Electricity is one of the most important and high-weightage chapters in Class 10 Physics. A large portion of the exam paper is based on numericals, and students who practice well can easily secure full marks in this chapter.
Many students understand the theory but lose marks in calculations due to lack of practice, formula confusion, or poor step presentation. The key to mastering this chapter is not just understanding concepts but applying them correctly in different types of questions.
In CBSE board exams, numericals from Electricity are almost guaranteed. These questions test your understanding of formulas, concepts, logical thinking, and accuracy.
In this detailed guide, we will solve the top 10 most important numericals covering all major concepts like resistance, Ohm’s law, power, electrical energy, and combination of resistors. Each question is explained step-by-step so that you can clearly understand the approach.
Numerical 1: Change in Resistance Due to Stretching
Question: If the length of a wire is increased by 20% by stretching, find the percentage change in its resistance and resistivity.
Solution:
We know the formula:
R = ρL / A
When a wire is stretched:
- Length increases
- Area decreases (because volume remains constant)
New length = 1.2L
New area = A / 1.2
Now,
R₂ = ρ × (1.2L) / (A / 1.2)
R₂ = ρL/A × (1.2 × 1.2)
R₂ = 1.44 R₁
This means resistance increases by 44%.
Important concept:
Resistivity does not change because it depends only on the material, not on dimensions.
Numerical 2: Resistance of Bulb
Question: A 60W bulb is used in India (220V) and USA (110V). Find the ratio of resistances.
Solution:
Formula:
P = V² / R
Since power is same for both cases:
220² / R₁ = 110² / R₂
Rearranging:
R₁ / R₂ = (220/110)² = 4
So,
R₁ = 4R₂
Conclusion:
Resistance in India is four times that in USA.
Concept insight:
At lower voltage (USA), resistance must be lower to maintain the same power rating.
Numerical 3: Equivalent Resistance of Circuit
Question: Find equivalent resistance when resistors are connected in series and parallel.
Solution:
Approach:
- First identify which resistors are connected in series (same current flows)
- Then identify which resistors are connected in parallel (same voltage across them)
- Simplify the circuit step-by-step instead of solving it in one go
Key formulas:
Series: R = R₁ + R₂ + R₃ …
Parallel: 1/R = 1/R₁ + 1/R₂ + 1/R₃ …
Example understanding:
If two resistors 2Ω and 3Ω are in series → R = 5Ω
If the same are in parallel → 1/R = 1/2 + 1/3 = 5/6 → R = 6/5 Ω
Tip:
Always simplify the circuit gradually. This avoids calculation mistakes and is the correct exam approach.
Numerical 4: Least Resistance Wire
Question: Which wire offers least resistance?
Concept:
R = ρL / A
To get minimum resistance:
- Length should be minimum
- Cross-sectional area should be maximum
Since area ∝ diameter², the wire with largest diameter and smallest length will have least resistance.
Exam tip:
This type of question is concept-based and frequently appears in MCQs.
Numerical 5: Number of Bulbs in Series
Question: How many 4V bulbs can be connected to 240V supply?
Solution:
Total voltage = n × voltage per bulb
240 = n × 4
n = 60 bulbs
Concept insight:
In series, voltage divides equally among identical bulbs.
Numerical 6: Current in Circuit
Question: Find current in circuit with total resistance 8Ω and voltage 4V.
Solution:
Using Ohm’s Law:
V = IR
I = V / R = 4 / 8 = 0.5 A
Important note:
Always write the formula first before substituting values.
Numerical 7: Energy Consumption and Cost
Question: A motor of 1100W runs for 5 hours daily for 6 days. Find energy consumed and cost.
Solution:
Convert power to kW:
1100 W = 1.1 kW
Time = 5 × 6 = 30 hours
Energy = Power × Time = 1.1 × 30 = 33 kWh
Cost = 33 × 5 = ₹165
Concept insight:
1 unit of electricity = 1 kWh
Numerical 8: Minimum Resistance Using Resistors
Question: Find minimum resistance using four 20Ω resistors.
Solution:
Minimum resistance occurs in parallel combination.
1/R = 1/20 + 1/20 + 1/20 + 1/20
1/R = 4/20 = 1/5
R = 5Ω
Important concept:
Parallel combination always reduces resistance.
Numerical 9: Resistance of Electric Kettle
Question: A kettle operates at 230V and 1000W. Find resistance.
Solution:
Use formula:
P = V² / R
R = V² / P
R = 230² / 1000
R = 52.9Ω
Tip:
Choose the correct formula based on given values.
Numerical 10: Ammeter Concept Question
Question: Why should an ammeter have low resistance?
Answer:
An ammeter is connected in series and is used to measure current. Since it is in series, the same current flows through it as in the entire circuit.
If the ammeter had high resistance:
- It would oppose the flow of current
- It would reduce the current in the circuit
- The measured value would not be accurate
Therefore, an ammeter must have very low resistance so that it does not disturb the circuit.
Key Formulas to Remember
- V = IR
- R = ρL/A
- P = VI
- P = V²/R
- P = I²R
- Energy = Power × Time
Tip:
Revise these formulas daily before solving numericals.
Exam Tips for Electricity Numericals
- Always write formula before calculation
- Maintain proper units throughout
- Show all steps clearly
- Avoid skipping calculations
- Practice circuit-based questions regularly
- Solve previous year questions multiple times
FAQs
Q1. How many numericals come from Electricity in CBSE boards?
Usually 2–3 direct numericals are asked in the board exam. In addition, MCQs and case-based questions also include numerical concepts, making this chapter extremely important for scoring.
Q2. Which topics are most important for numericals?
Topics like Ohm’s law, resistance, series and parallel combinations, electric power, and electrical energy are the most important. These topics are frequently repeated in board exams.
Q3. How can I improve speed in solving numericals?
Practice regularly and solve a variety of questions. Focus on understanding concepts instead of memorizing steps. With consistent practice, your speed and accuracy will improve automatically.
Q4. Do I need to memorize formulas?
Yes, formulas are essential for solving numericals quickly. However, you should also understand how the formulas are derived so that you can apply them correctly in different situations.
Q5. Are circuit diagrams important?
Yes, circuit diagrams are extremely important. Many questions are based on identifying series and parallel combinations. Practicing circuit diagrams will help you solve complex numericals easily.
Q6. What is the most common mistake students make in Electricity numericals?
Students often forget to convert units (like watts to kilowatts) or use incorrect formulas. Another common mistake is skipping steps, which can lead to loss of marks even if the final answer is correct.
Q7. How should I revise Electricity before exams?
Start with formulas, then solve important numericals, and finally practice previous year questions. Focus more on weak areas and revise them multiple times.
Conclusion
Electricity numericals are highly scoring if practiced properly. Instead of fearing numericals, focus on understanding concepts, applying formulas correctly, and practicing regularly.
Solve these important questions multiple times, analyze your mistakes, and improve your approach. With the right strategy and consistent practice, you can easily score full marks in this chapter.
Stay consistent, stay confident, and keep practicing.
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