Electricity - CIRCUIT DIAGRAM and OHM'S LAW

CIRCUIT DIAGRAM

    We know that an electric circuit comprises a cell (or a battery), a plug key, electrical component(s), and connecting wires. It is often convenient to draw a schematic diagram, in which different components of the circuit are represented by the symbols conveniently used. Conventional symbols used to represent some of the most commonly used electrical components are given in Table below-

PICTURE:-1

    The electronic circuit symbols mainly involve wires, power supplies, resistors, capacitors, diodes, transistors, meters, switches, sensors, logic gates, audio devices, and other components.

PICTURE:- 2

OHM’S LAW

    Is there a relationship between the potential difference across a conductor and the current through it? Let us explore with an Activity.

In 1827, a German physicist Georg Simon Ohm (1787–1854) found out the relationship between the current I, flowing in a metallic wire and the potential difference across its terminals. He stated that the electric current flowing through a metallic wire is directly proportional to the potential difference V, across its ends provided its temperature remains the same. This is called Ohm’s law. In other words –

V ∝ I 

               or  V/I = constant = R 

or V = IR

In Eq., R is a constant for the given metallic wire at a given temperature and is called its resistance. It is the property of a conductor to resist the flow of charges through it. Its SI unit is ohm, represented by the Greek letter Ω. According to Ohm’s law, 

R = V/I 

 If the potential difference across the two ends of a conductor is 1 V and the current through it is 1 A, then the resistance R, of the conductor is 1 Ω. That is, 

1 ohm = 1 volt /1 ampere

Also from this Eq. we get

 I = V/R

It is obvious from the above Eq. that the current through a resistor is inversely proportional to its resistance. If the resistance is doubled the current gets halved. In many practical cases it is necessary to increase or decrease the current in an electric circuit. A component used to regulate current without changing the voltage source is called variable resistance. In an electric circuit, a device called rheostat is often used to change the resistance in the circuit.

Certain components offer an easy path for the flow of electric current while the others resist the flow. We know that motion of electrons in an electric circuit constitutes an electric current. The electrons, however, are not completely free to move within a conductor. They are restrained by the attraction of the atoms among which they move. Thus, motion of electrons through a conductor is retarded by its resistance. A component of a given size that offers a low resistance is a good conductor. A conductor having some appreciable resistance is called a resistor. A component of identical size that offers a higher resistance is a poor conductor. An insulator of the same size offers even higher resistance.

FACTORS ON WHICH THE RESISTANCE OF A CONDUCTOR DEPENDS

    It is observed that the ammeter reading decreases to one-half when the length of the wire is doubled. The ammeter reading is increased when a thicker wire of the same material and of the same length is used in the circuit. A change in ammeter reading is observed when a wire of different material of the same length and the same area of cross-section is used. On applying Ohm’s law Eqs., we observe that the resistance of the conductor depends (i) on its length, (ii) on its area of cross-section, and (iii) on the nature of its material. Precise measurements have shown that resistance of a uniform metallic conductor is directly proportional to its length (l) and inversely proportional to the area of cross-section (A). That is,

                                                           R ∝ l .................................... (1)

and                                        R ∝ 1 /A ................................(2)

Combining Eqs.1 and 2 we get       R ∝ l / A 

or,                                         R = ρ l / A 

where ρ (rho) is a constant of proportionality and is called the electrical resistivity of the material of the conductor. The SI unit of resistivity is Ω m. It is a characteristic property of the material. The metals and alloys have very low resistivity in the range of 10^–8 Ω m to 10^–6 Ω m. They are good conductors of electricity. Insulators like rubber and glass have resistivity of the order of 10^12 to 10^17 Ω m. Both the resistance and resistivity of a material vary with temperature. Table 12.2 reveals that the resistivity of an alloy is generally higher than that of its constituent metals. Alloys do not oxidize (burn) readily at high temperatures. For this reason, they are commonly used in electrical heating devices, like electric iron, toasters etc. Tungsten is used almost exclusively for filaments of electric bulbs, whereas copper and aluminium are generally used for electrical transmission lines.

Example 1 (a) How much current will an electric bulb draw from a 220 V source, if the resistance of the bulb filament is 1200 Ω?

(b) How much current will an electric heater coil draw from a 220 V source, if the resistance of the heater coil is 100 Ω?

Solution (a) We are given V = 220 V; R = 1200 Ω. 

From Eq., we have the current I = 220 V/1200 Ω = 0.18 A. 

(b) We are given, V = 220 V, R = 100 Ω. From Eq., we have the current I = 220 V/100 Ω = 2.2 A.

Note the difference of current drawn by an electric bulb and electric heater from the same 220 V source!

Example 2. The potential difference between the terminals of an electric heater is 60 V when it draws a current of 4 A from the source. What current will the heater draw if the potential difference is increased to 120 V?

Solution;- We are given, potential difference V = 60 V, current I = 4 A. According to Ohm’s law,                           R=V/I

                                    =60V/4A

                                    = 15Ω

When the potential difference is increased to 120 V the current is given by 

current = V/R =120 V/15 = 8A.

The current through the heater becomes 8 A. 

FOR SELF EVALUATION CLICK THE LINK GIVEN AND ATTEMPT THE TEST 

https://docs.google.com/forms/d/1XwUTP21TuAqqZ34odo4V8nJePtH9YP4bsLXVgZaxyjk/edit?usp=sharing

REFERENCE

Picture 1- https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.istockphoto.com%2Fvector%2Fset-of-electronic-circuit-symbols-gm602316366-103526411&psig=AOvVaw0WPOBZ_H8Qmk4fI9uW8MVu&ust=1602614088088000&source=images&cd=vfe&ved=0CAIQjRxqFwoTCKCr_-zYr-wCFQAAAAAdAAAAABAD

Picture 2- https://www.google.com/url?sa=i&url=https%3A%2F%2Fdepositphotos.com%2Fvector-images%2Fresistor.html&psig=AOvVaw0WPOBZ_H8Qmk4fI9uW8MVu&ust=1602614088088000&source=images&cd=vfe&ved=0CAIQjRxqFwoTCKCr_-zYr-wCFQAAAAAdAAAAABAJ