What’s a Circuit | Physics


We cannot imagine a single day in our life without electricity. Electricity plays a vital role in our modern society. So, what constitutes this electric current? How does it flow in a circuit? Are you guys curious to know about an electric circuit? In this lesson, we’ll get acquainted with the basic terminology used in electric circuits and also find out, what constitutes the flow of current in a circuit. Just like water flowing in a river constitutes the water current, charges flowing in a conductor is electric current. Electric current is defined as charge flowing in a particular area per unit time. Or, it can also be defined as the rate of flow of charges. A closed and a continuous path for the flow of electric current is called an electric circuit. An electric circuit comprises of metallic wires and electrical components, in which electrons constitute the flow of charges. At the time when electricity was first observed, electrons were not known to scientists. Therefore, the flow of positive charge was considered to be the flow of current. From that time onwards, by convention it is considered that the flow of current in a circuit would be opposite to the direction of the flow of negative charge or electrons. If a charge Q flows through a conductor in a time t, then current I is defined as Q divided by t. The SI unit of charge is Coulomb. Since, one electron carries a charge of 1.6 into ten raised to power minus 19 Coulombs, one Coulomb is the charge constituted by 6.25 into ten raised to power 18 electrons. The SI unit of current is Ampere and 1 Ampere is a current flowing through a conductor when one Coulomb of charge flows in one second. Therefore, 1 Ampere is equal to 1 Coulomb per second. The instrument used to measure electric current is called an ammeter. Since one ampere is a large value of current, smaller values of current are generally expressed in the form mili-Ampere that is 10 raised to the power minus 3 or micro ampere that is ten raised to the power minus 6. Let us have a look at a typical schematic diagram of an electric circuit, which consists of a bulb, a battery, an ammeter and connecting wires. Note that, the current flows from the positive terminal of the battery to the negative terminal through the ammeters and the bulb. In order to make the construction of the circuit diagram easy and convenient, the electrical components are represented by symbols. Just like in the electric circuit, we just saw there are many other commonly used symbols. Let’s have a look at them one by one. An electric cell is represented by two straight lines. The longer line denotes the positive terminal and the shorter line denotes the negative terminal. When two or more electric cells are joined end-to-end, it forms a battery. And the symbol for the battery is as shown. A plug key or a switch is represented by two brackets. If there is a dot in between, it represents a closed switch and an open bracket represents an open switch. The symbol for a wire joint is represented in the form of a T. The symbols for a wire crossing ammeter, resistor, voltmeter, rheostat are as follows. We’ll discuss each of the electrical component in detail as we proceed with the lessons. For now, just keep these symbols in mind, so that you can identify the electrical component in any circuit. Before that, let’s have a deeper look into the flow of charges in a conductor. In a conductor the atoms are very closely spaced. The flow of charges in a conductor is very different from that in empty space. The electrons move with their drift velocity that is of the order of 1 mm/s. So, how is that the bulb lights up immediately, when you switch it on. It cannot be that electrons start from one end of the wire and reach the other end. This is because, the physical drift of electrons is a very slow process. You try to figure out the answer. We’ll discuss it in out next lesson. So, let us quickly recap what we learned in this lesson. We defined the term electric current and saw the schematic diagram of an electric circuit. We also saw the symbols of the commonly used electrical components in a circuit and we also discussed about the flow of electrons in a conductor.

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