Current Electricity – Key Concepts Flashcards

Twenty question-and-answer style flashcards covering core formulas, definitions, and principles from Class 12 Physics Chapter 3: Current Electricity.

Electric current is the rate of flow of charge through a cross-section; for steady current I = q / t.

What is the definition of electric current (I) and its formula for a steady current?

Ampere (A).

What is the SI unit of electric current?

They undergo random thermal motion, producing zero net current.

How do electrons move inside a conductor when no external electric field is applied?

Ohm’s Law: V ∝ I, or V = IR, where R is the resistance— the proportionality constant of a conductor.

State Ohm’s Law and define resistance.

R = ρ l / A; it is directly proportional to length and inversely proportional to area.

How does resistance (R) depend on a conductor’s length (l) and cross-sectional area (A)?

Resistivity is a material property governing resistance; metals have low ρ (≈10⁻⁸ Ω·m) while insulators have extremely high ρ.

What is resistivity (ρ) and how do conductors differ from insulators in terms of ρ?

Current density j = I / A; in a conductor j = σE, where σ is conductivity.

Define current density (j) and give its relation to electric field (E).

v_d = (e E τ) / m, with e = electron charge, τ = relaxation time, m = electron mass.

Write the formula for electron drift velocity (v_d) under an electric field.

I = n e A v_d, where n = number of electrons per unit volume and A = area.

What is the relationship between current (I) and drift velocity (v_d) in a conductor?

Mobility is drift velocity per unit electric field: μ = |v_d| / E = e τ / m.

Define mobility (μ) and provide its formula.

It fails for materials with non-linear V–I characteristics (e.g., semiconductors, diodes) and for devices whose behavior changes with current direction.

Give two major limitations of Ohm’s Law.

For metals, ρ increases with temperature (positive coefficient); for semiconductors, ρ decreases as temperature rises.

How does resistivity vary with temperature for metals compared to semiconductors?

P = IV = I²R = V² / R.

State three equivalent expressions for electrical power dissipated in a resistor.

Because power loss Pc ∝ I²R = (Pt / V)² R; increasing V lowers current and thus greatly reduces transmission losses.

Why is electric power transmitted at very high voltages over long distances?

EMF (E) is the open-circuit potential difference; terminal voltage V = E − I r, where r is internal resistance.

Define electromotive force (EMF) of a cell and give the relation for terminal voltage when current flows.

I_max = E / r (occurs when external resistance is zero).

What is the maximum current a cell can deliver?

Total EMF = nE; total internal resistance = n r.

For n identical cells each of EMF E and internal resistance r connected in series, what are the total EMF and total internal resistance?

The bridge is balanced when (R1 / R2) = (R3 / R4), yielding zero current through the galvanometer.

What is the balance condition of a Wheatstone bridge?

R_x = (R2 R3) / R4, using the known resistors in the balance ratio.

When a Wheatstone bridge is balanced, how can an unknown resistance R_x be found?

A meter bridge is a practical form of Wheatstone bridge that measures an unknown resistance by sliding a contact along a 1-meter wire until the bridge balances.

What is a meter bridge and on which principle does it operate?