4.2 Electrical quantities

4.2.4 Resistance

1. Understanding Resistance

1. Definition of Resistance
   • Resistance is the opposition to the flow of electric current. It is measured in ohms (Ω).
   • Formula:
   • where:
     R = resistance (Ω)
     V = potential difference (volts)
     I = current (amperes)
   • Ohm’s Definition: One ohm is the resistance when 1 volt causes 1 ampere of current to flow. $$1Ω=1{{V}\over{A}}$$
2. How Resistance Affects Circuits
   • A low resistance allows more current to flow, while high resistance reduces current.
   • Resistance depends on the shape and material of the conductor:
     • Longer wires have higher resistance.
     • Wires with larger diameters have lower resistance.

Example

A simple circuit consists of a cell, an ammeter, a voltmeter, and a resistor. The ammeter detects a current of 0.04A, and the voltmeter detects a voltage of 10V. Calculate the resistance of the resistor.

Answer:

• use of Equation
• Check what you have given with
  • Current = 0.04A
  • Voltage = 10V
• substitute values into equation and rewrite the equation
• 
• R = 250Ω

2. Measuring Resistance

1. Using a Circuit to Measure Resistance
   • Components required:
     • Variable Power supply
     • Ammeter (to measure current)
     • Voltmeter (to measure potential difference)
   • Steps:
     • Connect the resistor in the circuit with an ammeter in series and a voltmeter across the resistor.
     • Record the readings of the current and voltage.
     • Use R=V/I to calculate resistance.
2. Important Connections
   • Ammeters are connected in series.
   • Voltmeters are connected in parallel across the component.

As you increase the voltage across a fixed resistor, the current flowing through it will increase proportionally.

Think of it like a water pipe. The resistance of the pipe is fixed. If you increase the water pressure (voltage), more water will flow through the pipe (current).

3. Factors Affecting Resistance

• Effect of Length and Thickness
  • Resistance increases with the length of the wire.
  • Resistance decreases as the diameter of the wire increases.

Think of electricity flowing like water through a pipe. A long, narrow pipe slows the flow more than a short, wide pipe.

Experiment:

Investigating Resistance

Aim: To investigate how the length and thickness of a wire affect its resistance.

Apparatus:

• Cell
• Insulated wires with crocodile clips
• Meter ruler
• Resistance wires (thin and thick)
• Heatproof mat
• Ammeter and voltmeter

Procedure:

1. Secure the resistance wire along the meter ruler using masking tape.
2. Connect the circuit with the ammeter in series and voltmeter in parallel across the resistance wire.
3. Attach one crocodile clip at the 0 cm mark of the ruler.
4. Attach the second clip at intervals of 10 cm along the wire.
5. Record the voltage (V) and current (I) at each interval.
6. Calculate resistance (R=V/I ​) for each length.
7. Repeat steps 3–6 with a thicker wire.
8. Plot a graph of resistance against the length of the wire for both thin and thick wires.

Conclusion:                     

• Resistance increases with wire length.
• Resistance decreases with greater wire diameter.