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. What is wrong with this common trap diagram?
L ----(Lamp)----[SW]---- N
Switching neutral is a classic trap. Exam logic expects switching on the live side in basic circuits.
2. This style of drawing usually tests your ability to:
L1 ---[Load A]--- N L2 ---[Load B]--- N
Separate branch paths with labeled loads are used to test circuit tracing and assignment accuracy.
3. Why do both L and N pass through the RCD/ELCB block?
L ---[MCB]---[RCD/ELCB]--- Branches N --------------[RCD/ELCB]--- Branches
Leakage devices compare line and neutral current balance; imbalance indicates leakage path.
4. What exam skill does this pair of notations test?
[Single-line] DB -> SW -> Lamp [Multi-line] L -> SW -> Lamp -> N
The candidate must translate between simplified and detailed conductor representations correctly.
5. If Outlet 1 fails open at its load, Outlet 2 is usually:
L ---+---(Outlet 1)
+---(Outlet 2)
N ---+---(Outlet 1/2)
In a parallel branch arrangement, one load open-circuit does not necessarily interrupt other branches.
6. Primary purpose of the PE line here is:
PE ------------------------(Metal case) L,N -----------------------(Appliance)
PE is for protective earthing and fault-current safety, not normal operating current.