Physics 2: Electricity and Magnetism: cram pack

Formula sheets, the key method steps for every topic and 10 practice problems, with full solutions at the back.

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1Formulas and methods

Coulomb’s law & electric fields

Coulomb’s law: F = k·|q₁q₂|/r²
Field of a point charge: E = k·|q|/r²
Constant: k = 8.99 × 10⁹ N·m²/C²
Force on a charge: F = q·E

Series & parallel resistors

Resistors in series: R_eq = R₁ + R₂ + …
Resistors in parallel: 1/R_eq = 1/R₁ + 1/R₂ + …
Two in parallel: R_eq = R₁R₂ / (R₁ + R₂)
Ohm's law: V = I·R
Power: P = V·I = I²R = V²/R

Method

Start at the end of the network furthest from the terminals.
Combine resistors in series by adding them, and resistors in parallel with 1/R = 1/R₁ + 1/R₂ + …
Redraw the circuit after each step.
Repeat until one resistor is left between the terminals.

RC & RL transients

Time constant: τ = RC | τ = L/R
Response: x(t) = x(∞) + [x(0) − x(∞)]·e^−t/τ
Capacitor at DC: open circuit; v_C cannot jump
Inductor at DC: short circuit; i_L cannot jump
Settled: t ≈ 5τ

Method

Find the initial value just after switching: a capacitor voltage and an inductor current cannot jump.
Find the final value long after switching: a capacitor acts as an open circuit, an inductor as a short circuit.
Find the time constant: τ = RC or τ = L/R, using the resistance seen by the capacitor or inductor.
Use x(t) = x(∞) + [x(0) − x(∞)]·e^−t/τ.

Magnetic fields & induction

Long straight wire: B = μ₀I/(2πr)
Centre of a flat coil: B = μ₀NI/(2R)
Long solenoid: B = μ₀nI, n = N/L
Constant: μ₀ = 4π × 10⁻⁷ T·m/A
Force on a wire: F = BIL·sin θ
Force on a charge: F = |q|vB·sin θ
Circular motion: r = mv/(|q|B), T = 2πm/(|q|B)
Parallel wires: F/L = μ₀I₁I₂/(2πd)
Flux: Φ = BA·cos θ
Faraday’s law: ε = −N·ΔΦ/Δt
Moving rod: ε = BLv
Generator: E₀ = NBAω, E_rms = E₀/√2
Inductance: ε = L·ΔI/Δt, L = μ₀N²A/l, W = ½LI²
Mutual inductance: ε₂ = M·di₁/dt, M = k√(L₁L₂)
Velocity selector and spectrometer: v = E/B, r = √(2mV/|q|)/B

Method

Convert every quantity to SI units: cm to m, cm² to m², mT and µT to T.
Pick the right formula: B = μ₀I/(2πr) for a wire, μ₀NI/(2R) for the centre of a coil, μ₀nI inside a solenoid; F = BIL·sin θ or |q|vB·sin θ for forces.
For induction, find the change of flux ΔΦ = Δ(BA·cos θ) and use ε = N·ΔΦ/Δt, or ε = BLv for a moving rod and NBAω for a rotating coil.
Find directions separately with the right-hand rules, and Lenz’s law for induced currents.

AC impedance & phasors

Impedances: Z_R = R, Z_L = jωL, Z_C = 1/(jωC)
Angular frequency: ω = 2πf
Magnitude and angle: |Z| = √(R² + X²), φ = atan(X/R)
Phasor Ohm’s law: V = I·Z

Method

Convert the frequency: ω = 2πf.
Find each reactance: X_L = ωL and X_C = 1/(ωC).
Add impedances like resistors, with Z = R + jX.
Find the magnitude |Z| = √(R² + X²) and the angle φ = atan(X/R), then use Ohm’s law with phasors.

2Practice problems

1.Coulomb’s law & electric fieldsMedium

Three point charges lie on a straight line: −2 µC on the left, −4 µC on the right 40 cm away, and −6 µC between them, 10 cm from the left charge. Find the magnitude of the net electric force F on the middle charge. Use k = 8.99 × 10⁹ N·m²/C².

q_L = −2 µCq_R = −4 µCq_M = −6 µCr_L = 10 cmr_R = 30 cm

F = N

2.Coulomb’s law & electric fieldsHard

Two identical small balls, each of mass 5 g, hang from the same point on insulating threads 40 cm long. They are given equal charges and each thread settles at 15° from the vertical. Use g = 9.81 m/s². Find the charge q on each ball in nC. Use k = 8.99 × 10⁹ N·m²/C².

m = 5 gL = 40 cmθ = 15°g = 9.81 m/s²

q = nC

3.Series & parallel resistorsMedium

Find the equivalent resistance R_eq seen by the source V₁.

