Potential Difference
Potential Difference
The potential Difference between two
points in an electric field is the work
done in bringing a charge of +1C from
one point to the other
Unit of Potential difference
• Joule per coulomb (J C-1)
• Or more commonly...
VOLT (V)
THE VOLT
• The potential difference between two points is
1 volt if.....
1 joule of work is done when 1 coulomb is
brought from one point to the other
Therefore......
• 1VOLT = 1 JOULE PER COULOMB
• POTENTIAL DIFFERENCE IS SOMETIMES
CALLED VOLTAGE.
Potential difference (p.d.) is the difference in
electrical potential energy - voltage - between
any two points in a circuit
• A voltmeter is used to measure the potential
difference in a circuit. They are connected in
parallel with the two points between which
the p.d. is to be measured, such as before and
after a bulb. In order to get an accurate
reading the voltmeter must have a high
resistance in order to keep the electrons from
diverting around the bulb. An ideal or perfect
voltmeter has infinite resistance.
number of
Work done = coulombs
transfered
X
work done in
transfering one
coulomb
Therefore:
W=QV
or
Example 1:
• The potential difference between two points is
9V. Find the work done in transferring a
charge of 3 C between two points.
•
•
•
•
Eqn: W= QV
Calculation:
Answer:
UNIT???:
Example 2:
• The potential difference between two points is
3 kV. Find the work done when a charge of
6 µC is moved from one point to the other.
Eqn:
Calculation:
Answer:
Units?
Example 3:
• An electron of charge 2.2 x 10 -19 loses 4 x 1016 J of energy as it moves from one point to
another. What is the potential difference
between the two points?
• Eqn: W=QV
• Answer:
• Unit?:
Zero Potential
• Scientists needed a point of reference and it
was agreed that the Earth could be used as a
convenient reference point to measure the
potential difference between other points and
the Earth. The potential difference between a
point and the Earth is called the potential of
that point.
Therefore........
• THE EARTH IS AT ZERO POTENTIAL.
• THE POTENTIAL DIFFERENCE
BETWEEN A POINT AND THE
EARTH IS CALLED THE
POTENTIAL OF THAT POINT
Potential difference and Electric Field
strength
If the electric field between point A
and Point B is weak, then the work
done in bringing +1 C from A to B is
small
The Battery
The negative terminal has an excess of negative
charge and the positive terminal has an excess
of positive charge. Therefore there is an electric
field between the terminals. The higher the
VOLTAGE between the terminals the stronger
the electric field.
Electric Current
• If the terminals are connected with a metal
conductor (eg _____________) there will be
an electric field in the conductor. Free
electrons move under the influence of this
field. The electrons move from neg terminal
through the wire and then to the pos
terminal. This flow of electrons is called an
electric current.
Electric current DEF:
• An electrical current is a flow of electric
charge.
Capacitance
• The potential of a conductor and the charge
on it are directly proportional to eachother
CAPACITANCE
• The Capacitance of a conductor is the ratio of
the charge on the conductor to its potential,
defined as: C = Q/V
• Where C is a constant
• The value of C depends on the size and shape
of the conductor.
• Symbol: C
• A capacitor is an electrical device used to store
charge (Q)
• Unit: Farad (F)
• Picture a water container.
• The amount of water the container can hold (its
capacity) will depend on – among other things –
how quickly the water level rises when water is
poured in; if the water level rises rapidly it
suggests the container must be quite narrow and
therefore may not hold much water.
• We would deduce that the container had a small
‘capacitance’.
• An electrical capacitor can be compared to this
water container and the rate at which the potential
of the capacitor increases gives us an indication of
how much charge the capacitor can hold; if
putting a small charge on it raises its potential
considerably, then its capacitance must be small.
• Remember ‘raises its potential considerably’
means that a lot more work needs to be done to
bring further charge up to it.
• So for example if a capacitor has a capacitance
of 2 farads, then putting a charge of 6
coulombs on it will increase its potential by 3
volts (from C = Q/V, so V = Q/C).
• However if the capacitor had a capacitance of
200 farads, putting a charge of 6 coulombs on
it would only raise its potential by 0.03 volts.
Definition of Capacitance
If, when asked to define Capacitance,
you answer that “it is used to store
charge”, you will get zero marks,
because this is not a definition.
Q = CV
1.[2007]
• Calculate the energy stored in a 5 μF capacitor
when a potential difference of 20 V is applied
to it.
1.[2005]
• A capacitor of capacitance 100 μF is charged
to a potential difference of 20 V. What is the
energy stored in the capacitor?
1.[2002]
• How much energy is stored in a 100 μF
capacitor when it is charged to a potential
difference of 12 V?