Current electricity

What is a complete circuit?

What is an ammeter?

What is an electric current?

What is the conventional current?

What is the electromotive force (e.m.f.)

What is the potential difference?

What is the kilowatt-hour?

What is the voltmeter?

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 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 the conventional current?

We have seen in a previous post that when you have a complete circuit an electric current will flow. We have also see that an electric current is due to a flow of electrons that moves from the negative terminal to the positive terminal of the power supply.

Fig 1 below shows how the electron flows from the negative terminal to the positive terminal of the power supply.

Fig 1

However in a circuit the direction of the electric current is not defined as moving from the negative terminal to the positive terminal just as the flow of electrons. It is defined as moving from the positive terminal to the negative terminal of the power supply as shown in fig 2 below.

 

 

 

 

 

 

Fig 2

So why is it that despite knowing that the electric current is due to a flow of electrons that moves from the negative terminal to the positive terminal as shown in fig 1we set the direction of the electric current as being from the positive terminal to the negative terminal as shown in fig 2.

The answer lies in convention. A convention is like an assumption. When electricity was first  discovered it was assumed that electricity is a flow of positive charges that the positive terminal to the negative terminal. Hence in all physics books at the time the electric current was taken as moving from the positive terminal to the negative terminal.

However it was discovered later on that in a circuit only electrons move. they move from the negative terminal to the positive terminal of the power supply. However the concept of conventional current was so ingrained that it could not be altered.

Hence as from now we will say that electric current is a flow of positive charges even though we know that it is in fact due to the flow of electrons.

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.