Electricity Key Terms

Question 1

direct current

Answer 1

When current flows in only one direction at all times. E.g. a battery.

Question 2

alternating current

Answer 2

When current changes direction every fraction of a second. E.g. the mains

Question 3

mains voltage

Answer 3

In the UK, the mains has a declared value of 230 V

Question 4

r.m.s. value

Answer 4

A sort of ‘average’ value of voltage or current for an a.c. supply.
It is also known as the ‘declared value’

Question 5

peak voltage

Answer 5

The maximum voltage in an a.c. supply.
For an a.c. voltage wave shown on an oscilloscope screen, it is given by the
crest of the wave.

Question 6

peak current

Answer 6

the maximum current in an a.c. supply

Question 7

y-gain

Answer 7

A setting on an oscilloscope which controls the voltage per centimetre.
It affects the vertical component of the wave and can be used to determine
the peak voltage of an a.c. supply.

Question 8

time-base

Answer 8

A setting on an oscilloscope which controls the time per centimetre.
It affects the horizontal component of the wave and can be used to determine
the frequency of an a.c. supply.

Question 9

period

Answer 9

The time taken for one wave to pass a point.
In terms of an oscilloscope trace, it is given by the time-base setting multiplied
by the number of divisions.

Question 10

frequency

Answer 10

The number of waves per second.

In terms of an a.c. supply, it is given by the inverse of the period.

Question 11

current

Answer 11

The electric charge transferred per unit time.

In a circuit, it is measured using an ammeter.

Question 12

potential difference

Answer 12

The energy given to each coulomb of charge that passes through a power
supply. It is also known as voltage.
In a circuit, it is measured using a voltmeter.

Question 13

resistance

Answer 13

An opposition to current flow.
In a circuit, it is measured using an ohmmeter.
It depends on the type, length, thickness and temperature of a material.

Question 14

Ohm’s law

Answer 14

The voltage across a resistor is directly proportional to the current passing
through it, as long as temperature remains constant. That is, V I.
This follows for any ohmic conductor.

Question 15

potential divider

Answer 15

A series circuit with two or more resistors in which each resistor receives a
share of the supply voltage.
The splitting of the voltage depends on the relative resistances of the resistors.

Question 16

Wheatstone Bridge

Answer 16

Two potential divider circuits connected in parallel, often with a voltmeter in
the middle.
The voltage across the voltmeter can be determined by calculating the voltage
across each of the bottom resistors and then calculating the difference
between the two.

Question 17

Power

Answer 17

The electrical energy transferred per second.
It can also be written in terms of current and voltage, current and resistance
and voltage and resistance.

Question 18

Electrical source

Answer 18

A device that supplies electrical energy e.g. a power supply.

Question 19

electromotive force (e.m.f.)

Answer 19

The energy supplied to each coulomb of charge which passes through a source.
It is measured in volts, V.

Question 20

open circuit

Answer 20

A circuit in which no current is flowing, i.e. the switch is open.

Question 21

internal resistance

Answer 21

The resistance that a power supply (or battery) has when current is flowing in a
circuit.

Question 22

real source

Answer 22

A source of electrical energy (battery) in series with a small internal resistance

Question 23

ideal source

Answer 23

A source with no internal resistance (i.e. it has only an e.m.f.).

Question 24

lost volts

Answer 24

The energy wasted inside an electrical source due to its internal resistance
when current is drawn from the source.

Question 25

Terminal Potential Difference (t.p.d.)

Answer 25

The voltage available at the terminals of an electrical source.
That is, the voltage remaining after the lost volts has been subtracted from the
e.m.f.

Question 26

External Resistance

Answer 26

The total resistance in a circuit (excluding the power supply).
This could be from a load resistor, R

Question 27

short circuit

Answer 27

When a battery is short-circuited (e.g. by connecting it to itself), the load
resistance, R, is effectively zero. Since Vtpd = IR, this means that Vtpd must also
be zero.
A short circuit can be dangerous as with no load resistance, current can
become incredibly high

Question 28

capacitor

Answer 28

A device that can store charge and therefore energy in an electrical circuit.

