ICSE Class 8 Physics Chapter 11 Current Electricity

11 Current Electricity

This chapter consists of the following topics:

Electricity in motion (the electric current), strength of current, unit of charge and current.

Electric potential, potential difference, unit of potential difference.

Ammeter and voltmeter.

Electric resistance, factors affecting the resistance.

Heat analogy and water analogy.

Ohm's law, definition of one ohm.

Combination of resistances in series and in parallel.

Heating effect of current with applications.

Electric bulb and electric fuse.

Power and rating of an electrical appliance.

Commercial unit of electrical energy.

Note: The chapter on Current Electricity has been purposely inducted in this book although it is not a part of the syllabus. This will make the students to understand the basics of Current Electricity (i.e., electricity in motion) which will be further useful in the learning of concepts of Electromagnet and Electromagnetic Induction.

Electric Current

When two charged conductors are joined together by a metallic wire, or are brought in contact, the free electrons flow from one conductor to the other. Electrons flow from the conductor with surplus of free electrons to the other conductor which is in deficit of free electrons. The motion of electrons stops when the concentration of electrons in both the conductors become equal.

Such moving electrons flowing in a particular direction constitute an electric current.

Strength Of An Electric Current

Strength of an electric current, through a conductor, is the amount of charge that flows through the conductor in one second. It is denoted by I.

Let Q charge flows through a conductor in time t, then,

Strength of current through conductor = \(\frac{\text{charge that flows}}{\text{time taken}}\)

\(I = \frac{Q}{t}\)

Unit Of Charge And Current (Electric Current)

The S.I. unit of charge is coulomb which is denoted by letter C and the unit of current is ampere which is denoted by letter A.

\(I = \frac{Q}{t}\)

\(\Rightarrow\) 1 Ampere = \(\frac{1 \text{ Coulomb}}{1 \text{ second}}\) or 1 A = \(\frac{1 C}{1 s}\)

Thus, strength of current through a conductor is said to be 1 ampere if 1 coulomb charge flows through it in 1 second.

Smaller units of current are:

Milli ampere (mA) \(\Rightarrow\) 1 mA = 10-3 A

Micro ampere (μA) \(\Rightarrow\) 1 μA = 10-6 A

Nano ampere (nA) \(\Rightarrow\) 1 nA = 10-9 A

Peco ampere (μμA or pA) \(\Rightarrow\) 1 pA = 10-12 A

Charge on one electron = 1-6 × 10-19 C

The instrument used to measure the strength of current through a conductor is called Ammeter. Ammeter is connected in series with the given circuit.

Currents of approximately 0-2A are potentially fatal, because they can make the heart to beat in an uncontrolled manner.

The energy sources we use to make electricity can be renewable or non-renewable, but electricity itself is neither renewable nor non-renewable.

Example 1

If 6 coulomb of charge flows through a conductor in 3 seconds, find the strength of electric current.

Solution:

Given: Charge (Q) = 6 C and time (t) = 3 s

\(\therefore\) Current (I) = \(\frac{Q}{t}\) = \(\frac{6 C}{3 s}\) = 2 Ampere

Example 2

An electric current of 5 ampere flows through a conductor for 12 seconds. Find the amount of charge.

Solution:

Given: Current (I) = 5 A and time (t) = 12 s

Since, \(I = \frac{Q}{t}\)

\(\Rightarrow\) \(Q = I \times t\) = 5A × 12 s = 60 C

\(\therefore\) Amount of charge = 60 C

Example 3

For how much time must a charge of 20 coulomb flow through a conductor to constitute a current of 4 ampere?

Solution:

Given: Charge (Q) = 20 C and Current (I) = 4 A

Since, \(I = \frac{Q}{t}\)

\(\Rightarrow\) \(t = \frac{Q}{I}\) = \(\frac{20 C}{4 A}\) = 5 s

\(\therefore\) Required time = 5 s

Teacher's Note

When you plug in a phone charger, electric current flows through the circuit at a rate measured in amperes. Understanding current helps us appreciate why overcharging can be dangerous.

Electric Potential

When two charged bodies are connected with the help of a conductor, excess electrons flow through the conductor from the body with higher concentration of electrons to the body with lower concentration of electrons as shown in Fig. 11.1.

The flow of excess electrons will take place from:

(i) a negatively charged conductor to a positively charged conductor as shown in Fig 11.1.

(ii) a negatively charged conductor to an uncharged conductor (see Fig. 11.2(a)).

