Electricity — 15 Most Important Questions
for Board Exam 2025-26

✅ Answer & Explanation

Electric current is defined as the rate of flow of electric charge through a conductor.

Where I = electric current, Q = charge (in coulombs), t = time (in seconds).

✅ Answer & Explanation

For a metallic conductor, when voltage (V) is plotted on the Y-axis and current (I) on the X-axis, the graph is a straight line passing through the origin (Ohm's Law).

✅ Answer & Explanation

For resistors in parallel:

Note: In parallel combination, equivalent resistance is always less than the smallest individual resistance.

✅ Answer & Explanation

Resistance depends on: length of conductor, cross-sectional area, nature of material (resistivity), and temperature.

✅ Answer & Explanation

Using the power formula:

✅ Answer & Explanation

Ohm's Law: At constant temperature, the electric current flowing through a conductor is directly proportional to the potential difference across its ends.

Circuit Diagram components: Battery (EMF source), Rheostat (variable resistance), Key (switch), Ammeter (in series to measure current), Voltmeter (in parallel across the resistor R).

✅ Answer & Explanation

Parallel combination is preferred because:

• Each appliance gets the same voltage (equal to the supply voltage), ensuring proper functioning.
• Each appliance can be switched on/off independently without affecting others.
• If one appliance fails, the others continue to work (no circuit break).
• The overall resistance is reduced, allowing more current to flow as needed.

✅ Answer & Explanation

Resistivity (ρ) is the resistance offered by a conductor of unit length and unit cross-sectional area.

SI Unit: Ohm-metre (Ω·m)

✅ Answer & Explanation

Given: I = 2 A, t = 5 min = 300 s, R = 10 Ω

Using Joule's Law of Heating:

✅ Answer & Explanation

Series Combination: Resistors are connected end-to-end so the same current (I) flows through each.

Let the potential difference across each be V₁, V₂, and V₃.

Key Properties:

• Same current flows through all resistors.
• Total voltage is distributed across resistors.
• Equivalent resistance is always greater than any individual resistance.

✅ Answer & Explanation

Parallel Combination: All resistors are connected across the same two points, so the same potential difference (V) applies to each.

Current Distribution: Each branch carries a current inversely proportional to its resistance.

✅ Answer & Explanation

When electric current flows through a resistor, the free electrons collide with the lattice ions, converting electrical energy into heat energy. This is the heating effect of electric current.

Derivation of Joule's Law:

Work done in moving charge Q through potential difference V:

Joule's Law: Heat produced is directly proportional to I², R, and t.

✅ Answer & Explanation

Given: R₁ = 3 Ω, R₂ = 6 Ω, R₃ = 9 Ω, V = 18 V

✅ Answer & Explanation

Electric Power: The rate at which electrical energy is consumed or dissipated by a device.

Summary of Power Formulas: P = VI = I²R = V²/R

Energy Calculation (per day):

✅ Answer & Explanation

Factors Affecting Resistance:

• Length (L): R ∝ L — Longer wire → higher resistance (more collisions)
• Cross-sectional area (A): R ∝ 1/A — Thicker wire → lower resistance (more paths for electrons)
• Nature of material (ρ): R = ρL/A — Each material has a unique resistivity
• Temperature: For metals, resistance increases with temperature

When wire is stretched to double length:

Volume remains constant: L × A = L' × A' → 2L × A' = L × A → A' = A/2