Recall and use the equation for electrical energy E = I V t

4.2.4 Resistance

⚡️ In this section we’ll explore how resistance affects the flow of electric current and how to calculate the energy used in a circuit. Think of electricity like water flowing through a pipe: the resistance is the pipe’s roughness or narrowness that slows the flow. Understanding this will help you solve exam questions and design simple circuits.

Key Equation

The electrical energy (in joules) used by a device is given by: $$E = I \, V \, t$$ where:

• $I$ – current in amperes (A)
• $V$ – voltage across the device in volts (V)
• $t$ – time the current flows in seconds (s)

Remember: 1 J = 1 W × 1 s, so this equation is just the product of power and time.

Understanding Resistance

Resistance ($R$) tells us how difficult it is for electrons to move through a material. The relationship between voltage, current and resistance is Ohm’s Law:

• $V = I \, R$
• $I = \dfrac{V}{R}$
• $R = \dfrac{V}{I}$

🔌 A higher resistance means less current for the same voltage, just like a narrower pipe lets less water through.

Practical Example

Suppose a light bulb of 60 W runs on a 12 V supply.

1. Find the current: $I = \dfrac{P}{V} = \dfrac{60}{12} = 5 \text{ A}$.
2. Use the energy equation for 10 minutes (600 s): $E = I V t = 5 \times 12 \times 600 = 36\,000 \text{ J}$.

The table below shows the calculation steps.

Parameter	Value	Units
Power	60	W
Voltage	12	V
Current	5	A
Time	600	s
Energy	36,000	J

Exam Tips

✏️ Always check the units: Current (A), Voltage (V), Time (s), Energy (J). ✏️ Convert time to seconds before plugging into the equation. ✏️ Use Ohm’s Law first if you’re given voltage and resistance but need current. ✏️ Show all steps in your answer – examiners look for clear reasoning. ✏️ Remember the relationship $E = P t$ and $P = I V$ to cross‑check your results. 💡 If a question asks for energy, you can also use $E = I^2 R t$ or $E = \dfrac{V^2}{R} t$ as alternative forms.