Current is the flow of electrons through a conductor. Electrons are negatively charged particles, so real current flows in the direction of increasing voltage.
Conventional current assumes moving charges are positive and moves from regions of high electric potential to regions of low electric potential. All currents on Test Day will be conventional.
The current that comes out of a battery is direct current, i.e., its magnitude stays constant over time: I = Δq / Δt.
Electromotive force (EMF) ε is the electric potential difference (or voltage) that drives current. An EMF source supplies electric potential energy to charges.
Kirchoff's Loop Rule: The sum of the voltage drops across a closed circuit has to equal the sum of the voltage gains across the circuit.
Voltage drops across all resistors in the circuit will equal the terminal voltage V gained by the battery.
The resistance R of a conductor is a measure of the resistance to current flow. Resistivity ρ of a material is a measure of its resistance to current flow. Higher resistivity means a given potential difference will produce a lower current across it.
The resistance of a resistor depends on the resistivity of the material: R = ρL / A, where A is the cross-sectional area through which the current flows and L is the length of the resistor.
Ohm's Law states the voltage drop across resistors is proportional to the current passing through them: V = IR.
The units of electric current are ampere. where 1 A = 1 C/s. The units of resistance are an ohm, where 1 Ω = 1 V/A.
Where two or more resistors are in series, the current must pass through all of them. The total resistance would be: R_tot = R_1 + R_2.
When two or more resistors are in parallel, the current is split between the different paths and the voltage across each path is the same. The total resistance would be: 1/R_tot = 1/R_1 + 1/R_2.
When placed in a circuit, a battery has its on internal resistance which contributes to the total resistance of the circuit. Non-zero values reduce the effective terminal potential of the battery.
When a battery has internal resistance r, the terminal potential V of the battery is less than the EMF: V = ε - Ir. The relationship between the current and the EMF is: I = ε / (R + r).
Two conductors with a potential difference between them, placed a short distance apart, can be used to store electrical energy as a capacitor. When two uncharged conductors are connected to opposite terminals of a battery, equal amounts of positive and negative charge move to each of the conductors.
The amount of charge that a capacitor can hold depends on the capacitance: C = Q/V. The units of capacitance are the Farad: 1 F = 1 C/V.
When a battery is hooked up to an uncharged capacitor, it must do work in charging the capacitor. When the capacitor is fully charged by a battery with terminal voltage V, the potential energy stored is: U = (1/2) CV^2, where C is the capacitance.
The parallel-plate capacitor consists of two float conducting plates, with surface area A separated by distance d. The capacitance is: C = ε_0 · A / d, where the permittivity of free space ε_0 = 8.85 x 10e-12 C^2 / (N · m^2).
When two or more capacitors are in series, each capacitor has to store the same amount of charge: 1/C_tot = 1/C_1 + 1/C_2. The voltage drop across capacitors in parallel are the same: C_tot = C_1 + C_2.
A dielectric is a non-conducting material place between the two plates of a capacitor (e.g., air, rubber, glass and certain plastics). When a dielectric is inserted between the plates of a capacitor, the capacitance increases: C_new = KC_old, where K is the dielectric constant of the material.
Alternating current (AC) current and voltage periodically reverse direction. In a simple sircuit with resistors, the current will also vary sinusoidally in time: I = V/R = (V_max/R) sin ωt = I_max sin ωt.
Because current and voltage change direction periodically, both average out to zero. The root-mean-square (rms) current and voltage are given by: I_rms = I_max/sqrt(2) and V_rms = V_max/sqrt(2).
Electric power is: P = IV = (I^2)R.
