Electric Charge and Force: every key term you need (+ practice quiz)

More AP Physics 1 guides

• Kinematics
• Forces and Newton's Laws
• Work, Energy and Power
• Momentum and Collisions
• Circular Motion and Gravitation

Electric Charge

Fundamental property (positive/negative); measured in coulombs (C); conserved and quantized (multiples of e).

Coulomb's Law

F = kq₁q₂/r²; electric force between charges proportional to charges, inversely proportional to distance squared.

Electric Field

E = F/q; force per unit charge; vector pointing from positive to negative; measured in N/C.

Potential Difference

V = W/q; energy per unit charge; measured in volts (V); same as voltage.

Capacitor

Device storing charge; C = Q/V; capacitance measured in farads (F); parallel plates: C = ε₀A/d.

Current

I = Q/t; rate of charge flow; measured in amperes (A); conventional current flows from + to -.

Resistance

R = V/I; opposition to current flow; measured in ohms (Ω); R = ρL/A (resistivity × length / area).

Ohm's Law

V = IR; voltage equals current times resistance; linear relationship for ohmic conductors.

Power

P = IV = I²R = V²/R; electrical power dissipated as heat (watts).

EMF

Electromotive force; energy per unit charge; source voltage (often ε or E); includes internal resistance.

Series Circuit

Components in single loop; current same everywhere; voltages add; total R = R₁ + R₂ + ...

Parallel Circuit

Components on separate branches; voltage same everywhere; currents add; 1/R_total = 1/R₁ + 1/R₂ + ...

Kirchhoff's Laws

Junction rule: current in = current out; loop rule: sum of voltages in loop = 0.

Magnetic Field

B field; created by moving charges; measured in tesla (T); detected using compass or magnetic force.

Magnetic Force

F = qvB sin θ; force on moving charge in magnetic field; perpendicular to both v and B.

Lorentz Force

F = q(E + v × B); total electromagnetic force on charge.

Ampere's Law

Current creates circular magnetic field; B increases with current, decreases with distance.

Faraday's Law

Changing magnetic flux induces EMF; ε = -N(ΔΦ/Δt); basis for electric generators.

Lenz's Law

Induced current opposes change causing it; determines direction of induced current/field.

Transformer

Device changing AC voltage; V₁/V₂ = N₁/N₂; uses mutual induction (changing field in primary coil).

Charge Conservation and Quantization

Net charge of an isolated system is constant, and charge comes in multiples of e = 1.6 × 10⁻¹⁹ C. Rubbing transfers electrons; it never creates charge.

Charging by Induction

A charged rod near a grounded conductor drives like charges to ground; removing the ground first, then the rod, leaves the conductor with charge opposite to the rod's. No contact needed.

Coulomb's Law Scaling

F ∝ q₁q₂/r². Doubling both charges quadruples the force; doubling the separation cuts it to one-fourth. Both charges feel equal-magnitude forces (third law), regardless of size.

Superposition of Electric Forces

The net force on a charge is the vector sum of forces from every other charge. Along a line, find where the pulls cancel; in 2D, resolve into components.

Resistivity and Geometry

R = ρL/A. Doubling the length doubles resistance; doubling the diameter quarters it (area ∝ d²). Stretching a wire to twice its length quadruples R since A halves too.

Ohmic vs Non-Ohmic

An ohmic resistor has a straight-line I-V graph through the origin. A light-bulb filament's resistance rises with temperature, so its I-V curve bends over.

Real Battery with Internal Resistance

Terminal voltage V = ε − Ir. Larger current draw means larger internal drop; a shorted battery delivers ε/r. The battery's power dissipates partly inside itself.

Brightness Reasoning

Bulb brightness tracks power P = I²R = V²/R. In series, equal current means the higher-R bulb is brighter; in parallel, equal voltage means the lower-R bulb is brighter.

Adding a Parallel Branch

Adding a resistor in parallel lowers total resistance, increases battery current, and (with internal resistance) slightly reduces terminal voltage. Existing parallel bulbs stay nearly the same brightness.

Adding a Series Resistor

Adding resistance in series raises total resistance and reduces the current everywhere, dimming every bulb in the loop.

Kirchhoff Junction Rule Logic

Current into a junction equals current out because charge does not accumulate in steady state. It is a statement of charge conservation, not energy conservation.

Kirchhoff Loop Rule Logic

The sum of potential changes around any closed loop is zero because potential is single-valued; this is energy conservation per unit charge.

Ammeter and Voltmeter Placement

An ammeter goes in series and has near-zero resistance; a voltmeter goes in parallel and has very high resistance. Swapping them shorts the circuit or reads nothing useful.

Energy Delivered by a Circuit

Energy = Pt = VIt. A kilowatt-hour is 3.6 × 10⁶ J. Compare bulbs by energy per second, not by voltage rating alone.

Equivalent Resistance Bounds

Series equivalent is larger than any single resistor; parallel equivalent is smaller than the smallest. Use this to sanity-check any network calculation.