Ohm's Law & Power
Three variables, two equations, once you can rearrange V=IR and P=IV confidently, you can answer the majority of EI calculation questions.
Formula Reference
- Ohm's Law: V = IR (Voltage = Current × Resistance)
- Rearranged: I = V/R and R = V/I
- Power (basic): P = IV (Power = Current × Voltage)
- Power (resistance form): P = I²R
- Power (voltage form): P = V²/R
- Energy: E = P × t (Power × time, units: Watt-hours or Joules)
What the ASVAB is actually testing
Ohm's Law and power calculations make up the core of the Electronics Information (EI) subtest. The questions are predictable: you get two values, asked for a third. The entire section rewards knowing three equations and being able to rearrange them quickly under time pressure.
Ohm's Law: the foundation
Cover the unknown → V = I × R
V = IR, Voltage equals Current times Resistance.
Think of it like water pressure in a pipe: voltage is the pressure, current is the flow rate, and resistance is how narrow the pipe is. More resistance, less current for the same voltage.
Memorize the triangle shortcut:
V
-----
I × R
Cover what you want to find, the remaining arrangement is your formula:
- Cover V → V = I × R
- Cover I → I = V / R
- Cover R → R = V / I
Power: two forms you need
Cover the unknown → P = I × E
P = IV works when you have both current and voltage.
When you only have current and resistance, skip the middle step: P = I²R
When you only have voltage and resistance: P = V²/R
All three give the same answer for the same circuit. Use whichever saves you a calculation step.
The doubling trap
Test makers love asking what happens to power when you change one variable:
- Double voltage (constant R) → power quadruples (P = V²/R, V is squared)
- Double current (constant R) → power quadruples (P = I²R, I is squared)
- Double resistance (constant V) → power halves (P = V²/R, R is in denominator)
Recognizing the squared relationship is faster than recalculating from scratch.
Units to keep straight
| Symbol | Quantity | Unit |
|---|---|---|
| V | Voltage | Volt (V) |
| I | Current | Ampere (A) |
| R | Resistance | Ohm (Ω) |
| P | Power | Watt (W) |
Don't let the variable V (voltage) and the unit V (Volt) trip you up, context makes them clear.
Study approach
Write out all three power formulas on a scratch card and drill substitution problems. The EI section moves fast, you want Ohm's Law rearrangements to be automatic so you can spend your time on the harder circuit-type questions.
Common Pitfalls
- ⚠Solving for the wrong variable, always identify what the question asks for before rearranging
- ⚠Mixing up units: Voltage (V, Volts), Current (I, Amperes/amps), Resistance (R, Ohms Ω), Power (P, Watts)
- ⚠Using P = IV when you only have V and R, switch to P = V²/R to avoid an extra step
- ⚠Forgetting that doubling current quadruples power dissipated in a resistor (P = I²R, current is squared)
Worked Examples
Q1: A circuit has a voltage of 12 V and a resistance of 4 Ω. What is the current?
Answer: I = V/R = 12 V ÷ 4 Ω = 3 A
Q2: A resistor draws 3 A at 12 V. How much power does it dissipate?
Answer: P = IV = 3 A × 12 V = 36 W. Check: P = I²R = 9 × 4 = 36 W ✓
Q3: A 60 W bulb runs on 120 V. What is its resistance?
Answer: P = V²/R → R = V²/P = (120)²/60 = 14400/60 = 240 Ω
Q4: If current through a resistor doubles, what happens to the power it dissipates?
Answer: P = I²R, current is squared, so doubling I multiplies power by 4. Power quadruples.