V₁ = 6 VR₁ = 12 ΩR₂ = 20 ΩR₃ = 6 ΩR₄ = 3 Ω

R_eq = Ω

4.Series & parallel resistorsHard

Find the current I_R2 through R₂.

V₁ = 9 VR₁ = 15 ΩR₂ = 25 ΩR₃ = 2 ΩR₄ = 15 ΩR₅ = 25 ΩR₆ = 6 Ω

I_R2 = A

5.RC & RL transientsMedium

The inductor carries no current before the switch closes at t = 0. Find the energy W stored in the inductor at t = 1 ms, in mJ.

V_s = 20 VR = 50 ΩL = 33 mHt = 1 ms

W = mJ

6.RC & RL transientsHard

A timer starts charging an uncharged capacitor through a resistor from a 15 V supply at t = 0. It must trigger when the capacitor voltage reaches 9.48 V, exactly 50 ms after the start. With R = 1 kΩ, find the capacitance C needed.

V_s = 15 VV_th = 9.48 VT = 50 msR = 1 kΩ

C = µF

7.Magnetic fields & inductionMedium

A metal rod 20 cm long slides at 10 m/s along two rails in a uniform field of 0.2 T into the page. The rails are joined by a resistor of 4 Ω; the rod and rails have negligible resistance. Find the power P dissipated in the resistor.

B = 0.2 TL = 20 cmv = 10 m/sR = 4 Ω

P = mW

8.Magnetic fields & inductionHard

Two coils with self-inductances of 40 mH and 200 mH have a coupling coefficient of 0.6. The current in the first coil changes at 50 A/s. Find the emf ε induced in the second coil.

L₁ = 40 mHL₂ = 200 mHk = 0.6di/dt = 50 A/s

ε = V

9.AC impedance & phasorsMedium

A series RLC circuit is connected to a 120 V (rms), 200 Hz supply. Find the real power P taken from the supply.

V = 120 V (rms)f = 200 HzR = 40 ΩL = 150 mHC = 2.2 µF

P = W

10.AC impedance & phasorsHard

A series RLC circuit is connected to a 24 V (rms), 1000 Hz supply. Find the rms current I.

V = 24 V (rms)f = 1000 HzR = 15 ΩL = 20 mHC = 10 µF

I = A

3Solutions

1.Coulomb’s law & electric fields

Answer: F = 8.391 N

Coulomb’s law: two point charges attract or repel with F = k·|q₁·q₂|/r², with k = 8.99 × 10⁹ N·m²/C². The field of a point charge is E = k·|q|/r², pointing away from a positive charge and towards a negative one. Forces and fields from several charges add as vectors.

Force from each charge

Right is positive.

F_L = +k·2·6 × 10⁻¹²/0.1² = 10.79 N

F_R = −k·4·6 × 10⁻¹²/0.3² = −2.397 N

Net force

F = 10.79 + (−2.397) = 8.391 N → |F| = 8.391 N

Answer

F = 8.391 N

The net force points to the right.

2.Coulomb’s law & electric fields

Answer: q = 250.4 nC

Electric force from the equilibrium

The tension's horizontal part balances F, its vertical part balances m·g.

F = m·g·tan θ = 0.005·9.81·tan 15° = 0.01314 N

Separation

r = 2·L·sin θ = 2·0.4·sin 15° = 0.2071 m

Charge

F = k·q²/r² → q = r·√(F/k) = 0.2071·√(0.01314/8.99 × 10⁹) = 2.504 × 10⁻⁷ C = 250.4 nC

Answer

q = 250.4 nC

3.Series & parallel resistors

Answer: R_eq = 18.21 Ω

Series and parallel resistors: in series the same current flows through each resistor and R = R₁ + R₂ + …. In parallel each resistor has the same voltage and 1/R = 1/R₁ + 1/R₂ + …. A ladder network is reduced one pair at a time until a single equivalent resistance remains.

Combine resistors from the far end

Start at the end farthest from the source and work back towards it. Resistors carrying the same current are in series; resistors across the same two nodes are in parallel.

R₃ + R₄ = 6 + 3 = 9 Ω

R₂ ∥ 9 = 20·9/(20+9) = 6.207 Ω

R_eq = R₁ + 6.207 = 12 + 6.207 = 18.21 Ω

Answer

R_eq = 18.21 Ω

4.Series & parallel resistors

Answer: I_R2 = 0.1268 A

Combine resistors from the far end

Start at the end farthest from the source and work back towards it. Resistors carrying the same current are in series; resistors across the same two nodes are in parallel.

