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Ohm's law and resistors in series and parallel: Class 10 practice

Ohm's law: V = IR. Read the method, follow the three worked examples, then try the six questions.

What you should know first

  • Fractions
  • Units: volt, ampere, ohm

The idea behind ohm's law and resistors in series and parallel

Ohm's law: V = IR. In series resistances add (R = R₁ + R₂ + …); in parallel their reciprocals add (1/R = 1/R₁ + 1/R₂ + …), so the total is smaller than the smallest resistor.

Step-by-step method

  1. Find the equivalent resistance first.
  2. Then use I = V/R for the total current.
  3. In series the current is the same; in parallel the voltage is the same.

Worked example 1 (Basic)

A current of 1.5 A flows through a 8 Ω resistor. Find the potential difference across it.

  1. V = IR = 1.5 × 8
  2. V = 12 V
  3. Answer: 12 V

Worked example 2 (Standard)

Resistors of 7 Ω, 12 Ω and 1 Ω are connected in series to a 20 V battery. Find the total resistance and current.

  1. R = 7 + 12 + 1 = 20 Ω
  2. I = 20 ÷ 20 = 1 A
  3. Answer: R = 20 Ω, I = 1 A

Worked example 3 (Challenge)

30 Ω and 15 Ω resistors are connected in parallel across 30 V. Find the equivalent resistance and total current.

  1. 1/R = 1/30 + 1/15 = 1/10
  2. R = 10 Ω
  3. I = 30 ÷ 10 = 3 A
  4. Answer: R = 10 Ω, I = 3 A

Common mistakes and misconceptions

  • Adding parallel resistances directly
  • Using the total voltage across one series resistor

Practice questions

  1. (Basic) A current of 1 A flows through a 14 Ω resistor. Find the potential difference across it.
  2. (Basic) A current of 3 A flows through a 5 Ω resistor. Find the potential difference across it.
  3. (Standard) Resistors of 8 Ω, 8 Ω and 7 Ω are connected in series to a 46 V battery. Find the total resistance and current.
  4. (Standard) Resistors of 5 Ω, 12 Ω and 3 Ω are connected in series to a 40 V battery. Find the total resistance and current.
  5. (Challenge) 12 Ω and 6 Ω resistors are connected in parallel across 36 V. Find the equivalent resistance and total current.
  6. (Challenge) 12 Ω and 4 Ω resistors are connected in parallel across 18 V. Find the equivalent resistance and total current.

Answer key with full working

  • Q1: V = IR = 1 × 14 → V = 14 V → Answer: 14 V
  • Q2: V = IR = 3 × 5 → V = 15 V → Answer: 15 V
  • Q3: R = 8 + 8 + 7 = 23 Ω → I = 46 ÷ 23 = 2 A → Answer: R = 23 Ω, I = 2 A
  • Q4: R = 5 + 12 + 3 = 20 Ω → I = 40 ÷ 20 = 2 A → Answer: R = 20 Ω, I = 2 A
  • Q5: 1/R = 1/12 + 1/6 = 1/4 → R = 4 Ω → I = 36 ÷ 4 = 9 A → Answer: R = 4 Ω, I = 9 A
  • Q6: 1/R = 1/12 + 1/4 = 1/3 → R = 3 Ω → I = 18 ÷ 3 = 6 A → Answer: R = 3 Ω, I = 6 A

Syllabus note

This skill is listed under "Electricity" for Class 10 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.

Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.

Related practice and notes

Frequently asked questions

Which chapter is ohm's law and resistors in series and parallel in?

"Electricity", Class 10 Physics (NCERT/CBSE). Other boards may place it differently.

How were the answers checked?

Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.

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