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 a capacitor?

A capacitor is simply an device that is composed of two metal conductors that are separated by an insulator. The insulator is called a dielectric. Fig 1 below shows a simplified version of a capacitor.

Fig 1

In fig 2 below two metal foils are separated by a paper dielectric. The three are them are then rolled as shown.

Fig 2

The fig 3 below shows a capacitor as sold commercially. It is a capsule that contains the rolled up aluminium foils.

Fig 3

The two plates are then connected to a power supply. The positive plate is connected to the positive terminal while the other negative plate is connected to the negative terminal as shown in fig 4 below.

Fig 5

After the capacitor is connected to the power supply, the capacitor will be charged by the power supply. That is charges will accumulate on the two plates. Positive charges will accumulate on the positive plate of the capacitor while negative charges will accumulate on the negative plate of the capacitor. The charges on the two plates will create an electric field between the two plates where the energy will be stored in the form of electric potential energy.

The fig 6 below shows a capacitor whose terminals are connected to a light bulb. 

When this happens the energy stored in the capacitor will be released i.e  is “discharged” into the bulb.  

Fig 6

Hence a capacitor can be simply said to be an electronic device that stores charges. It is used in rectification circuits that changes alternating current to direct current, in tuning circuits and in filter circuits to filter out direct current.

What is the kilowatt-hour?

The kilowatt-hour is the unit in which usage of electricity is measured in the home.

You would remember that the SI unit of energy is the Joule (J)  as you have seen in  work done, gravitational potential energy or kinetic energy.

Now as you know the rating of home appliances is its power and the unit of power is the Watt (W).

Now if a kettle with a rating of 1000 W is used for two hours, then the amount of energy used will be calculated as shown:

Power = Energy used / time taken

Energy used = power * time taken  = 1000 * (2 * 3600)

                    = 7200000 J

                     = 7.2 MW (Megawatt)

However as you can see such calculation is difficult and beyond someone that is not a Physicist.

Hence in order to make matter more simple we use the Kilowatt-hour.

If we use the example above we would need to use the equation below:

Energy used in kilowatt-hour = power of appliance in Kilowatt  x time in hour

Hence the energy used = 2 kW x 2 h

= 4  kilowatt-hour (kWh)

As you can see it is quite simple to determine the the energy used in kilowatt-hour.