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 inWork done = coulombs X transfering

one transfered coulomb

Therefore:

W = Q V 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 10-16 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 CapacitanceIf, 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?