Transcript Electricity > Did you know?

Potential and
Capacitance
Potential Difference (V)
 Potential difference between two points
is the work done to transfer a one
coulomb of charge between the points.
i.e
V=W
Q
 Unit Volt V or J C-1
 Volt The p.d. between two points 1 volt if
one joule of work is done bringing one
coulomb of charge from one point to the
other
 Potential at a point is the p.d. between a
point and the Earth, where the Earth is at
zero potential
Measuring Potential
Voltmeter
Electroscope
The spread of the leaves is a
measure of the potential
between the leaves and the
case.
Two oppositely charged plates are 5cm apart. The electric
field strength between them is 2000N/C.
(i) What is the force on 1C placed between the plates?
(ii) Find the work done in bringing the charge from one plate
to another.
(iii) What is the potential difference between the plates.
The potential difference between two plates is 2000V. An
electron is released from one plate and moves to the
other. What is its speed when is reaches the second plate.
(e=1.6x10-19 C and m=9.1 x 10-31Kg)
Capacitance
Potential is proportional to charge
VαQ
CV=Q
C is capacitance
Capacitance (C)
 Capacitance is the ratio of the charge on a conductor to its
p.d.
i.e. C = Q
V
 Unit Farad F or C V-1
Capacitor stores charge
Symbol
The presence of a negative plate
reduces the potential of the positive
plate and then more charge can be
stored on this plate. This effectively
increases capacitance C.
Q
C
V
Q=charge
V=potential
Energy Stored in a Charged Capacitor
W = ½ CV
2
 Experiment to show that capacitors store energy.
Connect an electrolytic capacitor to a d.c. supply. This charges the
capacitor. Disconnect the capacitor from the power supply and
connect to a bulb. The bulb lights showing that there is energy stored
in a capacitor.
Conduction in Capacitors
Capacitors conduct AC.
Capacitors do not conduct DC.
AC
The bulb stays lit.
The bulb lights for a short while
as charge builds up on the
capacitor, then it goes out as
capacitors do not conduct DC.
A Capacitor
Factors affecting capacitance
1. Capacitance is proportional to area of
overlap.
2. Capacitance is inversely proportional to
distance between plates.
3. Capacitance is proportional to the
permittivity of the dielectric
Parallel Plate Capacitor
C =A
d
A = area of overlap of plates
d = distance between plates
 = permittivity of dielectric (insulator between plates)
Experiment investigating the factors
affecting capacitance.
Apparatus: multimeter set to read capacitance, 2 metal
plates
Method: Connect the plates to the multimeter. Reduce the
overlap area and note that the capacitance drops.
Increase the distance between the plates and note that
the capacitance decreases.
Conclusion: Capacitance is proportional to area of overlap
and inversely proportional to the distance between the
plates.
Capacitors
Uses of Capacitors
 Tune radio stations
(variable capacitor)
 Smooth out variations in
d.c. (Allow a.c. to flow but
block d.c.)
 Camera flash
Potential Difference (V)
 This can also be said to be the energy lost by 1
coulomb as it moves between 2 points in a circuit
Note:
W = VQ
W = VQ
t
t
P = VI
Divide both sides by t (time)
(P = W and I = Q )
t
t
Electromotive Force
 e.m.f. (E) is a voltage applied to a cct.
 Unit Volt
Sources of potential/voltage/emf
Simple
Cell
Primary Cell
Secondary Cell
Thermocouple
Mains
Electric cell is a device that
converts chemical energy into
electrical energy and is a source of
e.m.f.
(e.m.f.)
Simple Cell
 Copper and zinc plates are
electrodes (negative cathode
e
e
and positive anode)
Anode
Cathode Copper sulphate with dilute
Cu
sulphuric acid is the
Zn
electrolyte
+
 This simple cell can’t be
Zn
recharged as the chemicals
Cu2+
Cu
are used up as a current
flows
Zn2+
Primary Cell
 This type of cell can’t be recharged.
 Also known as dry cells as the electrolyte tends to be a
chemical paste.
Secondary Cell
 This is a cell that can be recharged
 Also known as an accumulator
 Car battery is a lead-acid accumulator