Physics terms in Fields

A device that converts kinetic energy into electrical energy by rotating a coil in a magnetic field (or a magnet inside coils), producing an alternating emf.

An emf that reverses direction periodically; a coil rotating at constant speed in a uniform field gives $\epsilon =NBA\omega \sin \omega t$.

The point of an orbit around the Sun that is farthest from the Sun, where the orbiting body moves slowest.

The mean distance between the Earth and the Sun, $1\text{ AU}=1.50\times {10}^{11}\text{ m}$.

The fluid resistance a satellite meets from the thin upper atmosphere; it removes energy, so the orbit shrinks and the satellite's speed increases.

Light emitted high in the atmosphere near the polar regions by atoms excited by charged particles from the Sun, guided there by the Earth’s magnetic field.

The charge of a particle divided by its mass, $q\mathrm{/}m$, in ${\text{C kg}}^{-1}$; it sets the radius and period of the particle’s path in a magnetic field.

Charging a conductor by touching it with a charged object, so that some of the charge spreads on to it; the conductor gains charge of the same sign.

Charging two different materials by rubbing them together, which transfers electrons from one to the other and leaves them with equal and opposite charges.

The principle that the total electric charge of an isolated system stays constant: charge can be transferred but never created or destroyed.

The constant in Coulomb's law, $k=1\mathrm{/}4\pi {\epsilon }_0=8.99\times {10}^9{\text{N m}}^2{\text{C}}^{-2}$.

The electric force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation: $F=k{q}_1{q}_2\mathrm{/}{r}^2$.

Uniform electric and magnetic fields at right angles to each other, arranged so that their forces on a moving charge act in opposite directions.

A pivoted frame that measures a magnetic flux density by balancing the magnetic force on a current-carrying wire against the weight of a known mass.

Connecting a conductor to Earth so that electrons can flow between them, which neutralises the conductor or removes charge of one sign from it.

Loops of induced current that flow inside a solid piece of metal when the magnetic flux through it changes; they oppose the change and dissipate energy in the metal.

The curving and spreading of field lines near the ends of parallel plates, where the field is weaker and no longer uniform.

A scalar property of particles such as protons and electrons that makes them exert electric forces on each other; it is positive or negative and measured in coulombs (C).

A region in which a charge experiences an electric force.

The electric force per unit charge on a small positive test charge, $E=F\mathrm{/}q$; a vector measured in ${\text{N C}}^{-1}$ or ${\text{V m}}^{-1}$.

The work done per unit charge in bringing a small positive test charge from infinity to a point, $V_{\mathrm{e}}=kQ\mathrm{/}r$ for a point charge; a scalar measured in volts.

The work done to assemble a system of charges from infinite separation; for two point charges $E_{\mathrm{p}}=k{q}_1{q}_2\mathrm{/}r$, positive for like charges and negative for unlike charges.

The sudden conduction of an insulator such as air when the electric field becomes strong enough to ionise it, producing a spark; for dry air about $3\times {10}^6{\text{V m}}^{-1}$.

The production of an emf in a circuit when the magnetic flux linking it changes.

A device that produces a narrow beam of electrons: electrons released from a heated cathode are accelerated through a potential difference towards an anode and pass through a hole in it.

The separation of charge in a conductor caused by a nearby charged object; with earthing it charges the conductor, without contact, with the opposite sign to the object.

A closed oval curve with two points inside it called foci; planetary orbits are ellipses with the Sun at one focus.

A surface on which every point has the same potential; no work is done moving along it, and field lines cross it at right angles.

The minimum launch speed for an unpowered object to escape completely from a body's gravitational field, $v_{\text{esc}}=\sqrt{2GM\mathrm{/}r}$.

A conducting enclosure; because the electric field inside a conductor's cavity is zero, it shields its contents from external electric fields.

The emf induced in a circuit equals the rate of change of the magnetic flux linkage through it: $\epsilon =-N\mathrm{\Delta }\mathrm{\Phi }\mathrm{/}\mathrm{\Delta }t$.

A line showing the direction of the force on a test mass (or charge) at each point; the closer the lines, the stronger the field.

A glass bulb containing an electron gun and gas at low pressure, in which an electron beam bent into a circle by a magnetic field is made visible by the glowing gas.

A rule for the direction of the magnetic force: with the first finger along the field and the second finger along the conventional current, the thumb points along the force.

The product $N\mathrm{\Phi }$ of the number of turns of a coil and the magnetic flux through each turn; measured in webers.

A sensitive ammeter, usually with its zero in the centre of the scale, used to detect small currents in either direction.

A sensor that detects ground vibrations: relative motion between a coil and a magnet, one of them held by a spring, induces an emf proportional to the speed of the ground.

A circular equatorial orbit with a period of one day, travelling west to east, so the satellite stays above the same point on the Earth; it is about 36 000 km above the surface.

The constant $G$ in Newton's law of gravitation, $6.67\times {10}^{-11}{\text{N m}}^2{\text{kg}}^{-2}$.

A region of space in which a mass experiences a force because of its mass.

The work done per unit mass to bring a small test mass from infinity to a point, $V_{\mathrm{g}}=-GM\mathrm{/}r$; a scalar measured in ${\text{J kg}}^{-1}$, zero at infinity.

A current driven round a complete circuit by an induced emf, $I=\epsilon \mathrm{/}R$.

A cooker in which a rapidly alternating magnetic field from a coil induces eddy currents in the metal base of a pan, heating the pan directly.

