Lecture 3 - Basic Electrical Units

Vocabulary flashcards covering basic electrical units, Ohm’s Law, passive sign convention, and related concepts from the lecture notes.

Voltage

Electrical potential difference between two points in a circuit; unit: Volts (V); 1 V = 1 joule per coulomb (1 J/C).

Current

Rate of flow of electric charge through a conductor; unit: Amperes (A); 1 A = 1 C/s; flows from high to low voltage.

Resistance

Measure of how a circuit impedes current flow; unit: Ohms (Ω); determined by material and geometry; higher resistance means less current for a given voltage.

Power

Rate of energy transfer in a circuit; unit: Watts (W); P = V × I; positive when energy is absorbed, negative when delivered.

Kilowatt-hour

Unit of energy equal to one kilowatt of power used for one hour; 1 kWh = 1000 W × 3600 s = 3.6 MJ.

Ohm's Law

Relationship between V, I, R: V = I × R; equivalently I = V / R and R = V / I.

Passive Sign Convention

Current is assumed to flow from higher potential to lower potential; power is positive when energy is absorbed (P = V × I > 0) and negative when delivered.

Voltage drop

Voltage difference across an element when current flows; for a resistor, V = I × R.

Reference direction

A chosen direction to define currents and voltages (e.g., i_ab points from node a to b); ensures consistent sign conventions.

Electron flow vs conventional current

Conventional current flows from high to low potential; electrons move opposite; energy transfer via the electromagnetic field propagating near the speed of light.

Energy transfer in circuits

Resistors absorb energy; sources deliver energy to the circuit.

Volt

Unit of voltage; symbol V; equals energy per unit charge (1 V = 1 J/C).

Ampere

Unit of current; symbol A; equals 1 coulomb per second (1 C/s).

Ohm

Unit of resistance; symbol Ω; equals 1 V per A.

Watt

Unit of power; symbol W; equals 1 volt × 1 ampere; also 1 J/s.

Elementary charge

Magnitude of the charge of a single electron or proton: e ≈ 1.602 × 10^-19 C.

Coulomb

Unit of electric charge; 1 C ≈ 6.242 × 10^18 elementary charges; 1 A = 1 C/s.

1 A = 1 C/s

Definition linking amperes to coulombs per second.

1 V = 1 J/C

Definition linking volts to joules per coulomb.

Current reference i_ab

Current variable iab points from node a to b; iba is the opposite; iab = - iba.

Voltage reference vab

Voltage vab is positive at node a and negative at node b; vab = - v_ba.

Power dissipation in a resistor (P = I^2R)

Power dissipated as heat in a resistor equals I^2 times R; another form is P = V^2 / R or P = V × I.

Power dissipation in a resistor (P = V^2 / R)

Alternative form of resistor power using voltage: P = V^2 / R.

Power formula (P = V × I)

General power relation: power equals voltage times current.