UNIT G485 Module 2 5.2.1 Capacitors 1
Candidates should be able to : INTRODUCTION
Define capacitance and the farad.
Electric current is a flow of charge, usually carried by electrons. In a
Select and use the equation :
Q = CV circuit the electrons drift along the conductors which make up the circuit.
If there is a gap in the circuit, we say that there is no current, but this is
State and use the equation for the total capacitance of two not completely true.
or more capacitors in series.
State and use the equation for the total capacitance of two In the circuit shown opposite, when
S is closed electrons are caused to
V
or more capacitors in parallel. + -
flow briefly in a clockwise direction. S
Solve circuit problems with capacitors involving series and Negative charge accumulates on one
parallel circuits. side of the gap which repels electrons
Explain that the area under a potential difference against on the other side. So negative charge
builds up on one side and positive
charge graph is equal to the energy stored by a capacitor.
charge on the other. The flow quickly - -
Select and use the equations for a charged capacitor : ++
stops when the p.d. across the gap ++ - -
(due to the separated charges)
gap
W = ½QV W = ½CV2 becomes equal to the supply voltage,V.
We can think of the wires on either side of the gap as ‘storing’ a tiny amount
Sketch graphs that show the variation with time of potential of charge. This charge storage is the basis of a device called a CAPACITOR.
difference, charge and current for a capacitor discharging
through a resistor.
A CAPACITOR is an arrangement of conductors and
Define the time constant of a circuit.
insulators designed to store electrical charge.
Select and use :
Time constant = CR
Analyse the discharge of a capacitor using equations of the
Although capacitors come in
form :
a huge variety of different
x = x0e-t/CR types, shapes and sizes, they
all basically consist of two
metal plates separated by
an insulating material which
Explain exponential decays as having a constant-ratio
is called the dielectric.
property.
Describe the uses of capacitors for the storage of energy in
applications such as flash photography, lasers used in nuclear
fusion and as back-up power supplies for computers.
, UNIT G485 Module 2 5.2.1 Capacitors 2
Once a capacitor has been charged, charged capacitor
it can be discharged by disconnecting
If capacitors were made in their
it from the supply and connecting the
basic form of two flat conducting
leads together.
plates separated by an insulator,
they would be excessively large
and cumbersome. We can observe a discharge by
R
connecting a charged capacitor
In order to make capacitors of a to an LED through a protective
size suitable for connection in a resistor.
circuit, the metal plates and the
dielectric are rolled into a The LED glows as the capacitor
cylindrical shape as shown in the discharges. The LED glows as
capacitor discharges
diagram opposite.
CAPACITANCE
CHARGING OF A PARALLEL-PLATE CAPACITOR The term CAPACITANCE is used in order to quantify the amount of charge
which can be stored by a given capacitor.
Two parallel, metal plates placed close to +Q -Q
each other form a capacitor. When such a The CAPACITANCE (C) of a capacitor is the amount of
capacitor is connected to a battery, one of charge stored per unit potential difference across it.
the plates gains electrons and so
becomes negatively charged.
CAPACITANCE = CHARGE
This causes an equal number of electrons POTENTIAL DIFFERENCE
to be repelled from the other plate, which
then becomes positively charged. The
(C)
arrival of electrons at one plate and the
repulsion of electrons from the other C = Q
occurs simultaneously.
V
(F) (V)
If one plate stores charge –Q, the other stores charge +Q and we say that charge Q
is stored.
The amount of charge stored depends on the POTENTIAL DIFFERENCE (VOLTAGE)
of the supply to which the plates are connected.
Candidates should be able to : INTRODUCTION
Define capacitance and the farad.
Electric current is a flow of charge, usually carried by electrons. In a
Select and use the equation :
Q = CV circuit the electrons drift along the conductors which make up the circuit.
If there is a gap in the circuit, we say that there is no current, but this is
State and use the equation for the total capacitance of two not completely true.
or more capacitors in series.
State and use the equation for the total capacitance of two In the circuit shown opposite, when
S is closed electrons are caused to
V
or more capacitors in parallel. + -
flow briefly in a clockwise direction. S
Solve circuit problems with capacitors involving series and Negative charge accumulates on one
parallel circuits. side of the gap which repels electrons
Explain that the area under a potential difference against on the other side. So negative charge
builds up on one side and positive
charge graph is equal to the energy stored by a capacitor.
charge on the other. The flow quickly - -
Select and use the equations for a charged capacitor : ++
stops when the p.d. across the gap ++ - -
(due to the separated charges)
gap
W = ½QV W = ½CV2 becomes equal to the supply voltage,V.
We can think of the wires on either side of the gap as ‘storing’ a tiny amount
Sketch graphs that show the variation with time of potential of charge. This charge storage is the basis of a device called a CAPACITOR.
difference, charge and current for a capacitor discharging
through a resistor.
A CAPACITOR is an arrangement of conductors and
Define the time constant of a circuit.
insulators designed to store electrical charge.
Select and use :
Time constant = CR
Analyse the discharge of a capacitor using equations of the
Although capacitors come in
form :
a huge variety of different
x = x0e-t/CR types, shapes and sizes, they
all basically consist of two
metal plates separated by
an insulating material which
Explain exponential decays as having a constant-ratio
is called the dielectric.
property.
Describe the uses of capacitors for the storage of energy in
applications such as flash photography, lasers used in nuclear
fusion and as back-up power supplies for computers.
, UNIT G485 Module 2 5.2.1 Capacitors 2
Once a capacitor has been charged, charged capacitor
it can be discharged by disconnecting
If capacitors were made in their
it from the supply and connecting the
basic form of two flat conducting
leads together.
plates separated by an insulator,
they would be excessively large
and cumbersome. We can observe a discharge by
R
connecting a charged capacitor
In order to make capacitors of a to an LED through a protective
size suitable for connection in a resistor.
circuit, the metal plates and the
dielectric are rolled into a The LED glows as the capacitor
cylindrical shape as shown in the discharges. The LED glows as
capacitor discharges
diagram opposite.
CAPACITANCE
CHARGING OF A PARALLEL-PLATE CAPACITOR The term CAPACITANCE is used in order to quantify the amount of charge
which can be stored by a given capacitor.
Two parallel, metal plates placed close to +Q -Q
each other form a capacitor. When such a The CAPACITANCE (C) of a capacitor is the amount of
capacitor is connected to a battery, one of charge stored per unit potential difference across it.
the plates gains electrons and so
becomes negatively charged.
CAPACITANCE = CHARGE
This causes an equal number of electrons POTENTIAL DIFFERENCE
to be repelled from the other plate, which
then becomes positively charged. The
(C)
arrival of electrons at one plate and the
repulsion of electrons from the other C = Q
occurs simultaneously.
V
(F) (V)
If one plate stores charge –Q, the other stores charge +Q and we say that charge Q
is stored.
The amount of charge stored depends on the POTENTIAL DIFFERENCE (VOLTAGE)
of the supply to which the plates are connected.
