Lesson page
Lesson 2: Ohm’s Law and Power
Read the lesson, then answer the multiple-choice practice set below. You get instant right or wrong feedback with explanation.
Electrical Calculations Electrical Theory Exam Practice
Lesson Content
This lesson is the main bridge between theory and exam calculations. Keep the formulas short and practice them until they are automatic.
Japanese Key Term
電力（でんりょく / power）
Hiragana
でんりょく
English Meaning
Power
Simple Explanation
Ohm's Law connects voltage, current, and resistance. Power tells you how fast electrical energy is being used.
Detailed Explanation
The exam often combines one formula with a short practical story. You may be asked to calculate current from voltage and resistance, then immediately calculate power from the answer. Take it in steps and keep the units visible.
Key Vocabulary
- 電圧（でんあつ / voltage）
- 電流（でんりゅう / current）
- 抵抗（ていこう / resistance）
- 電力（でんりょく / power）
Formula
V = I × RnP = V × I
Worked Example
If V = 100 V and I = 0.5 A, then P = 100 × 0.5 = 50 W.
Exam Tips
Write the unit next to every answer. Many exam mistakes are unit mistakes, not math mistakes.
Common Exam Trap
Do not use kilowatts and watts without converting. 1 kW = 1000 W.
Site Reality in Japan
In real electrical work, power information helps you match equipment ratings and breaker capacity.
Practice Question
A heater uses 2 A at 200 V. What is its power?
Answer
400 W
Explanation
Use P = V × I. 200 × 2 = 400 W.
1. A bad joint has 0.5 ohms resistance. If 10 A flows, power loss at the joint is:
P = I^2 R = 100 x 0.5 = 50 W.
2. What is the meaning of 力率?
力率 means power factor.
3. In a series AC circuit, if total voltage is 100 V and resistor voltage is 80 V, the power factor is closest to:
cos(phi) = Vr / V = 80 / 100.
4. Which notation means single-phase three-wire?
1phi3W is single-phase three-wire.
5. Which notation means three-phase three-wire?
3phi3W is standard three-phase three-wire notation.
6. WP on a symbol means:
WP in this exam context indicates weatherproof.