Question 29

capacitance

Answer 29

The ability of a capacitor to store charge.
More precisely, it is the charge stored per volt of potential difference across a
capacitor.
It is given by the gradient of the line on a Q-V graph

Question 30

RC Circuit

Answer 30

A circuit containing both a resistor and a capacitor.

Question 31

Conductor

Answer 31

A material that allows electrons to flow through it and therefore conduct
electricity e.g. metals and carbon

Question 32

Insulator

Answer 32

A material that does not allow electrons to flow through it and therefore does
not conduct electricity e.g. rubber, plastic and glass.

Question 33

Semiconductor

Answer 33

A material that behaves as an insulator when pure, but can be made to conduct
by adding an impurity or exposing it to heat, light etc e.g. silicon and
germanium.

Question 34

Band theory

Answer 34

In isolated atoms, electrons occupy discrete energy levels (orbits).
However, when atoms are brought together to form solids, these energy levels
interact with each other and become grouped into bands.
These bands represent a continuous range of energies, but there are some
groups of energy that are not allowed (band gaps).
Discussions around band theory usually involve the terms conduction band,
valence band and band gap.

Question 35

Conduction band

Answer 35

The band above the valence band.
Electrons can jump into this band from the valence band when they have
enough energy.
It is not completely full of electrons meaning that electrons can move freely.
This means that the band can conduct when an electric field is applied.

Question 36

valence band

Answer 36

The band that the outermost electrons can occupy.

Electrons will jump from this band into the conduction band when excited.

Question 37

doping

Answer 37

The addition of an impurity atom to an intrinsic semiconductor during
manufacture, in order to increase its conductivity.

Question 38

valency

Answer 38

The number of bonds an atom is able to make due to the electrons in its outer shell

Question 39

pure/intrinsic semiconductor

Answer 39

A semiconductor that has not been doped; it is in its purest form.

Question 40

extrinsic semiconductor

Answer 40

A semiconductor that has been doped with an impurity atom.

Question 41

n-type doping

Answer 41

The addition of a Group V element like arsenic to an intrinsic semiconductor is
called n-type doping.
Most conduction takes place by the movement of free electrons, which are
negatively charged.
The majority charge carriers are therefore electrons.

Question 42

p-type doping

Answer 42

The addition of a Group III element like boron to an intrinsic semiconductor is
called p-type doping.
Most conduction takes place by the movement of holes, which are positively
charged.
The majority charge carriers are therefore holes.

Question 43

hole

Answer 43

The absence of an electron.
We can think of holes as positively charged particles that move in the opposite
direction to electrons.

Question 44

p-n junction diode

Answer 44

The junction formed when a piece of p-type material is grown together with a
piece of n-type material.

Question 45

depletion region

Answer 45

The region around the p-n junction that consists of ‘filled holes’ i.e. no free
charge carriers. It is also known as the depletion layer.

Question 46

biasing

Answer 46

Applying a voltage to a p-n junction to make it behave in a particular way.

Question 47

forward bias

Answer 47

When the negative terminal of the cell is connected to the n-type and the
positive terminal of the cell is connected to the p-type.
The diode conducts because the depletion layer has been removed.
Forward bias therefore reduces the electric field in the p-n junction.

Question 48

reverse bias

Answer 48

When the negative terminal of the cell is connected to the p-type and the
positive terminal of the cell is connected to the n-type.
The diode will not conduct since the depletion layer width is increased.
Reverse bias therefore increases the electric field in the p-n junction.

Question 49

LED

Answer 49

A forward biased p-n junction diode that emits photons due to recombination.

Question 50

recombination

Answer 50

When electrons drop energy level from the conduction band to the valence
band (i.e. when electrons ‘fall’ into holes).

Question 51

solar cell

Answer 51

A p-n junction that produces a potential difference when photons are
absorbed.
It is used to store solar energy

Question 52

photovoltaic effect

Answer 52

The name given to the production of a voltage when a solar cell absorbs
incident photons.