AP Physics C: E&M key terms

A fundamental property of matter that causes it to experience a force when placed in an electromagnetic field. Charge comes in positive and negative types and is conserved in isolated systems.

The rule giving the magnitude of the electrostatic force between two point charges as proportional to the product of the charges and inversely proportional to the square of the distance between them, directed along the line joining them.

A vector field assigning to each point in space the force per unit positive charge that a test charge would experience there. Field lines point away from positive charge and toward negative charge.

A scalar measure of the number of electric field lines passing through a surface, computed as the surface integral of the field dotted with the area vector. It depends on field strength, area, and orientation.

A statement that the net electric flux through any closed surface equals the enclosed charge divided by the permittivity of free space. It links charge distribution to field and is especially useful with symmetric configurations.

The constant epsilon-naught appearing in Coulomb's law and Gauss's law, approximately 8.85 times ten to the minus twelve coulombs squared per newton meter squared. It sets the strength of the electric force in vacuum.

The rule that the net electric field or force at a point from several sources is the vector sum of the individual contributions computed as if each source acted alone. It allows complex charge distributions to be handled piece by piece.

A pair of equal and opposite charges separated by a small distance. In a uniform field it experiences a torque but no net force, and it produces a characteristic field falling off faster than a point charge's.

A model treating charge as spread smoothly along a line, over a surface, or through a volume, described by linear, surface, or volume charge density. Fields are found by integrating contributions from infinitesimal elements.

The electric potential energy per unit positive charge at a point, equal to the work per unit charge needed to bring a test charge from a reference point to that point. It is a scalar measured in volts.

The energy stored in a system of charges because of their positions relative to one another, equal to the work done by an external agent in assembling the configuration. For two point charges it varies inversely with separation.

The difference in electric potential between two points, equal to the work per unit charge done by the electric field on a charge moving between them. It drives current through a circuit element.

A surface on which the electric potential has the same value everywhere. It is always perpendicular to electric field lines, and no work is required to move a charge along it.

The line integral of the electric force along a path, equal to the charge times the potential difference between endpoints with a sign convention. For a conservative electrostatic field it is path independent.

A vector field whose line integral between two points depends only on the endpoints, not the path, and whose closed-loop integral is zero. The electrostatic field is conservative, which permits defining a potential.

A material containing charges free to move, so that in electrostatic equilibrium the internal field is zero, excess charge resides on the surface, and the surface is an equipotential. Metals are typical examples.

The ratio of the magnitude of charge stored on either plate of a capacitor to the potential difference between the plates. It depends only on geometry and the dielectric material, measured in farads.

A device consisting of two conductors separated by an insulator that stores charge and energy when a potential difference is applied. Common geometries include parallel plates, cylinders, and spheres.

An insulating material placed between capacitor plates that becomes polarized, reducing the internal field for a given charge and increasing capacitance by a factor called the dielectric constant.

The potential energy held by a charged capacitor, equal to one half the capacitance times the square of the voltage, or equivalently one half the charge times the voltage. It resides in the electric field between the plates.

The rate at which electric charge passes through a cross section, measured in amperes. Conventional current is taken in the direction positive charge would flow, opposite to electron drift.

A measure of how strongly a component opposes current, equal to the potential difference across it divided by the current through it. It depends on material resistivity, length, and cross-sectional area.

The relationship stating that for certain materials the current through a component is directly proportional to the potential difference across it, with resistance as the constant of proportionality, provided temperature and other conditions stay fixed.

The energy per unit charge supplied by a source such as a battery or generator to drive current around a circuit, often called emf. It equals the open-circuit potential difference of an ideal source.

The small resistance inherent in a real voltage source, which causes its terminal potential difference to drop below the emf as current increases. It limits the maximum current the source can deliver.

Two conservation-based principles for circuits: the junction rule states that charge entering a node equals charge leaving, and the loop rule states that potential changes around any closed loop sum to zero.

The rate at which electrical energy is converted to thermal energy in a resistor, equal to current times voltage, or current squared times resistance, or voltage squared divided by resistance.

A vector field that exerts forces on moving charges and on magnetic materials, measured in teslas. Field lines form closed loops, and the force on a moving charge is perpendicular to both velocity and field.

The force on a charge moving through a magnetic field, equal to the charge times the cross product of velocity and field. It is always perpendicular to velocity, so it does no work.

The net force on a straight segment of wire in a magnetic field, equal to current times length vector crossed with the field. Its direction follows from the right-hand rule.

A rule giving the magnetic field contribution from a small segment of current-carrying wire as proportional to current times length crossed with the unit vector to the point, divided by distance squared.

A statement that the line integral of the magnetic field around a closed loop equals the permeability of free space times the net current passing through the loop. It is most useful when symmetry makes the field constant along a path.

The constant mu-naught appearing in Ampere's law and magnetic force expressions, approximately four pi times ten to the minus seven tesla meters per ampere. It sets the strength of magnetic effects in vacuum.

A scalar measure of the magnetic field passing through a surface, computed as the surface integral of the field dotted with the area vector. It is the quantity whose change drives induced emf.

A statement that the induced emf around a loop equals the negative rate of change of magnetic flux through the loop. Changing field strength, loop area, or orientation all produce induction.

The principle that an induced current flows in the direction whose magnetic field opposes the change in flux that produced it. The minus sign in Faraday's law expresses this opposition.

The current driven around a conducting loop by a changing magnetic flux, as described by Faraday's law. Its direction is set by Lenz's law and its size depends on the loop's resistance.

The proportionality between the magnetic flux through a circuit and the current producing it, measured in henries. It quantifies how strongly a changing current induces an opposing emf in a coil.