AP Inter Physics (BIEAP) key terms

An expression showing which base quantities make up a derived quantity and with what powers, written with square brackets such as [M L T⁻²] for force. It is used to check equations and convert units.

The rule that every term in a physically correct equation must have the same dimensions. It allows checking whether a formula can be right, though it cannot supply numerical constants.

The digits in a measured value that carry real information about precision, counted from the first non-zero digit and including one estimated digit. The result of a calculation is rounded to match the least precise input.

The smallest measurement an instrument can read, found by dividing the value of one main-scale division by the number of vernier or screw divisions. It sets the limit on the uncertainty of a reading.

The absolute error of a measurement expressed as a fraction of the accepted value and multiplied by a hundred. For products and quotients, relative errors of the factors add.

The velocity of one body as observed from another moving body, obtained by vector subtraction of the observer's velocity from the object's velocity. It explains why two trains passing seem to move faster.

Two-dimensional motion of a body launched into the air and then moving only under gravity, treated as independent horizontal uniform motion and vertical uniformly accelerated motion. Range is greatest at a 45-degree launch angle.

Motion along a circle at constant speed, in which velocity keeps changing direction so a centripetal acceleration v²/r acts towards the centre. The angular velocity remains constant.

The constant acceleration g of a freely falling body near Earth's surface, about 9.8 m s⁻² and directed downward. It is independent of the mass of the falling body in the absence of air resistance.

The tendency of a body to resist any change in its state of rest or uniform motion, measured by its mass. Greater mass means greater inertia and a smaller acceleration for the same applied force.

The net force on a body equals the rate of change of its momentum, which reduces to F = ma when mass is constant. Force and acceleration are always in the same direction.

The product of a force and the time over which it acts, equal to the change in momentum it produces. Extending the contact time reduces the force, as in airbags and catching a ball by moving the hands back.

The product of a body's mass and its velocity, a vector quantity measured in kg m s⁻¹. In an isolated system the total momentum before and after a collision is unchanged.

The net inward force needed to keep a body moving in a circle, of magnitude mv²/r, supplied by friction, tension, gravity or a normal reaction. It does no work because it is perpendicular to velocity.

Raising the outer edge of a curved road so that a component of the normal reaction supplies the centripetal force. At the design speed no friction is needed to keep a vehicle on the curve.

The net work done on a body by all forces equals the change in its kinetic energy. It holds even when forces vary and gives a shortcut for finding speeds without analysing time.

The energy a body possesses because of its motion, equal to ½mv², a scalar measured in joules. It depends on the square of speed, so doubling speed quadruples it.

A force for which the work done between two points is independent of the path taken, so it can be described by a potential energy. Gravity and spring force are examples; friction is not.

A collision in which both total momentum and total kinetic energy are conserved, as between ideal billiard balls. In two dimensions the bodies may leave at an angle whose sum is 90 degrees when masses are equal.

Oscillation in which the restoring force, and hence acceleration, is directly proportional to displacement from equilibrium and directed towards it. Displacement follows a sine or cosine function of time.

The time taken for one complete oscillation of a periodic motion, the reciprocal of frequency. For a simple pendulum it depends only on length and g, not on the mass of the bob.

A large-amplitude response that occurs when the driving frequency of an external periodic force matches the natural frequency of an oscillator. Damping reduces the sharpness and height of the resonance peak.

A periodic variation in loudness heard when two sound waves of slightly different frequencies superpose, with beat frequency equal to the difference of the two frequencies. It is used to tune musical instruments.

The apparent change in the observed frequency of a wave when the source, the observer or the medium moves relative to the others. The pitch rises as a source approaches and falls as it recedes.

Complete reflection of light back into a denser medium when it strikes the boundary at an angle greater than the critical angle. It requires light travelling from denser to rarer medium and makes optical fibres work.

The angle of incidence in the denser medium for which the refracted ray grazes the boundary at 90 degrees, given by sin C = 1/n. Beyond it, total internal reflection takes place.

The ratio of the speed of light in vacuum to its speed in a medium, or equivalently the sine of the angle of incidence to the sine of the angle of refraction. It measures how strongly a medium bends light.

A vector for a pair of equal and opposite charges, equal to charge times separation, directed from negative to positive charge and measured in coulomb metre. It determines the torque a dipole feels in a field.

The force per unit positive charge experienced at a point in space, a vector measured in N C⁻¹ or V m⁻¹. Field lines run from positive to negative charge and never cross each other.

The work done per unit positive charge in bringing it from infinity to a point, a scalar measured in volts. Potential difference between two points equals the work per unit charge moved between them.

The ability of a conductor or capacitor to store charge per unit potential difference, C = Q/V, measured in farads. For a parallel-plate capacitor it equals ε₀A/d when no dielectric fills the gap.

The small average velocity along a conductor acquired by free electrons under an applied field, typically of order 10⁻⁴ m s⁻¹. Current equals the product of charge density, area and drift velocity.

At constant temperature the current through a metallic conductor is directly proportional to the potential difference across it, so V = IR. Devices obeying it have a straight-line current-voltage graph.

A property of the material of a conductor, equal to resistance times area divided by length, measured in ohm metre. It rises with temperature for metals and falls for semiconductors.

The total current entering a junction in a circuit equals the total current leaving it, a statement of conservation of charge. It is applied together with the loop rule to solve multi-loop circuits.

The force on a charge q moving with velocity v in a magnetic field B, of magnitude qvB sin θ and directed perpendicular to both v and B. It does no work and provides the centripetal force in circular motion.

The number of field lines passing through a surface, equal to B A cos θ, measured in weber. A changing flux through a circuit is what induces an electromotive force.

The production of an electromotive force in a conductor whenever the magnetic flux linked with it changes. The induced emf equals the negative rate of change of flux, and induced current opposes the change.

The emission of electrons from a metal surface when light of frequency above a threshold falls on it. It shows light behaves as particles, since emission depends on frequency and is instantaneous.

The minimum energy needed to remove an electron from the surface of a metal, usually quoted in electron volts. The maximum kinetic energy of a photoelectron is photon energy minus the work function.

The wavelength associated with a moving particle, equal to Planck's constant divided by its momentum. It is measurable for electrons but negligibly small for everyday objects.

The time in which half of the radioactive nuclei in a sample decay, independent of the amount present. After n half-lives the fraction remaining is (1/2)ⁿ.

The difference between the total mass of the nucleons in a nucleus and the mass of the nucleus itself, appearing as binding energy through E = mc². It explains why energy is released in fission and fusion.

The boundary formed when p-type and n-type semiconductors are joined, with a depletion region and a built-in potential barrier. It conducts easily under forward bias and hardly at all under reverse bias.

A heavily doped p-n junction designed to operate safely in reverse breakdown, where the voltage across it stays nearly constant. It is used as a voltage regulator, always connected in reverse bias.

A circuit that converts alternating current into direct current, using one diode for half-wave and four in a bridge for full-wave output. A capacitor filter smooths the pulsating output.

The process of superimposing an information signal onto a high-frequency carrier wave so it can be transmitted efficiently. Amplitude, frequency and phase are the quantities commonly varied.