Ohm’s law: one triangle for every question

Resistance is the squeeze

Volts are the pressure. Amps are the flow. Now squeeze the hose. That squeeze is resistance: how hard the hose fights the flow. We measure it in ohms.

Same pressure, tighter squeeze, and the flow drops to a trickle. Let go, and the water rushes out again.

The formula: E = I × R

That’s Ohm’s law: pressure equals flow times resistance. On the exam it’s written E = I × R.

E is voltage. It stands for electromotive force, a fancy name for the push. I is current, in amps, and R is resistance, in ohms.

The other two formulas come from the same one: current is I = E ÷ R and resistance is R = E ÷ I. The triangle below gives you all three.

One triangle, three moves

Put E on top, because the push drives everything. I and R sit side by side on the bottom. It’s the same trick as the pie for watts: cover the one you want, and the other two tell you what to do.

• Cover E, and I sits next to R. Side by side, multiply: 2 amps through a 10-ohm resistor is 2 × 10, which is 20 volts.
• Cover I, and you see E over R. Over and under, divide: 200 volts across 100 ohms is 200 ÷ 100, which is 2 amps.
• Cover R, and you see E over I. Divide again: a circuit that draws 4 amps from a 12-volt source is 12 ÷ 4, which is 3 ohms.

All nine sums on the exam

Nine of the twelve Ohm’s law questions are sums, three for each move. Find what’s asked for, cover it, and do what the triangle says.

The trap: the wrong move

The wrong answers on the exam are what you get from the wrong move. Take 200 volts across 100 ohms. Multiply instead of dividing and you get 20,000 amps, and it’s one of the choices.

20,000 amps through a squeezed hose? That’s a fire hose through a drinking straw. Check every answer against the hose: more pressure, more flow; more squeeze, less flow. If it doesn’t make sense with water, it’s the wrong move.

• For 90 volts and 3 amps, multiplying gives 270 ohms, and that’s a choice too. Dividing gives 30 ohms. Check it: 3 amps times 30 ohms is 90 volts. It fits.

Three things to remember

1. Volts push, amps flow, ohms resist.
2. E equals I times R, with E on top.
3. Side by side, multiply. Over and under, divide.

Every question this covers

12 questions from the 2026–2030 Technician pool, word for word. On the exam the choices may come in a different order.

1. T5D01 What formula is used to calculate current in a circuit?

   1. A I = E x R
   2. B I = E / R Answer
   3. C I = E2 x R
   4. D I = E2 / R

2. T5D02 What formula is used to calculate voltage in a circuit?

   1. A E = I x R Answer
   2. B E = I / R
   3. C E = I2 x R
   4. D E = I2 / R

3. T5D03 What formula is used to calculate resistance in a circuit?

   1. A R = E x I
   2. B R = E / I Answer
   3. C R = E + I
   4. D R = E - I

4. T5D04 What is the resistance of a circuit in which a current of 3 amperes flows when connected to 90 volts?

   1. A 3 ohms
   2. B 30 ohms Answer
   3. C 1/30 ohm
   4. D 270 ohms

5. T5D05 What is the resistance of a circuit for which the applied voltage is 12 volts and the current flow is 1.5 amperes?

   1. A 18 ohms
   2. B 0.125 ohms
   3. C 8 ohms Answer
   4. D 13.5 ohms

6. T5D06 What is the resistance of a circuit that draws 4 amperes from a 12-volt source?

   1. A 3 ohms Answer
   2. B 16 ohms
   3. C 48 ohms
   4. D 8 ohms

7. T5D07 What is the current in a circuit with an applied voltage of 120 volts and a resistance of 80 ohms?

   1. A 9600 amperes
   2. B 200 amperes
   3. C 0.667 amperes
   4. D 1.5 amperes Answer

8. T5D08 What is the current through a 100-ohm resistor connected across 200 volts?

   1. A 20,000 amperes
   2. B 0.5 amperes
   3. C 2 amperes Answer
   4. D 100 amperes

9. T5D09 What is the current through a 24-ohm resistor connected across 240 volts?

   1. A 2400 amperes
   2. B 0.1 amperes
   3. C 10 amperes Answer
   4. D 5760 amperes

10. T5D10 What is the voltage across a 2-ohm resistor if a current of 0.5 amperes flows through it?

    1. A 1 volt Answer
    2. B 0.25 volts
    3. C 2.5 volts
    4. D 1.5 volts

11. T5D11 What is the voltage across a 10-ohm resistor if a current of 1 ampere flows through it?

    1. A 1 volt
    2. B 10 volts Answer
    3. C 11 volts
    4. D 9 volts

12. T5D12 What is the voltage across a 10-ohm resistor if a current of 2 amperes flows through it?

    1. A 8 volts
    2. B 0.2 volts
    3. C 12 volts
    4. D 20 volts Answer

Keep going

• Practice every question on HamStudy
• How to get your license, step by step
• Watts: the same hose, for power

Questions from the NCVEC 2026–2030 Technician question pool, with the errata of February 19, 2026.