What is the voltmeter?

The voltmeter is a device that is used to measure the electromotive force (e.m.f.) or the potential difference across a circuit component.

Fig 1 below shows a voltmeter that is connected to a dry cell.

Fig 1

The voltmeter is always connected in parallel to the power supply. That is the positive terminal of the power supply is connected to the positive terminal of the voltmeter. The negative terminal of the power supply is likewise connected to the negative terminal of the voltmeter.

Please note that by convention the positive terminal is red while the negative terminal is black.

Fig 2  is the same circuit but this time using symbols.

Fig 2

Should there be another circuit element like a bulb the voltmeter is connected in parallel to the bulb, if you want to measure the potential difference across the bulb,  of across the power supply if you want to measure the potential difference across the power supply while it is supplying current to the bulb.Again with the positive terminal of the power supply connected to the positive terminal of the voltmeter and the negative terminal of the power supply connected to the negative terminal of the voltmeter. Fig 3 below illustrates the two concepts.

Fig 3

In addition to the voltmeter in Fig 1 there are also voltmeters that are dual range.  That is they can measure voltages over two different ranges. A typical high school voltmeter has the 0 – 5 V and 0 – 15 V ranges. An examples can be seen in fig 4 below.

Fig 4

The way to use the voltmeter is to connect the negative terminal of the voltmeter to the negative terminal of the power supply and positive terminal of the power supply to either the 5 V positive terminal and the 15 V positive terminal depending on which range to be used. Fig 5 shows the voltmeter being used over the 0 – 5 V range while fig 6 shows the voltmeter being used over the 0 –15 V range.

Fig 5

Fig 6

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