What is an ammeter?

An ammeter is a device that is used to measure the electric current. Now the ammeter can either be digital or analogue. It can be a single range or dual range.

Digital ammeter

The digital ammeter is one on which there is a screen and you can read the value of the electric current. An  example of a digital ammeter is in fig 1 below. It the easiest to use as you just to connect it and read it.

Fig 1

Analogue ammeter

The analogue ammeter is the one that was used in the old days. It has a pointer in it that will indicate the current in the circuit. It is more difficult to use and the chance of committing a parallax error is greater. However it does not require maintenance like the digital ones that require a dry cell in it to work. The dry cell require frequent replacing. Fig 2 below is an example of an analogue ammeter.

 

Single range

A single range ammeter is one that ca measure electric current within a particular range. For example it can measure from 0 to 5 A. However there are ammeters that can measure current on two different range. For example it can measure current on the 0 to 1 A range. However if the current being measured exceed the 1 A vale it can be switched to a higher range like the 0 to 5 A. An examples of a single range ammeter is in fig 3 and that of a double range is in fig 4.

Fig 3 A single range ammeter

Fig 4 A dual range ammeter.

How to use a dual range ammeter?

Most ammeter as you can see above have only two terminals and you just plug the wires in. However a dual range ammeter has three terminals. So how would you plug in the wires in that case? I would take the case of an ammeter that have two ranges 0 – 1 A and 0 – 5 A. If you want to use the 0 – 1 A range then you would connect the wires as in the fig 5 below.

Fig 5 Connecting an ammeter on the first range.

However if you want to connect the Ammeter on the second range then you would have to connect the wires as shown in fig 6 below.

Fig 6 Connecting an ammeter on the second range

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