CBSE Class 10 Physics Electricity Notes and Important Questions (2025–2026 Board Exams)
Mastering CBSE Class 10 Physics Electricity is one of the highest-yield investments you can make for your 2025–2026 science board exams. Electricity contributes significantly to the physics portion of the CBSE Class 10 syllabus, testing students on foundational definitions, circuit diagram interpretation, derivations, and multi-step numerical problems. Whether you are preparing for periodic tests or your final board exam 10 assessment, this comprehensive guide covers complete revision notes, essential formulas, and official board questions with step-by-step solutions.
Key Concepts & Formula Notes
Chapter 11 (Electricity) of the NCERT Class 10 Science textbook deals with the behavior of electric charges, circuit parameters, and thermal effects. Below is a structured breakdown of every fundamental concept you must master.
1. Electric Current and Potential Difference
Electric Current (I): The rate of flow of electric charges through a conductor. Quantitatively, it is expressed as:
I = Q / t
- SI Unit: Ampere (A). 1 Ampere = 1 Coulomb per second (1 A = 1 C s-1).
- Direction: By convention, the direction of electric current is taken as opposite to the flow of electrons (positive to negative terminal).
- Measurement: Measured using an Ammeter, which is always connected in series in a circuit because it has very low resistance.
Electric Potential Difference (V): The amount of work done (W) in moving a unit positive charge (Q) from one point to another in an electric field:
V = W / Q
- SI Unit: Volt (V). 1 Volt = 1 Joule per Coulomb (1 V = 1 J C-1).
- Measurement: Measured using a Voltmeter, which is always connected in parallel across the component because it has very high resistance.
2. Ohm's Law and Factors Affecting Resistance
Ohm's Law: At constant temperature and physical conditions, the electric current flowing through a conductor is directly proportional to the potential difference applied across its ends.
V ∝ I ⇒ V = I × R
Where R is the electrical resistance of the conductor, measured in Ohms (Ω).
Factors on which resistance depends:
- Length of the conductor (l): R ∝ l (Resistance increases with length).
- Cross-sectional area (A): R ∝ 1/A (Thicker wires have less resistance).
- Nature of the material: Characterized by electrical resistivity (ρ).
- Temperature: Resistance of metallic conductors increases with rising temperature.
R = ρ × (l / A)
Resistivity (ρ) has the SI unit Ω·m and is an intrinsic material property independent of dimensions.
3. Series vs. Parallel Resistor Combinations
Understanding how resistors behave when grouped together is crucial for numerical problem solving in CBSE Physics:
- Resistors in Series:
- Current remains the same through all resistors (I = constant).
- Potential difference divides: V = V1 + V2 + V3.
- Equivalent Resistance: Rs = R1 + R2 + R3 + …
- Rs is always greater than the highest individual resistance.
- Resistors in Parallel:
- Potential difference remains equal across each branch (V = constant).
- Total current splits: I = I1 + I2 + I3.
- Equivalent Resistance: 1/Rp = 1/R1 + 1/R2 + 1/R3 + …
- Rp is always smaller than the smallest individual resistance.
- Advantage in domestic circuits: If one appliance fails, other appliances continue functioning independently, and each gets full supply voltage.
4. Heating Effect of Electric Current (Joule's Law)
When an electric current passes through a purely resistive conductor, electrical energy is converted entirely into thermal energy. According to Joule's Law of Heating, the heat generated (H) is:
H = I2Rt
This law establishes that heat generated is:
- Directly proportional to the square of current (I2) for a given resistance.
- Directly proportional to resistance (R) for a given current.
- Directly proportional to time (t) for which current flows.
5. Electric Power and Commercial Energy Units
Electric Power (P): The rate at which electrical energy is consumed or dissipated in an electrical circuit.
P = V × I = I2R = V2 / R
- SI Unit: Watt (W). 1 W = 1 Volt × 1 Ampere = 1 Joule/second.
- Commercial Unit of Electric Energy: Kilowatt-hour (kWh), commonly known as a 'unit'.
- Conversion: 1 kWh = 1000 W × 3600 s = 3.6 × 106 Joules (J).
Important CBSE Questions with Answers
These questions are curated from official CBSE question banks and frequently appear in board examinations.
Q1. Define the SI unit of electric current. What does one ampere mean?