(iii) an uncharged conductor to a positively charged conductor (see Fig. 11.2 (b)).

Thus, electric potential is the electric condition which determines the direction of flow of charge (electrons) from one body to another body.

The conductor with higher concentration of electrons is said to be at a lower potential and the conductor with lower concentration of electrons is said to be at a higher potential. On connecting, electrons flow from the conductor at lower potential to the conductor at higher potential. Conventionally, we say that, electric current flows from the body at a higher potential to the body at a lower potential; for this reason, the electric current is called conventional current as shown in Fig. 11.3.

For a battery, electronic current (flow of electrons) is from negative terminal to positive terminal, whereas the conventional current flows from positive terminal to negative terminal outside the battery.

In electrical circuits, the direction of conventional current is marked and not the direction of electronic current.

We detect the direction of electric current with the help of Galvanometer.

Potential Difference

Potential difference between any two points in an electric field is the amount of work done in moving a unit positive charge from one point to the other.

Suppose W joule of work is done in bringing Q coulomb of positive charge from one point to the other. Then the potential difference (V) between the two points will be given by:

Potential difference = \(\frac{\text{Work done}}{\text{Charge moved}}\)

\(V = \frac{W}{Q}\)

Unit Of Potential Difference

SI unit of potential difference is volt. Since, the SI unit of work is joule and the SI unit of charge is coulomb

\(\therefore\) 1 volt = \(\frac{1 \text{ Joule}}{1 \text{ Coulomb}}\)

Thus, if 1 joule of work is done in moving 1 coulomb of positive charge between two points in an electric field, the potential difference between these two points is 1 volt.

The bigger units of potential difference are kilovolt and megavolt.

1 Kilovolt = 1000 volt.

1 Megavolt = 1000000 volt = 10^6 volt

We measure potential difference between the two given points by an instrument called Voltmeter. Voltmeter is connected in parallel to given circuit.

Voltmeter: A glass or a plastic container containing two electrodes and an electrolytic solution is called voltameter. Voltameter is also called an electrolytic cell. It is different from voltmeter.

LED (Light Emitting Diode): LED can be used for lighting in place of an electric bulb or CFL, due to their high energy efficiency and low power consumption. LED glows even when a small electric current flows through it. LEDs are available in different colours.

Note: Electric circuit - The path along which an electric current can flow is called electric circuit.

Example 4

60 J of work is done in flowing 12 coulomb of positive charge between two points in an electric field. Find the potential difference between these two points.

Solution:

Given: Work (W) = 60 J and charge moved (Q) = 12 C

\(\therefore\) Potential difference = \(\frac{\text{Work done}}{\text{Charge moved}}\) = \(\frac{60 J}{12 C}\) = 5V

Example 5

How much work is said to be done in moving 15 coulomb of positive charge between two points with a potential difference of 3-2 volt.

Solution:

Given: Charge (Q) = 15 C and potential difference (V) = 3-2 V

Since, \(V = \frac{W}{Q}\)

\(\Rightarrow\) \(W = V \times Q\) = 3-2 V × 15 C = 48 J

\(\therefore\) Required work done = 48 J

Teacher's Note

A battery's voltage rating tells you how much energy it can provide to move charges through a circuit. Higher voltage means more work done per unit charge.

Description Of Ammeter And Voltmeter

Ammeter: The current flowing in a circuit is measured by an ammeter. It has a circular scale graduated in amperes as shown in Fig. 11.5. An ammeter is connected in series in a circuit. The terminal (+) is connected to the positive terminal of the cell and the terminal (-) is connected to the negative terminal of the cell through the other electrical components.

The Figs. 11.6(a) and 11.6(b) show the connection of ammeter in a circuit.

Voltmeter: The potential difference between the two points of a circuit is measured by a voltmeter. It has a graduated scale in volts as shown in Fig. 11.7. A voltmeter is connected in parallel across the two points between which the potential difference is to be measured. The terminal (+) is connected to the point which is connected to the positive terminal of the cell and the terminal (-) is connected towards the negative terminal of the cell. Figure 11.8 shows the connection of voltmeter in parallel in a circuit to find out the potential difference across a bulb.

Classroom discussion: Do you think that an electric car which uses rechargeable batteries does not waste energy or cause pollution? Discuss.

Electrical Resistance

There is always some obstruction to the flow of current, when it flows through a conductor like a metal wire.

This obstruction in the flow of current is provided by the material of the conductor and is called its electrical resistance.

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