Construction Electrician exam: Ohm's law and power

Key points

• V = I x R. Cover the value you want and the other two show the formula.
• P = V x I, which also gives P = I squared x R and P = V squared / R.
• Resistors in series add up. In parallel, the total is less than the smallest branch.
• Doubling current through a fixed resistance gives four times the power.
• Keep units straight: kilowatts to watts and milliamps to amps before you calculate.

Study tip

Write down the two values you are given before choosing a formula. Most errors come from using the wrong pair.

Try these

1. A 120 V circuit draws 10 A. The resistance is:

   1. 1.2 ohms
   2. 12 ohms
   3. 120 ohms
   4. 1200 ohms
   Show answer

   B. 12 ohms R = V / I = 12 ohms.

2. A 1500 W heater on 240 V draws about:

   1. 3.1 A
   2. 6.25 A
   3. 12.5 A
   4. 16 A
   Show answer

   B. 6.25 A I = P / V = 6.25 A.

3. Two 10 ohm resistors in parallel total:

   1. 20 ohms
   2. 10 ohms
   3. 5 ohms
   4. 2.5 ohms
   Show answer

   C. 5 ohms Equal resistors in parallel: R / n.

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