"Master Electricity in physics: top PYQ Solutions for exam success"

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Past Year Questions (PYQ)
1. CBSE 2023:
Two resistors of \(R_1 = 4 \, \Omega\) and \(R_2 = 6 \, \Omega\) are connected in series. Calculate the equivalent resistance. If the potential difference across \(R_1\) is \(2 \, V\), what will be the total potential difference across the combination?

2. CBSE 2022:
What is the resistance of a conductor through which \(1 \, C\) charge flows when a potential difference of \(5 \, V\) is applied across it for 5 seconds? Also, calculate the current flowing through the conductor.

3. CBSE 2021:
An electric iron of \(1000 \, W\) is operated at \(220 \, V\). Calculate the current drawn and the resistance of the heating element.

4. CBSE 2020:
Explain the role of a fuse in an electric circuit. Why is it necessary to use a fuse with a proper rating in a circuit?

5. CBSE 2019:
A wire of uniform cross-sectional area carries a constant current. If the length of the wire is doubled by stretching it, what will be the new resistance of the wire? Assume the resistivity remains the same.

6. CBSE 2018:
A circuit consists of a battery, a bulb, and a key. Draw a labeled circuit diagram for the setup. What will happen to the bulb’s brightness if the resistance in the circuit is increased?

These questions cover a broad spectrum of concepts within the Electricity chapter and align with the type of competency-based questions expected in the 2025 board exam.

Electricity - CBSE 2025 Board Exam Short Notes

#### 1. Electric Current (I):
**Definition**: The flow of electric charge (electrons) through a conductor.
**Formula**: \( I = \frac{Q}{t} \)
Where \( I \) = current (in amperes, A), \( Q \) = charge (in coulombs, C), \( t \) = time (in seconds, s).

#### 2. Potential Difference (V):
**Definition**: The work done to move a unit charge between two points in a circuit.
**Formula**: \( V = \frac{W}{Q} \)
Where \( V \) = potential difference (in volts, V), \( W \) = work done (in joules, J), \( Q \) = charge (in coulombs, C).

#### 3. Ohm’s Law:
**Statement**: The current through a conductor is directly proportional to the potential difference across it, provided the temperature remains constant.
**Formula**: \( V = IR \)
Where \( V \) = voltage, \( I \) = current, \( R \) = resistance (in ohms, \( \Omega \)).

#### 4. Resistance (R):
**Definition**: The opposition to the flow of current in a conductor.
**Formula**: \( R = \frac{V}{I} \)
SI unit: Ohms \( (\Omega) \).
**Factors Affecting Resistance**:
1. **Length of the conductor**: \( R \propto L \)
2. **Cross-sectional area**: \( R \propto \frac{1}{A} \)
3. **Material**: Different materials have different resistivities.
4. **Temperature**: Resistance increases with temperature for conductors.

#### 5. Resistivity (\( \rho \)):
**Definition**: The resistance of a material of unit length and unit cross-sectional area.
**Formula**: \( R = \rho \frac{L}{A} \)
Where \( \rho \) = resistivity, \( L \) = length, \( A \) = cross-sectional area.
**SI Unit**: \( \Omega \, \text{m} \).

#### 6. Series and Parallel Circuits:
**Series Circuit**:
\( R_{\text{total}} = R_1 + R_2 + R_3 + \dots \)
Current is the same through all resistors, voltage divides.

**Parallel Circuit**:
\( \frac{1}{R_{\text{total}}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + \dots \)
Voltage is the same across all resistors, current divides.

#### 7. Electric Power (P):
**Definition**: The rate at which electrical energy is consumed or produced.
**Formulas**:
1. \( P = VI \)
2. \( P = I^2 R \)
3. \( P = \frac{V^2}{R} \)
SI Unit: Watt (W).

#### 8. Energy Consumption:
**Electrical Energy**: \( E = Pt \)
Where \( E \) = energy (in joules, J or kilowatt-hour, kWh), \( P \) = power (in watts, W), \( t \) = time (in seconds or hours).

#### 9. Heating Effect of Current:
**Joule’s Law of Heating**: \( H = I^2 R t \)
Where \( H \) = heat produced (in joules, J), \( I \) = current, \( R \) = resistance, \( t \) = time.

#### 10. Electric Fuse:
**Definition**: A safety device that protects an electrical circuit from excessive current by melting and breaking the circuit when the current exceeds a safe limit.
**Material**: Low melting point material like tin-lead alloy