Home | | Science 9th std | Electric circuit diagram

# Electric circuit diagram

To represent an electrical wiring or solve problem involving electric circuits, the circuit diagrams are made.

Electric circuit diagram

To represent an electrical wiring or solve problem involving electric circuits, the circuit diagrams are made. The four main components of any circuits namely the, (i) cell, (ii) connecting wire, (iii) switch and (iv) resistor or load are given above. In addition to the above many other electrical components are also used in an actual circuit. A uniform system of symbols has been evolved to describe them. It is like learning a sign language, but useful in understanding circuit diagrams. ## 1. Some common symbols inthe electrical circuit

Some of the symbols are shown in Table 1. ## 2.Different electrical circuits Look at the two circuits, shown in Figure 2.15. In Figure A two bulbs are connected in series and in Figure B they are connected in parallel. Let us look at each of these separately.

## Series circuits

Let us first look at the current in a series circuit. In a series circuit the components are connected one after another in a single loop. In a series circuit there is only one pathway through which the electric charge flow. From the above we can know that the current I all along the series circuit remain same. That is in a series circuit the current in each point of the circuit is same.

Now, for example, let us consider three resistors of resistances R1, R2 and R3 that are connected in series. When resistors are connected in series, same current is flowing through each resistor as they are in a single loop. If the potential difference applied between the ends of the combination of resistors is V, then the potential differences across each resistor R1, R2 and R3 are V1, V2 and V3 respectively as shown in Figure 2.16.

The net potential difference, V = V1 + V2 + V3 By Ohm’s law, V1 = IxR1; V2 = IxR2; V3 = IxR3; and V = IxRS

where R3 is the equivalent or effective resistance of the series combination.

Hence, (IxRs) =(IxR1)+ (IxR2) + (IxR3) = Ix(R1 + R2 + R3)

RS = R1 + R2 + R3

Thus, the equivalent resistance of a number of resistors in series connection is equal to the sum of the resistance of individual resistors.

Suppose, n resistors are connected in series, then the equivalent resistor is,

RS = R1 + R2 + R3 + . . . . . . + Rn

Exercise 2.7

Look at the series circuit below. a. What is the effective resistance of the three resistors?

b. What is the current measured by ammeter A1 and ammeter A2?

c. What is the potential difference across each resister?

Solution:

a) Effective resistance R 5 R1 1 R2 1 R3

= 2 + 4 1 6 = 12V

b) Since, V= 6 V and effective resistance is 12V

I= V/R = 6V/12V = 0.5A

As the same current flows through both the resistors, both the ammeters A1 and A2 will show the same current of 0.5A.

c) Let V1, V2 and V3 be the potential difference across the 2V, 4V, 6V resisters respectively, then

V1 = I 3 R1 = 0.5A 3 2V = 1V

V2 = I 3 R2 = 0.5A 3 4V = 2 V

V3 = I 3 R3 = 0.5A 3 6V = 3 V

Now, we can see that V = V1+V2+V3 = 6 V

## Parallel circuits

In parallel circuits, the components are connected to the e.m.f source in two or more loops. In a parallel circuit there is more than one path for the electric charge to ow. In a parallel circuit the sum of the individual current in each of the parallel branches is equal to the main current owing into or out of the parallel branches. Also, in a parallel circuit the potential difference across separate parallel branches are same. Consider three resistors of resistances R1, R2 and R3 connected in parallel. A source of e.m.f with voltage V is connected to the parallel combination of resistors. A current I entering the combination gets divided into I1, I2 and I3 through R1, R2 and R3 respectively as shown in Fig. 2.17.

The total current I is, I = I1 + I2 + I3

By Ohm’s law, I1 = V/R1; I2 = V/R2; I3 = V/R3; and I = V/RP

where RP is the equivalent or effective resistance of the parallel combination.

(V/RP) = (V/R1) + (V/R2) + (V/R3) = Vx(1/R1 +1/R2 +1/R2)

1/RP = 1/R1 +1/R2 +1/R2

Thus, the reciprocal of the effective resistance of resisters in parallel (1/RP) is equal to the sum of the reciprocal of all the individual resistance.

### Exercise 2.8

Figure shows three resistors of values 1 Ω, 3 Ω , and 6 Ω connected in parallel to a 6 V dry cell. (a) What is the effective resistance of the three resistors?

(b) What is the p.d. across each resistor?

(c) What is the current measured by ammeters A1, A2 and A3?

### Solution:

(a) 1/Rp = 1/R1 + 1/R2 + 1/R3

1/Rp = 1/1 + 1/3 + 1/6

1/Rp = 9/6

Rp = 0.667 W

(b) As the resistors are in parallel, the p.d. across each resistor is equal,

p.d. = 6V

(c) (i) I = V/R = 6 V/6 Ω = 1 A

Current measured by ammeter A1 is 1 A

(ii) Current through 3 Ω resistor

= 6 V/3 Ω = 2 A

Current measured by ammeter A2

= 1 A + 2 A = 3 A

(iii) Current through the 1 Ω resistor

= 6 V/1 Ω = 6 A

Current measured by ammeter A3

= 6 A + 3 A = 9 A

Alternatively, since V = 6 V and effective resistence R = 0.667 W, current measured by ammeter

A3 = 6 V / 0.667 Ω = 9 A

Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail
9th Science : Electric charge and electric current : Electric circuit diagram |