Physics: Electricity & Magnetism Flashcards

An RC circuit has resistance R and capacitance C. The time constant τ represents the time for the capacitor to charge to approximately what fraction of its maximum charge?

Answer: 63%

After one time constant τ = RC, the charge reaches Q₀(1 − e⁻¹) ≈ 0.632 Q₀, or about 63% of maximum.

A uniform electric field E points in the +x direction. The electric potential V decreases most rapidly in the:

Answer: +x direction

The electric field points in the direction of greatest decrease in potential, so potential decreases most rapidly in the +x direction (direction of E).

Which Maxwell equation describes the fact that magnetic monopoles do not exist?

Answer: ∇·B = 0

∇·B = 0 (Gauss's Law for magnetism) states that the net magnetic flux through any closed surface is zero, implying no magnetic monopoles.

A wire of length L carrying current I is placed in a uniform magnetic field B perpendicular to the wire. The magnitude of the force on the wire is:

Answer: BIL

The force on a current-carrying wire is F = BIL sin θ; with θ = 90°, F = BIL.

Two capacitors C₁ = 3 μF and C₂ = 6 μF are connected in series. The equivalent capacitance is:

Answer: 2 μF

For series capacitors: 1/C_eq = 1/3 + 1/6 = 1/2, so C_eq = 2 μF.

The displacement current in Maxwell's equations was introduced to:

Answer: Correct Ampère's Law so that it satisfies charge conservation

Maxwell added the displacement current term ε₀∂E/∂t to Ampère's Law to ensure that ∇·J + ∂ρ/∂t = 0 (charge conservation) is satisfied.

A charged particle enters a region with crossed electric and magnetic fields (E perpendicular to B). For the particle to travel in a straight line, its speed must be:

Answer: v = E/B

For straight-line motion, the electric and magnetic forces must balance: qE = qvB, giving v = E/B.