What is the potential difference?

As we have seen it  a previous post a power supply has an electromotive force (e.m.f). The e.m.f. is the energy that the power supply will provide to 1 C of charge to make the 1 C of charge move around the circuit.

Now in a circuit there may be circuit components. Fig 1 below shows a circuit where there is a power supply and a bulb.

Fig 1

Now the 1 C is moving from the positive terminal and is moving around the circuit. While passing through the bulb it will move form the point A to the point B. Now the 1 C of charge has electrical energy. When it will pass through the bulb it will lose the electrical energy which will be transformed into another form.We say that the energy is dissipated.

Hence we can define the potential difference between point two points is the energy dissipated when 1 C of charge move from one of the point to the other.

Hence if the potential difference across the bulb is 3 V it means that when 1 C of charge flows from point A to point B 3 J of energy is dissipated in the bulb.

Hence if the potential difference across a circuit element is V and Q charge flows in the circuit element then the energy dissipated W can be calculated using the equation

W = VQ

What is electro-motive force (e.m.f.)?

As you would remember from this post on complete circuit a power supply is needed in order to move charges in the circuit.

Thus if we consider that an electric current is a flow of positive charges according to the conventional definition of current, for the positive charges to move from the positive terminal to the negative terminal of the power supply they need energy. It is this energy that is supplied by the power supply to the positive charges. The positive charges can thus use this energy to move around the circuit. It is this energy that the positive charges possesses that is called electrical energy.

Fig 1

Fig 1  shows a closed circuit. If the circuit is closed then an electric current will flow as indicated from the positive terminal of the power supply to the negative terminal of the power supply.

Fig 2

In fig 2 you can see 1 c of charge leaving the positive terminal of the power supply. Now for this 1 C of charge the power supply will provide a certain amount of energy. 

Suppose that for every one coulomb of charge that leaves the power supply the power supply provides 10 J of energy. Then the electromotive force (e.m.f.)  of the power supply will be 10 Volt (V).

Hence if the e.m.f. of a dry cell is 1.5 V it means that when the dry cell is in a circuit for every 1 C of charge then the power supply will provide 1.5 J of energy.

Hence we can define the electromotive force as the energy that the power supply provide to a unit charge to move it around the circuit.

Hence if the a charge Q is moved around a circuit and the power supply supplied W amount of energy, the the e.m.f. E can be calculated as shown:

E  = W/Q

Example

If 4 C of charge  needs 10 J of energy is provided by the power supply. Calculate the e.m.f. of the power supply?

E = W/Q

= 10 /4

2.5 J

what is an electric current?

As we have seen in an earlier post, an electric current will only flow if their is a complete circuit. You have also seen that an electric current is also due to the flow of electrons.

Fig 1 below show a complete circuit. As you can see from the circuit electrons will flow through the bulb on its way to the positive terminal of the power supply.

Fig 1

Now depending on the circuit a certain number of electrons will flow through the bulb every second.

Hence if we know the number of electrons that is flowing through the bulb every second, then it means that we cal calculate the amount of charge that flow through the bulb in one second.

Example 1

If the number of electrons that flows through the bulb in 10 s is 3.0x10²² and the charge of one electron is 1.6 x 10^-19 C,

Calculate (i) the amount of charge that flows through the bulb in 10 s.

                    (ii) the amount of charge that flows through the bulb in 1 s.

Ans

(i) The amount of charge flowing though the bulb in 10 s is

Q = 3.0 x 10²² *1.6x10^-19

    = 4800 C

(ii) The amount of charge flowing through the bulb in 1s is

Q = 4800/10 =480 C

Now the quantity electric current is defined as the rate of flow of electric charge.

The unit of electric current is the Ampere (A) .

Thus if we are able to determine the rate of flow of electric charge or the amount of charge that flows in a circuit element like the bulb every second it means that we have determined the electric current.

If the amount of charge  flowing through the bulb every second is 480 C then

The electric current = 480 C /s or 480 A.

Thus we can conclude that the electric current is merely an indication of the amount of charge flowing per second in a circuit element.  The more charge flowing per second the greater the electric current flowing through the circuit element.