Current Electricity — Flashcards
Punjab Board · Class 12 · Physics
30 flashcards for Current Electricity (Punjab Board Class 12 Physics) to test yourself on key terms and facts.
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Define electric current and state its SI unit.
Answer
Electric current (I) is defined as the amount of charge flowing through a cross-section of a conductor per unit time. Mathematically: I = Q/t, where Q is charge in coulombs and t is time in seconds. S…
A copper wire has 8.5 × 10²⁸ free electrons per m³. If a current of 2.0 A flows through a cross-sectional area of 2.0 × 10⁻⁷ m², calculate the drift velocity of electrons.
Answer
Given: n = 8.5 × 10²⁸ m⁻³, I = 2.0 A, A = 2.0 × 10⁻⁷ m², e = 1.6 × 10⁻¹⁹ C Formula: I = neAv_d, so v_d = I/(neA) Step 1: Calculate denominator = 8.5 × 10²⁸ × 1.6 × 10⁻¹⁹ × 2.0 × 10⁻⁷ = 8.5 × 1.6 × 2.0…
State Ohm's Law and explain what happens when voltage across a conductor doubles.
Answer
Ohm's Law: V = IR, where V is potential difference in volts, I is current in amperes, and R is resistance in ohms. This states that current is directly proportional to voltage if resistance remains co…
A nichrome wire has a resistance of 80 Ω at 27°C. The temperature coefficient of resistance is 0.0004 °C⁻¹. Calculate its resistance at 227°C.
Answer
Given: R₀ = 80 Ω at T₀ = 27°C, α = 0.0004 °C⁻¹, T = 227°C Formula: R_T = R₀[1 + α(T - T₀)] Step 1: Calculate temperature change: ΔT = T - T₀ = 227 - 27 = 200°C Step 2: Calculate α(ΔT) = 0.0004 × 200 =…
Derive the relationship between resistance (R), resistivity (ρ), length (l), and cross-sectional area (A).
Answer
Starting from: Resistance is proportional to length (R ∝ l) and inversely proportional to area (R ∝ 1/A). Combining: R ∝ l/A Introducing proportionality constant ρ (resistivity): R = ρl/A Where: R = r…
Why do electrons move with constant drift velocity instead of continuously accelerating under the electric field?
Answer
In a conductor, free electrons experience electric force (F = -eE) and accelerate initially. However, they constantly collide with positive ions in the lattice at random intervals. After each collisio…
Calculate the power dissipated in a 5 Ω resistor when 3 A of current flows through it.
Answer
Given: R = 5 Ω, I = 3 A Formula: P = I²R Step 1: P = (3)² × 5 = 9 × 5 = 45 W Alternative formula check: V = IR = 3 × 5 = 15 V Using P = V²/R = (15)²/5 = 225/5 = 45 W ✓ Using P = VI = 15 × 3 = 45 W ✓ A…
Two identical resistors are connected in series. If the equivalent resistance is 100 Ω, what is the resistance of each resistor?
Answer
Given: Two identical resistors in series, R_eq = 100 Ω, R₁ = R₂ = R (unknown) Formula for series: R_eq = R₁ + R₂ Step 1: Since resistors are identical: R_eq = R + R = 2R Step 2: 100 = 2R Step 3: R = 1…
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