Know that the p.d. across an electrical conductor increases as its resistance increases for a constant current

4.3.3 Action and Use of Circuit Components

Objective

Understand that the potential difference (p.d.) across an electrical conductor increases as its resistance increases when the current is kept constant.

1️⃣ What is Potential Difference?

The potential difference, or voltage, is the electric “pressure” that pushes electrons through a conductor. Think of it like the water pressure that forces water through a pipe.

2️⃣ Ohm’s Law – The Basic Relationship

Ohm’s Law links voltage (V), current (I), and resistance (R):

\$V = IR\$

Where:

  • \$V\$ = potential difference (volts, V)

  • \$I\$ = current (amperes, A)

  • \$R\$ = resistance (ohms, Ω)

3️⃣ How Resistance Affects Voltage (for Constant Current)

If the current \$I\$ stays the same, increasing the resistance \$R\$ will increase the voltage \$V\$ proportionally, because of the equation \$V = IR\$.

Analogy: Imagine a garden hose (current) that carries water. If you narrow the hose (increase resistance), you need higher water pressure (voltage) to keep the same flow rate.

4️⃣ Real‑World Examples

  1. 🔌 Light Bulbs: A bulb with higher resistance (e.g., a dimmer bulb) requires a higher voltage to produce the same current as a lower‑resistance bulb.

  2. 🧪 Resistors in a Circuit: In a series circuit, each resistor drops a portion of the total voltage. A higher resistance resistor drops more voltage.

  3. ⚡️ Heating Elements: Heating coils have high resistance; they need a high voltage to produce the same current that generates heat.

5️⃣ Examination Tips

✔️ Key Formula: \$V = IR\$ – remember the order: Voltage equals Current times Resistance.

✔️ Constant Current: When the current is fixed, focus on how changing resistance changes voltage.

✔️ Units: Voltage (V), Current (A), Resistance (Ω). Check that all units match when solving.

✔️ Diagram: Draw a simple circuit with a battery, a resistor, and a meter. Label \$I\$, \$R\$, and \$V\$ clearly.

✔️ Example Question: “If a 2 A current flows through a resistor and the voltage across it is 10 V, what is the resistance?” – Use \$R = V/I\$.

6️⃣ Quick Practice

Current (A)Resistance (Ω)Voltage (V)
1.54\$1.5 \times 4 = 6\$
32\$3 \times 2 = 6\$

Use this table to check your calculations: plug in the current and resistance, then multiply to find the voltage. If your answer matches the table, you’ve got the concept down!