Answer: The SI unit of electric current is the ampere (A). One ampere is defined as the flow of one coulomb of electric charge through a given cross-section of a conductor per second. Mathematically, I = Q / t, so 1 A = 1 C / 1 s.
Q2. Define 1 watt of power. How is electrical power related to voltage and current?
Answer: 1 watt (W) is the rate of energy consumption in an electrical circuit when a current of 1 ampere flows through a component across a potential difference of 1 volt (1 W = 1 J s-1). Electrical power is related to voltage and current by the relation: P = V × I (Power = Voltage × Current).
Q3. State Ohm's Law. Write the formula and define each term.
Answer: Ohm's Law: The electric current flowing through a metallic conductor is directly proportional to the potential difference applied across its terminals, provided its temperature and other physical conditions remain constant.
Formula: V = I × R
- V: Potential difference across the conductor (measured in Volts, V).
- I: Electric current flowing through the conductor (measured in Amperes, A).
- R: Resistance of the conductor (measured in Ohms, Ω).
Q4. A resistor of resistance R is connected across a battery of 6V. If the current is 0.5 A, find the value of R.
Answer:
Given:
- Potential difference (V) = 6 V
- Current (I) = 0.5 A
Applying Ohm's Law (V = I × R):
R = V / I = 6 / 0.5 = 12 Ω
Therefore, the value of resistance R is 12 Ω.
Q5. Explain why tungsten is used for making filaments of electric lamps.
Answer: Tungsten is specifically chosen for incandescent bulb filaments because:
- It possesses an exceptionally high melting point (approximately 3380°C), allowing it to glow white-hot without melting.
- It has relatively high resistivity, enabling it to generate intense thermal radiation and emit visible light efficiently when current passes through it.
- It does not oxidize (burn) easily at elevated operating temperatures in an inert gas environment (argon/nitrogen).
Q6. Calculate the total resistance when three resistors of 2Ω, 3Ω and 5Ω are connected in series.
Answer:
Given resistors in series: R1 = 2 Ω, R2 = 3 Ω, R3 = 5 Ω.
For a series combination:
Rtotal = R1 + R2 + R3
Rtotal = 2 + 3 + 5 = 10 Ω
The total equivalent resistance of the series network is 10 Ω.
Q7. Two resistors of 4Ω and 6Ω are connected in parallel. Find the equivalent resistance.
Answer:
Given resistors in parallel: R1 = 4 Ω, R2 = 6 Ω.
For a parallel combination:
1 / Rp = 1 / R1 + 1 / R2
1 / Rp = 1/4 + 1/6 = (3 + 2) / 12 = 5/12
Rp = 12 / 5 = 2.4 Ω
The equivalent resistance of the parallel combination is 2.4 Ω.
Q8. Why does the heat produced in a resistor depend on current, resistance and time? State the law.
Answer: When an electric current flows against resistance, electrical work is done to maintain the drift of charge carriers against collisions in the lattice, and this work converts directly into heat energy. This dependency is defined by Joule's Law of Heating:
H = I2Rt
According to this law, the amount of heat (H) produced is:
- Directly proportional to the square of current (I2) flowing through the resistor.
- Directly proportional to the resistance (R) of the conductor.
- Directly proportional to the time duration (t) for which current flows.
How to Prepare for This Topic in CBSE Class 10 Physics
Scoring a perfect 100% in the electricity section of your board exam 10 requires a balanced strategy between theoretical clarity and numerical accuracy:
- Memorize Standard Unit Definitions: CBSE board evaluators look for precise keywords. Ensure you write exact SI unit relations when asked to define 1 Volt, 1 Ampere, 1 Ohm, or 1 Watt.
- Practice Circuit Diagrams with Correct Polarities: Always label the positive and negative terminals of batteries, ammeters, and voltmeters, and clearly mark the direction of conventional current flow with arrows.
- Master Equivalent Resistance Reductions: Practice mixed circuit diagrams where resistors are arranged in hybrid series-parallel combinations. Always identify nodes and parallel branches first before simplifying.
- Watch Out for Unit Conversions in Numericals: Always convert minutes or hours into seconds when calculating charge (Q = I × t) or heat (H = I2Rt). For commercial energy calculations, convert power into kilowatts (kW) and time into hours (h).
Where to Practice More
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