Each planet moves in an ellipse with the Sun at one focus.

The line joining a planet to the Sun sweeps out equal areas in equal intervals of time, so the planet moves fastest when closest to the Sun.

For bodies orbiting the same central mass, the square of the orbital period is proportional to the cube of the semi-major axis (the radius for a circular orbit): $T^2\propto r^3$.

The direction of an induced emf is such that the current it would produce opposes the change in flux that causes it; a consequence of the conservation of energy.

An orbit a few hundred to about two thousand kilometres above the Earth's surface, with a period of roughly 90 to 120 minutes.

A region in which a magnet, a magnetic material or a moving charge experiences a magnetic force.

A line showing the direction in which the north pole of a small compass would point at each point; the lines form closed loops, running from N to S outside a magnet.

For a flat area $A$ in a uniform field, $\mathrm{\Phi }=BA\cos \theta$, where $\theta$ is the angle between the field and the normal to the area; measured in webers.

The force per unit charge per unit speed on a charge moving at right angles to a magnetic field, $B=F\mathrm{/}qv$; a vector measured in tesla.

The force a magnetic field exerts on a moving charge, $F=qvB\sin \theta$, perpendicular to both the velocity and the field.

One of the two ends of a magnet, north or south, where its field is most concentrated; like poles repel and unlike poles attract, and poles always come in pairs.

An instrument that separates ions by their charge-to-mass ratio, using the different radii of their paths in a magnetic field, to measure the masses and abundances of isotopes.

An experiment (1909–1913) in which charged oil drops were held stationary in a uniform electric field, showing that charge is quantised and measuring the elementary charge.

The force on a current-carrying conductor in a magnetic field, $F=BIL\sin \theta$, used in electric motors and loudspeakers.

A point where the fields of two or more bodies cancel, so the resultant field strength is zero.

Every point mass attracts every other with a force proportional to the product of the masses and inversely proportional to the square of their separation: $F=G{m}_1{m}_2\mathrm{/}{r}^2$.

The total (kinetic plus gravitational potential) energy of a body in a circular orbit, $E_{\text{total}}=-GMm\mathrm{/}2r$; it is negative because the body is bound.

The time for one complete orbit; for a circular orbit $T^2=4{\pi }^2{r}^3\mathrm{/}GM$.

The speed of a body in a circular orbit of radius $r$ around a mass $M$, $v=\sqrt{GM\mathrm{/}r}$, found by setting the gravitational force equal to the centripetal force.

The point of an orbit around the Sun that is closest to the Sun, where the orbiting body moves fastest.

The constant ${\mu }_0=4\pi \times {10}^{-7}{\text{T m A}}^{-1}$ that sets the strength of the magnetic field produced by a current in a vacuum.

A property $\epsilon$ of a medium that determines the electric force between charges in it; the larger the permittivity, the weaker the force.

The constant ${\epsilon }_0=8.85\times {10}^{-12}{\text{C}}^2{\text{N}}^{-1}{\text{m}}^{-2}$ that sets the strength of electric forces between charges in a vacuum.

A charge whose size is negligible compared with the distances involved, so that it can be treated as concentrated at a single point.

A body whose size is negligible, so that all its mass can be treated as being at one point.

The rate of change of potential with distance, $\mathrm{\Delta }V\mathrm{/}\mathrm{\Delta }r$; the field strength is minus the potential gradient.

The fact that electric charge exists only in whole-number multiples of the elementary charge, $q=ne$.

A field whose lines point straight towards (or away from) a single central point, such as the field outside a spherical mass; its strength falls with distance.

The ratio ${\epsilon }_{\mathrm{r}}=\epsilon \mathrm{/}{\epsilon }_0$ of a medium's permittivity to that of a vacuum; the factor by which the medium reduces the force between charges.

A rule for the field of a current: grip the wire with the right hand, thumb along the conventional current, and the curled fingers give the direction of the magnetic field lines.

The emf induced in a coil by a change in its own current, which changes its own flux linkage; by Lenz’s law it opposes the change in current.

Half the longest diameter of an ellipse; for an orbit, the mean of the closest and farthest distances from the central body.

A metal ring on the axle of a generator, connected to one end of the coil, on which a fixed brush slides so that the coil stays connected to the external circuit as it turns.

The continuous stream of charged particles, mainly protons and electrons, flowing outwards from the Sun.

A long coil of wire wound as a helix; a current in it produces a strong, uniform magnetic field inside and a field like a bar magnet's outside.

The SI unit of magnetic flux density: the field that exerts a force of 1 N on a 1 m length of wire carrying 1 A at right angles to it; $1\text{ T}=1{\text{N A}}^{-1}{\text{m}}^{-1}$.

A small positive charge used to detect and measure an electric field, small enough not to disturb the charges producing the field.

A mass small enough not to disturb the field it is placed in, used to define the field strength or potential at a point.

The release of electrons from the surface of a heated metal, when some electrons gain enough energy to escape.

An instrument that measures a very small force from the angle through which it twists a rod hanging on a thin wire; Cavendish used one to measure $G$ in 1798.

A field with the same magnitude and direction at every point, shown by parallel, equally spaced field lines.

An arrangement of crossed electric and magnetic fields that lets through undeflected only particles with speed $v=E\mathrm{/}B$.

The SI unit of magnetic flux: $1\text{ Wb}=1{\text{T m}}^2$, equivalent to $1\text{ V s}$.

The sensation of having no weight when a body and its surroundings are in free fall together, so no support force acts on it; gravity still acts.