R₅ + R₆ = 25 + 6 = 31 Ω

R₄ ∥ 31 = 10.11 Ω

R₃ + 10.11 = 12.11 Ω

R₂ ∥ 12.11 = 8.158 Ω

R_eq = R₁ + 8.158 = 15 + 8.158 = 23.16 Ω

Current from the source

The source sees only R_eq, so Ohm’s law gives the current it delivers. That current flows through R₁.

I_R1 = V₁/R_eq = 9/23.16 = 0.3886 A

Current through R₂

Subtract the drop across R₁ to get the voltage at node A, which is the voltage across R₂.

V_A = V₁ − I_R1·R₁ = 9 − 0.3886·15 = 3.17 V

I_R2 = V_A/R₂ = 3.17/25 = 0.1268 A

Answer

I_R2 = 0.1268 A

5.RC & RL transients

Answer: W = 1.607 mJ

First-order transients: in a circuit with one capacitor or one inductor, every voltage and current moves exponentially from its initial value to its final value: x(t) = x(∞) + [x(0) − x(∞)]·e^−t/τ, with τ = RC or τ = L/R. After about 5τ the circuit has settled.

Time constant and final current

Long after the switch closes the inductor behaves like a short circuit, so the final current is set by R alone.

τ = L/R = 33 mH / 50 Ω = 0.66 ms

i(∞) = V_s/R = 20/50 = 0.4 A

Current at the given time

i(t) = 0.4·(1 − e^−1/0.66) = 0.3121 A

Stored energy

W = ½·L·i² = ½·33·0.3121² = 1.607 mJ

Answer

W = 1.607 mJ

6.RC & RL transients

Answer: C = 50.02 µF

Required time constant

9.48 = 15·(1 − e^−50/τ) → τ = 50/ln(15/(15 − 9.48))

τ = 50/0.9997 = 50.02 ms

Capacitance

kΩ times µF gives milliseconds.

C = τ/R = 50.02/1 = 50.02 µF

Answer

C = 50.02 µF

7.Magnetic fields & induction

Answer: P = 40 mW

Electromagnetic induction: the flux through a loop is Φ = BA·cos θ, with θ between B and the normal of the loop. Faraday's law: a change of flux induces emf = −N·dΦ/dt in a coil of N turns, and Lenz's law says the induced current flows so that its own field opposes the change. A rod moving across a field gives emf = BLv. A coil rotating at ω gives e = NBAω·sin ωt, peak NBAω. Self-inductance: emf = −L·di/dt, with L = μ₀N²A/l for a long solenoid, and the energy stored is W = ½LI². Two coupled coils: emf₂ = −M·di₁/dt with M = k√(L₁L₂).

Motional emf

ε = BLv = 0.2·0.2·10 = 0.4 V

Current

I = ε/R = 0.4/4 = 0.1 A

Power

P = εI = 0.4·0.1 = 0.04 W

Answer

P = 40 mW

8.Magnetic fields & induction

Answer: ε = 2.683 V

Mutual inductance

M = 0.6·√(40·200) = 53.67 mH

Induced emf

ε = M·di/dt = 0.05367·50 = 2.683 V

Answer

ε = 2.683 V

9.AC impedance & phasors

Answer: P = 18.22 W

AC circuits with phasors: resistors, inductors and capacitors have impedances R, jωL and 1/(jωC), with ω = 2πf. In series, impedances add like resistances, but as complex numbers. Rms current and voltage follow Ohm’s law with |Z|.

Reactances

X_L = 188.5 Ω, X_C = 361.7 Ω

Impedance

|Z| = √(40² + (−173.2)²) = 177.8 Ω

Real power

Inductors and capacitors only store and return energy; the resistor absorbs all of the real power.

I = 120/177.8 = 0.675 A

P = I²·R = 0.675²·40 = 18.22 W

Answer

P = 18.22 W

10.AC impedance & phasors

Answer: I = 0.2167 A

Reactances

ω = 2πf = 2π·1000 = 6283 rad/s

X_L = ωL = 6283·0.02 = 125.7 Ω

X_C = 1/(ωC) = 1/(6283·10·10⁻⁶) = 15.92 Ω

Impedance

In series the impedances add as complex numbers: Z = R + j(X_L − X_C).

|Z| = √(R² + (X_L − X_C)²) = √(15² + (109.7)²) = 110.8 Ω

Current

The same current flows through every element of a series circuit.

I = V/|Z| = 24/110.8 = 0.2167 A

Answer

I = 0.2167 A

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