Topic 07: Electric and Magnetic Fields

Question 1

Capacitance

Answer 1

The ratio of the charge stored in a capacitor to the potential difference between the two sides of the capacitor

Question 2

Coulomb’s law

Answer 2

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 3

Dielectric

Answer 3

An insulating material placed between capacitor plates that prevents charge from crossing between the plates. The dielectric becomes polarised when the capacitor is charged and changes the capacitance of the capacitor

Question 4

Eddy current

Answer 4

Small closed loops of current within a conductor or magnet. In a transformer, these currents act against the magnetic flux that generates a current in the secondary coil, making the transformer less efficient and heating the core

Question 5

Electric field

Answer 5

A region of space in which charged particles experience either an attractive or repulsive force depending on the charge of the particle and direction of the field

Question 6

Electric field lines

Answer 6

A way of representing an electric field in a diagram, they point in the direction that a positive charge would experience a force, point positive to negative

Question 7

Electric field strength

Answer 7

Defined at a point. The electrostatic force experienced by a unit positive charge at that point in the field

Question 8

Electric potential

Answer 8

Defined at a point. The work done moving a unit positive charge from an infinite distance away to that point

Question 9

Electric potential difference

Answer 9

The work done moving a unit positive charge between two points

Question 10

Electromagnetic induction

Answer 10

When a current carrying conductor moves relative to a magnetic field, an EMF is induced in the conductor

Question 11

Electrostatic force

Answer 11

The force generated between charges due to Coulomb’s law

Question 12

Equipotential

Answer 12

Planes of points where the electric potential is the same, no work is done moving charge along these lines

Question 13

Faraday’s law

Answer 13

The magnitude of the induced EMF is directly proportional to the rate of change of magnetic flux linkage

Question 14

Fleming’s left hand rule

Answer 14

The relative direction of motion, field direction, and current direction in the motor effect can be represented by the thumb, first finger, and second finger of the left hand respectively

Question 15

Frequency

Answer 15

The number of complete oscillations of the current (or voltage) in an A.C. circuit per second

Question 16

Lamination

Answer 16

When a transformer’s core is made up of sets of layers to prevent eddy currents

Question 17

Lenz’s law

Answer 17

The direction of an induced current is such that it opposes the current that created it

Question 18

Magnetic field

Answer 18

A region of space in which magnetic materials and moving electric charges feel a force

Question 19

Magnetic flux

Answer 19

The magnetic field passing through a given area. Given by the product of the magnetic flux density and the area

Question 20

Magnetic flux density

Question 21

Magnetic flux linkage

Answer 21

The magnetic flux of a coil found by the product of the magnetic flux and number of turns of a coil

Question 22

Motor effect

Answer 22

The force felt by a current carrying wire in a magnetic field

Question 23

Oscilloscope

Answer 23

A device used to monitor periodically time-changing quantities, such as voltage or current, in an A.C. circuit

Question 24

Peak value

Answer 24

The maximum voltage or current that an A.C. Current achieves

Question 25

Period

Answer 25

The time taken for the current (or voltage) in an A.C. circuit to complete one full cycle

Question 26

Radial field

Answer 26

A field with field lines originating from a point like spokes on a wheel

Question 27

Root-mean-square value

Answer 27

For an A.C. circuit, RMs is the value of the voltage or current that would produce the same power dissipation in a D.C. Circuit as the A.C. Circuit, It is defined as the peak voltage divided by square root 2

Question 28

Time constant

Answer 28

Describes the rate of charge or discharge of a capacitor. It is the time taken for the capacitor to charge from zero to 63% of its full value or to discharge from full to 37% of its full value

Question 29

Transformer

Answer 29

A device used to increase or decrease the voltage with two sets of coils with different numbers of turns wrapped around a magnetic core. The transformer is step-up if the number of coils on the secondary coil is greater than the number of turns on the primary coil. The transformer is step-down if the number of turns on the secondary coil is fewer than the number on the primary coil

Question 30

Uniform field

Answer 30

A field with parallel field lines