight line through the origin).
Plot a graph of resistance (on the y-axis) against length (on the x-axis) and draw a line of best fit.

Hazards & Safety

Heating of wire burns skin:
- Avoid touching the wire when the circuit is on; allow time for cooling.
Mild electric shock:
- Use a low-voltage supply and keep equipment dry.

Investigating Factors that Affect Resistance in Circuits 2

This experiment aims to explore how the total resistance of a circuit changes when resistors are connected in series and when they are connected in parallel.

Apparatus

Power supply (around 4 V)
Ammeter
Voltmeter
Two fixed-value resistors (R₁ and R₂)
Connecting wires and crocodile clips
Switch

Variables

Independent variable:
- The way the resistors are connected (single, series, or parallel)
Dependent variable:
- Total resistance of the circuit
Control variables:
- Voltage of the power supply
- Type and temperature of resistors used

Method - Single Resistor Test

Build a simple circuit with the power supply, one resistor (R₁), an ammeter in series, and a voltmeter across the resistor.
Close the switch and record the readings of current and potential difference.
Repeat with the second resistor (R₂).

Method - Series and Parallel

Series:
- Reconnect the circuit with both resistors joined in series.
- Keep the voltmeter in parallel across both resistors together.
- Record the current and voltage once the switch is closed.
Parallel:
- Rearrange the resistors so that R₁ and R₂ are in parallel.
- Ensure the voltmeter is connected across both resistors.
- Take the new current and voltage readings
For every setup, calculate total resistance using the formula: $R=\frac{V}{I}$

Results and Analysis

In Series:
- The total resistance increases and equals the sum of both resistors: $RTotal=R_1 +R_2$
- This happens because the current passes through each resistor in turn, so resistance adds up.
In Parallel:
- The total resistance is smaller than the smallest individual resistor: $\frac{1}{RTotal}=\frac{1}{R_1} +\frac{1}{R_2}$
- This is because the current splits into separate paths, allowing more total charge to flow.

Safety Precautions

Do not touch the resistors or wires while current is flowing–they can become very hot.
Turn off the power immediately if you notice burning smells or smoke.
Keep liquids away from the circuit to prevent short circuits or electric shocks.
Use low voltage to reduce the risk of overheating and ensure safe handling of the apparatus.