| Electric charge |
A fundamental property of matter that causes it to experience a force when placed in an electromagnetic field. Charge comes in two polarities (positive and negative) and is quantized in integer multiples of the elementary charge \(e = 1.602 \times 10^{-19}\) C. Like charges repel; unlike charges attract. |
| Coulomb |
The SI unit of electric charge, symbol C. One coulomb is the amount of charge transported by a current of one ampere in one second. One coulomb equals approximately \(6.242 \times 10^{18}\) elementary charges (protons or electrons). |
| Current (ampere) |
The rate of flow of electric charge past a cross-section of a conductor, defined as \(I = dQ/dt\). The SI unit is the ampere (A), equal to one coulomb per second. Current is a signed quantity; its reference direction must be specified when writing circuit equations. |
| Conventional current |
The agreed-upon convention in circuit analysis where positive current flows from the positive terminal of a source, through the external circuit, to the negative terminal — opposite to the physical direction of electron flow. Conventional current direction is used in all standard circuit laws. |
| Voltage |
The electric potential difference between two points in a circuit, representing the energy per unit charge required to move a positive test charge between those points. Defined as \(V = dW/dQ\) and measured in volts (V), where 1 V = 1 J/C. |
| Electromotive force |
The energy per unit charge supplied by a source (battery, generator) that drives current around a circuit. EMF, measured in volts, is not literally a force but describes the source's ability to do work on charges. A real source has internal resistance that reduces the terminal voltage under load. |
| Power |
The rate at which energy is transferred or dissipated in a circuit element, given by \(P = VI = I^2R = V^2/R\) for a resistor. The SI unit is the watt (W), where 1 W = 1 J/s. Power is positive when absorbed (load) and negative when delivered (source). |
| Energy |
The capacity to do work, measured in joules (J) in SI units. In electric circuits, energy consumed or stored is \(W = Pt\) for constant power, or the integral \(W = \int p\, dt\) for time-varying power. Energy is also commonly expressed in watt-hours (Wh) or kilowatt-hours (kWh) for utility billing. |
| Resistance |
A property of a material or device that opposes the flow of electric current, converting electrical energy into heat. Resistance \(R\) is measured in ohms (Ω), defined by \(R = V/I\). For a cylindrical conductor it depends on resistivity, length, and cross-sectional area. |
| Conductance |
The reciprocal of resistance, \(G = 1/R\), measuring how easily current flows through an element. The SI unit is the siemens (S). Conductances in parallel add directly, which can simplify analysis of parallel networks. |
| Electrical ground |
The reference node in a circuit assigned a potential of 0 V, relative to which all other node voltages are measured. Ground does not need to be connected to physical earth; it is a mathematical reference datum chosen for analytical convenience. |
| Node |
A point in a circuit where two or more elements are connected together. All points connected by ideal (zero-resistance) conductors belong to the same node. Identifying nodes is the first step in the node voltage method of circuit analysis. |
| Branch |
A single circuit element (or group of elements in series) connected between two nodes. The number of branches, nodes, and loops in a network are related by Euler's formula for planar graphs: \(b = l + n - 1\). |
| Open circuit |
A break in a circuit path that presents theoretically infinite resistance between two points, so no current can flow through that path. A voltage may appear across the open terminals, but the current is zero. |
| Short circuit |
A zero-resistance connection between two circuit points, which forces the voltage across those points to zero and allows theoretically unlimited current to flow. Short circuits can be intentional (as in circuit analysis) or accidental (as in faults causing excessive current). |