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Current Electricity

Punjab Board · Class 12 · Physics

Quick revision notes for Current Electricity — Punjab Board Class 12 Physics. Key concepts, formulas, and definitions for last-minute revision.

45 questions30 flashcards5 concepts

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A standard circuit diagram of a Wheatstone bridge, showing four resistors, a galvanometer, and a battery, highlighting the balanced condition.
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Key Topics to Revise

1

Electric Current and Current Density

  • Electric current is the net rate of flow of electric charge through a cross-section of a conductor.
  • For steady current: I = q/t, where q is the net charge flowing in time t.
  • For non-steady (instantaneous) current: I(t) = lim(ΔQ/Δt) as Δt → 0, which is the derivative dQ/dt.
2

Ohm's Law and Resistance

  • Ohm's Law: V ∝ I, or V = IR, where R is the resistance of the conductor.
  • Resistance R depends on the material and dimensions of the conductor: R = ρl/A.
  • ρ (rho) is the resistivity (or specific resistance) of the material. SI unit: Ω·m.
3

Drift Velocity, Relaxation Time, and Mobility

  • In the absence of an electric field, free electrons in a conductor move randomly due to thermal motion. Their average velocity is zero — no net current.
  • When electric field E is applied, electrons experience a force F = eE in the direction opposite to E (since electrons are negative).
  • Electrons accelerate between collisions with fixed positive ions. The average time between successive collisions is called relaxation time (τ).
4

Temperature Dependence of Resistivity

  • For metals, resistivity increases with temperature: ρ_T = ρ₀[1 + α(T - T₀)]
  • α is the temperature coefficient of resistivity. For metals, α is positive (resistance increases with temperature).
  • Physical reason: As temperature increases, τ (relaxation time) decreases due to more frequent collisions, so ρ = m/(ne²τ) increases.

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Full Notes

Key Concepts

Electric current (I) is defined asWhen an electric field is appliedOhm's Law states that the potentialResistance R depends on bothWhen current flows through a resistor

Frequently Asked Questions

What are the important topics in Current Electricity for Punjab Board Class 12 Physics?
Key topics in Current Electricity include Flowchart showing how current is defined from charge flow, the formula, units, and practical examples, Flowchart illustrating the free electron model in conductors with and without electric field, Flowchart showing Ohm's law and its three forms for calculating V, I, or R. These are the concepts Punjab Board Class 12 examiners draw on most — study them first, then practise related questions.
How to score full marks in Current Electricity — Punjab Board Class 12 Physics?
Understand the core concepts first, then work through the 45 practice questions available for this chapter. Revise formulas and definitions regularly, and use flashcards for quick recall before the exam.

Sources & Official References

Content is aligned to the official syllabus. Refer to the board website for the latest curriculum.

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