Fields and their Consequences

Question 1

Back Emf

Answer 1

An emf generated by the coil in an AC generator that acts against the potential difference that has been supplied to the motor.

Question 2

Capacitance (C)

Answer 2

The charge stored per unit pd in a capacitor.

Question 3

Capacitor

Answer 3

An electrical component that stores charge. A parallel-plate capacitor is made of two parallel conducting plates with an insulator between them (dielectric).

Question 4

Coulomb’s Law

Answer 4

The size of the force that acts between two point charges is proportional to the product of their charges and inversely proportional to the square of their separation. It is attractive for opposite charges and repulsive for like charges.

Question 5

Cyclotron

Answer 5

A particle accelerator made up of two D shaped electrodes positioned opposite each other. The electric field changes direction each time a particle moves from one electrode to the other, causing the particle to accelerate.

Question 6

Dielectric

Answer 6

An insulating material placed between the two plates of a capacitor in order to increase the amount of charge it can store.

Question 7

Electrical conductor

Answer 7

A material that contains free electrons that are able to move throughout the material and carry charge.

Question 8

Electrical insulator

Answer 8

A material that does not contain any free electrons. All electrons in an insulator are part of the individual atoms that make it up.

Question 9

Electric field

Answer 9

A region surrounding a charged object which causes a force to be exerted on any charged object placed within the field.

Question 10

Electric field strength (E)(at a point in the field)

Answer 10

The force per unit positive charge exerted on a charged object placed at that point in the field. This is a vector acting in the same direction as the force on a positive charge.

Question 11

Electric potential (V)(at a point in the field)

Answer 11

The work done per unit charge on a positive test charge in bringing it from infinity to that point in the field.

Question 12

Electromagnetic induction

Answer 12

When an emf is induced in a wire/conducting rod when it is moved relative to a magnetic field.

Question 13

Equipotential

Answer 13

A surface of constant potential. No work is done by the field when an object moves along an equilibrium.

Question 14

Escape velocity

Answer 14

The minimum velocity required by an object to be able to escape a gravitational field a mass when projected vertically from its surface.

Question 15

Faraday’s Law

Answer 15

The magnitude of the induced emf is equal to the rate of change of flux linkage through the circuit.

Question 16

Field line / line of force

Answer 16

A line representing the path that a North Pole (magnetic field), positive charge (electric field) or mass (gravitational field) would take when placed within the field.

Question 17

Force field

Answer 17

An area in which an object will experience a non-contact force.

Question 18

Geostationary satellite

Answer 18

A satellite that orbits above the equator with a 24 hour period, so it will always remain above the same position on the Earth. They orbit approximately 36,000km above the surface of the Earth.

Question 19

Gravitational field

Answer 19

A region surrounding a mass in which any other object with mass will experience an attractive force.

Question 20

Gravitational field strength

Answer 20

The force per unit mass exerted on a small test mass placed within the field.

Question 21

Gravitational potential (V)(at a point in the field)

Answer 21

The work done per unit mass required to move a small test mass from infinity to that point.

Question 22

Gravitational potential energy

Answer 22

The component of a object’s energy due to its position in a gravitational field.

Question 23

Kepler’s Third Law

Answer 23

The square of an object’s orbital period (T) is directly proportional to the cube of its orbital radius (r).

Question 24

Lenz’s Law

Answer 24

An induced current is always in a direction so as to oppose the change that caused it.

Question 25

Magnetic field

Answer 25

A region surrounding a magnet or current-carrying wire that will exert a force on any other magnet or current-carrying wire placed within it.

Question 26

Magnetic flux

Answer 26

A value which describes the magnetic field or field lines passing through an area. It is the product of magnetic flux density and the perpendicular area it passes through.

Question 27

Magnetic flux density (B)

Answer 27

The force per unit current per unit length on a current-carrying wire placed at 90º to the field lines. Sometimes also referred to as the magnetic field strength.

Question 28

Magnetic flux linkage

Answer 28

The magnetic flux multiplied by the number of turns, N, of the coil.

Question 29

Motor effect

Answer 29

When a current-carrying wire is placed within a magnetic field (non-parallel to the field lines) and experiences a force perpendicular to both the wire and the field lines.

Question 30

Permittivity of free space

Answer 30

A measure of the ability of a vacuum to allow an electric field to pass through it.

Question 31

Polarised

Answer 31

An atom/molecule becomes polarised when an external electric field causes the negative electron cloud to be shifted in the opposite direction to the positive nucleus - the charges are pulled in opposite directions. (This is what happens to the molecules of the dielectric in a capacitor).

Question 32

Potential gradient

Answer 32

The change of potential per metre at a point in the field.

Question 33

Radial field

Answer 33

A field in which the field lines are all directed towards a single point (e.g. the centre of a planet or a point charge).

Question 34

Relative permittivity

Answer 34

The ratio of charge stored in a capacitor with the dielectric to charge stored without the dielectric. Also sometimes referred to as the dielectric constant.

Question 35

Step-down transformer

Answer 35

A device made of two insulated wires coiled around an iron core in which the output voltage is smaller than the input voltage due to the secondary coil having fewer turns than the primary coil.

Question 36

Step-up transformer

Answer 36

A device made of two insulated wires coiled around an iron core in which the output voltage is greater than the input voltage due to the secondary coil having more turns than the primary coil.

Question 37

Synchronous orbit

Answer 37

An orbit in which the period of the orbit is equal to the rotational period of the object that it is orbiting.

Question 38

Time constant

Answer 38

The time taken for a capacitor to discharge to 37% (e to the power of -1) of its initial charge. The time constant is equal to the product of the capacitance and the resistance of the fixed resistor (that the capacitor is being discharged through).

Question 39

Uniform field

Answer 39

A field in which all of the field lines are parallel and equally spaced - field strength is equal in all areas of the field.