Kirchhoff's junction rule
Answer: The total current entering a junction equals the total current leaving it.
Explanation
Conservation of charge.
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Physics: Electricity and Circuits (High School)
CircuitsAC/DCInductionMagnetismChargeCurrentPotential
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Circuits
What does a fuse or circuit breaker do?
Answer: Breaks the circuit when current exceeds a safe level, preventing overheating and fires.
Explanation
Both protect wiring, not people; GFCIs protect people.
What is EMF and internal resistance of a battery?
Answer: EMF is the battery's ideal voltage; internal resistance causes the terminal voltage to drop under load: V = EMF − Ir.
Explanation
Terminal voltage equals EMF only with no current.
What does a voltmeter measure and how is it connected?
Answer: Potential difference; connected in parallel across the component.
Explanation
Ammeters measure current and are connected in series.
Kirchhoff's loop rule
Answer: The sum of potential changes around any closed loop is zero.
Explanation
Conservation of energy.
What happens to the other bulbs when one bulb in a series circuit burns out?
Answer: They all go out because the circuit is broken.
Explanation
In parallel, the others stay lit.
How do resistors combine in parallel?
Answer: 1/R_total = 1/R1 + 1/R2 + …; each has the same voltage; currents add.
Explanation
Total resistance is less than the smallest branch.
How do resistors combine in series?
Answer: R_total = R1 + R2 + …; the same current flows through each; voltages add.
Explanation
Total resistance increases.
AC/DC
What is the difference between AC and DC?
Answer: DC flows in one direction at constant voltage (batteries); AC reverses direction periodically (wall outlets, 60 Hz in the US).
Explanation
Transformers only work with AC.
Induction
What does a transformer do?
Answer: Changes AC voltage using two coils: Vs/Vp = Ns/Np.
Explanation
Step-up transformers raise voltage for transmission to reduce I²R losses.
How does a generator work?
Answer: Rotating a coil in a magnetic field changes the flux, inducing an alternating EMF.
Explanation
A motor is a generator in reverse.
What does Lenz's law say?
Answer: The induced current flows in a direction that opposes the change in flux that produced it.
Explanation
Explains the minus sign in Faraday's law and conservation of energy.
State Faraday's law of induction.
Answer: A changing magnetic flux through a loop induces an EMF: EMF = −N ΔΦ/Δt.
Explanation
Flux Φ = BA cosθ.
Magnetism
What does the right-hand rule give for a straight current-carrying wire?
Answer: Thumb along the current, fingers curl in the direction of the magnetic field circling the wire.
Explanation
Field strength decreases with distance from the wire.
What is the force on a current-carrying wire in a magnetic field?
Answer: F = BIL sinθ
Explanation
Basis of electric motors.
What is the force on a charge moving in a magnetic field?
Answer: F = qvB sinθ, directed by the right-hand rule, perpendicular to both v and B.
Explanation
A charge moving parallel to B feels no force.
What produces a magnetic field?
Answer: Moving electric charges (currents) and magnetic materials.
Explanation
Field lines form closed loops from north to south outside a magnet.
Charge
What is Coulomb's law?
Answer: F = k·q1·q2 / r², the force between two point charges; k ≈ 8.99 × 10⁹ N·m²/C².
Explanation
Like charges repel, opposite charges attract.
Conductors vs insulators?
Answer: Conductors let charge move freely (metals); insulators do not (rubber, glass).
Explanation
Semiconductors are in between.
What is the elementary charge?
Answer: The charge of a proton, e ≈ 1.6 × 10⁻¹⁹ C; an electron has −e.
Explanation
Charge is quantized in multiples of e.
What is an electric field and what is its unit?
Answer: The force per unit positive charge at a point: E = F/q, measured in N/C (or V/m).
Explanation
Field lines point away from positive charges and toward negative.
Current
Electrical power formulas
Answer: P = IV = I²R = V²/R
Explanation
Measured in watts; energy = power × time (kWh on electric bills).
What factors determine a wire's resistance?
Answer: R = ρL/A: resistivity of the material, length, and cross-sectional area.
Explanation
Longer and thinner wires have more resistance; resistance rises with temperature in metals.
State Ohm's law.
Answer: V = IR: voltage equals current times resistance.
Explanation
Ohmic materials have constant resistance.
Define electric current and its unit.
Answer: The rate of flow of charge: I = Q/t, measured in amperes (1 A = 1 C/s).
Explanation
Conventional current flows from positive to negative.
Potential
What is a capacitor and what does capacitance measure?
Answer: A device that stores charge on two separated conductors; C = Q/V, in farads.
Explanation
Energy stored: U = ½CV².
What is electric potential (voltage)?
Answer: Electric potential energy per unit charge, measured in volts (1 V = 1 J/C).
Explanation
Potential difference drives current.
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