Electricity Key Terms Flashcards
direct current
When current flows in only one direction at all times. E.g. a battery.
alternating current
When current changes direction every fraction of a second. E.g. the mains
mains voltage
In the UK, the mains has a declared value of 230 V
r.m.s. value
A sort of ‘average’ value of voltage or current for an a.c. supply.
It is also known as the ‘declared value’
peak voltage
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.
peak current
the maximum current in an a.c. supply
y-gain
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.
time-base
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.
period
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.
frequency
The number of waves per second.
In terms of an a.c. supply, it is given by the inverse of the period.
current
The electric charge transferred per unit time.
In a circuit, it is measured using an ammeter.
potential difference
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.
resistance
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.
Ohm’s law
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.
potential divider
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.
Wheatstone Bridge
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.
Power
The electrical energy transferred per second.
It can also be written in terms of current and voltage, current and resistance
and voltage and resistance.
Electrical source
A device that supplies electrical energy e.g. a power supply.
electromotive force (e.m.f.)
The energy supplied to each coulomb of charge which passes through a source.
It is measured in volts, V.
open circuit
A circuit in which no current is flowing, i.e. the switch is open.
internal resistance
The resistance that a power supply (or battery) has when current is flowing in a
circuit.
real source
A source of electrical energy (battery) in series with a small internal resistance
ideal source
A source with no internal resistance (i.e. it has only an e.m.f.).
lost volts
The energy wasted inside an electrical source due to its internal resistance
when current is drawn from the source.
Terminal Potential Difference (t.p.d.)
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.
External Resistance
The total resistance in a circuit (excluding the power supply).
This could be from a load resistor, R
short circuit
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
capacitor
A device that can store charge and therefore energy in an electrical circuit.
capacitance
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
RC Circuit
A circuit containing both a resistor and a capacitor.
Conductor
A material that allows electrons to flow through it and therefore conduct
electricity e.g. metals and carbon
Insulator
A material that does not allow electrons to flow through it and therefore does
not conduct electricity e.g. rubber, plastic and glass.
Semiconductor
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.
Band theory
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.
Conduction band
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.
valence band
The band that the outermost electrons can occupy.
Electrons will jump from this band into the conduction band when excited.
doping
The addition of an impurity atom to an intrinsic semiconductor during
manufacture, in order to increase its conductivity.
valency
The number of bonds an atom is able to make due to the electrons in its outer shell
pure/intrinsic semiconductor
A semiconductor that has not been doped; it is in its purest form.
extrinsic semiconductor
A semiconductor that has been doped with an impurity atom.
n-type doping
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.
p-type doping
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.
hole
The absence of an electron.
We can think of holes as positively charged particles that move in the opposite
direction to electrons.
p-n junction diode
The junction formed when a piece of p-type material is grown together with a
piece of n-type material.
depletion region
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.
biasing
Applying a voltage to a p-n junction to make it behave in a particular way.
forward bias
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.
reverse bias
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.
LED
A forward biased p-n junction diode that emits photons due to recombination.
recombination
When electrons drop energy level from the conduction band to the valence
band (i.e. when electrons ‘fall’ into holes).
solar cell
A p-n junction that produces a potential difference when photons are
absorbed.
It is used to store solar energy
photovoltaic effect
The name given to the production of a voltage when a solar cell absorbs
incident photons.
