Ohm's Law without the panic

Ohm's Law is the single highest-value thing on the syllabus. It appears directly in questions, and it hides inside questions about power supplies, antennas, meters and safety. Learn it properly once and it keeps paying.

The relationship

Voltage equals current times resistance.

V = I × R

• V — voltage, in volts. The push.
• I — current, in amperes. The flow.
• R — resistance, in ohms. The opposition.

Rearranged two ways:

I = V ÷ R        R = V ÷ I

You do not need to memorise three formulas. Memorise one and rearrange it. If algebra makes you nervous, cover the quantity you want in the triangle V over I R, and what remains is the calculation.

Worked example

A 13.8 V supply feeds a 6 Ω load. What current flows?

I = V ÷ R = 13.8 ÷ 6 = 2.3 A

Power

Power in watts is voltage times current:

P = V × I

Substituting Ohm's Law gives two more forms that save time in the exam:

P = I² × R        P = V² ÷ R

Use whichever form matches the two quantities the question gives you. If you are told current and resistance, reach for I²R rather than working out the voltage first.

The trap worth knowing

Because power depends on the square of voltage or current, doubling either one quadruples the power. Examiners like this because it catches people who assume everything is linear.

If the voltage across a fixed resistor doubles, the power dissipated increases four times — not two.

Where this shows up elsewhere

• Power supplies — a transceiver drawing 9 A at 13.8 V consumes about 124 W of DC input, which is not the same as its RF output.
• Safety — shock severity depends on the current through your body, and current depends on the resistance of your skin and the path. Wet skin lowers resistance, so the same voltage drives far more current.
• Feedlines — power lost in a cable turns into heat, calculated with I²R.

Practise this

Do the Ohm's Law drill until you can do these in your head at exam pace. Getting fast at this one topic frees up time for the questions that genuinely need thinking.