Short answer: Distinguish voltage, current, resistance and power before calculating: voltage is potential difference, current is the rate of charge flow, resistance opposes current and power is the rate of energy transfer. For the simple resistive DC example below, use V = I x R and P = V x I, keeping each quantity’s unit with the number.
Give each number its quantity and unit
The DOE electrical science handbook covers these basic relationships.
| Quantity | Symbol used here | Unit |
|---|---|---|
| Voltage | V | volt |
| Current | I | ampere |
| Resistance | R | ohm |
| Power | P | watt |
Write “12 V” instead of an isolated “12.” This keeps the given values distinct when you rearrange a formula.
Solve the ideal resistive DC example
Given a hypothetical 12 V DC source and a fixed 6 ohm resistor:
- I = V / R = 12 / 6 = 2 A.
- P = V x I = 12 x 2 = 24 W.
- Cross-check: P = I squared x R = 2 squared x 6 = 24 W.
The example uses an ideal fixed resistance. It does not select a real component rating or authorize building or measuring a live circuit.
Show how you selected the relationship
Write four lines: the given values, the formula, the substituted values and the answer with its unit. Then use the second relationship to check the result.
For AC motor and transformer questions, select the relationships appropriate to the stated system. Start by identifying the model and what the question asks.
Texas exam-study references on this site use the 2026 NEC. Keep physical quantities, calculation steps and the applicable code rule visible as distinct parts of the solution.
Set up the voltage-current-power solution
Set up the voltage-current-power solution
Use the paper example to check that every number stays attached to the right quantity.
- I named the quantity requested
- I wrote every given value with its unit
- I selected a formula for the stated model
- I showed the substitution
- I checked the answer using another relationship
Keep the formula and cross-check with the answer
Write enough of the solution for another learner to follow the formula and the units.
Given values | Quantity requested | Formula | Substitution | Answer and unit | Check —|—|—|—|—|— | | | | |
Work three electrical-quantity questions
1. What is current for 12 V across a fixed 6 ohm resistor?
- A. 2 A
- B. 72 A
- C. 0.5 A
2. What is power in the same example?
- A. 6 W
- B. 24 W
- C. 12 W
3. What comes first in an AC motor calculation question?
- A. Use the DC resistor formula for every AC motor quantity
- B. Reuse the DC example’s answer without checking the requested quantity
- C. Identify the stated system and the relationship appropriate to the quantity
Work three electrical-quantity questions: answers
- A. I = V / R = 12 / 6 = 2; the alternatives multiply or reverse the ratio.
- B. P = 12 x 2 = 24; use both voltage and current.
- C. Select the model before substituting values.
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Log your Texas Journeyman hours free. A free Tsai Electro account keeps your supervised hours, your online practice scores and your exam date in one place. Start with a five-question check to see which topic to practice next.
Carry the four-line solution habit into exam calculations. The Texas Journeyman Electrician Exam Prep Book (2026 NEC) includes eight calculation lessons and worked answers that let you follow the given values, rule and steps. Use the chapter exercises to practice the method before moving to the two full mock exams.
Why keep the units on every line?
Units show which physical quantity each value describes and make a reversed ratio easier to spot.
How do I check the power result?
For the ideal resistive example, compare V x I with I squared x R; both give 24 W.
