Class 7 Science Electric Current and its Circuits Chapter Notes

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This Class 7 Science chapter notes page on Electric Current and its Circuits covers the symbols of electric components, electric circuits and their parts, electric charge, electric current, potential difference, Ohm's law, series and parallel combinations of resistances, the heating and magnetic effects of current, electromagnets and the electric bell, followed by the chapter exercise with answers.

Topics Covered

1. Introduction
2. Symbols of electric components
3. Electrical circuit
4. Electric charge, electric current and potential difference
5. Ohm's law and resistance
6. Combination of resistances
7. Heating effect of current
8. Magnetic effect of electric current
9. Electromagnets and electric bell
10. Exercises

4.1 Introduction

Electricity is important in our daily life. Our household contains television, fan, AC, tubelights, etc. All these appliances need electricity to work.

4.2 Symbols of Electric Components

Some common electric components can be represented by symbols. Table 4.1 shows some electric components and their symbols. An electric cell has a positive terminal and a negative terminal. In the symbol of the electric cell, the longer line represents the positive terminal and the thicker, shorter line represents the negative terminal.

For a switch, the 'ON' position and the 'OFF' position are represented by the symbols as shown. The wires used to connect the various components in a circuit are represented by lines.

Table 4.1 Symbols for Some Electric Circuit Components

[Figure: Electric cell picture beside its symbol, electric bulb picture beside its symbol, switch in ON position beside its symbol, switch in OFF position beside its symbol, battery picture beside its symbol, wire picture beside its symbol, as printed in Table 4.1]

Notice that the positive terminal of one cell is connected to the negative terminal of the next cell. Such a combination of two or more cells is called a battery.

Fig. 4.1 (a) A battery of two cells (b) A battery of four cells

Fig. 4.2 Connecting two cells together to make a battery

Fig. 4.3 An electric circuit (cell, switch and bulb joined by wires on a board)

Fig. 4.4 Circuit diagram of electric circuit shown in fig. 4.3

In the bulb, there is a thin wire, called the filament, which glows when an electric current passes through it. When the bulb gets fused, its filament is broken.

4.3 Electrical Circuit

An electrical circuit is an electrical network which provides an electrical path for the flow of electrical current and electrical charge. An electrical circuit generally consists of various electrical components and an electrical load.

The components of the circuit shown above are:
(i) Battery/Cell (B): A cell or battery acts as the source of charge/current in an electrical circuit.
(ii) Electrical switch (S): An electrical switch is a device which is used to either make or break the path for the flow of current.
(iii) Electrical load (RL): The external element or object across which all the electrical measurements or calculations are made. This is the chief element which is used for the conversion of electrical energy into the required form of energy for use. Example: bulb, heater, etc.
(iv) Voltmeter (V): An electrical instrument that is used to measure the potential difference between two points in an electrical circuit. A voltmeter is always connected in parallel to the two points across which the potential difference is to be calculated.
(v) Ammeter (A): An electrical instrument that is used to measure the electrical current flowing through a load or electrical path. An ammeter is always connected in series with the electrical path through which the current is to be measured.

Note:
(i) In a battery, the positive terminal is considered to be the starting point of charge/current, while the negative terminal is the ending point.
(ii) Outside the battery, current flows from the positive to the negative terminal, whereas the direction is reversed inside the battery.
(iii) The switch may be in 'ON' or 'OFF' state, depending on whether it is closed or open. A closed switch is in the ON state, and an open switch is in the OFF state.


(iv) Whether the current or charge will flow through the electrical load or not is decided by the topology of the electrical circuit and the position of the electrical switch.

4.4 Electric Charge and Electric Current

(i) Electric charge: Electric charge can be defined as the physical property that arises due to deficiency or excess of electrons in a substance. This property is responsible for electrical effects.
(ii) Electric current: Electric current is defined as the rate of flow of charge. Mathematically,
Electric current, I = \( \frac{Q}{t} \), where Q is the charge and t is the time.
Electric current is a scalar quantity. Its SI unit is ampere (A).

4.5 Electrical Potential Difference

The electrical potential difference between two points is the compelling force which causes the electrical charges, and hence the electric current, to flow through an electrical path. Theoretically, it is defined as the amount of work done in moving a unit positive charge between two points without imparting any kinetic energy to it.
\( V_A - V_B = \frac{W_{AB}}{q} \), where V_A - V_B is the potential difference between two points A and B, q is the unit positive charge, and W_AB is the amount of work done in moving the charge between points A and B.
The SI unit of electrical potential difference is volt (V).

Electrical Potential

The electric potential at a point is defined as the amount of work done in bringing a unit positive charge from infinity to that point. The SI unit of electric potential is the same as that of potential difference, that is, volt (V).

Note:
(i) Electrical potential difference is a practical term, whereas electric potential is a referential term in which one point is considered to be at infinity.
(ii) Electric current and positive charges flow from high potential to low potential in an electrical circuit, outside the battery.
(iii) Electrons (negative charges) flow from low potential to high potential. Therefore, the direction of current is taken to be opposite to the direction of flow of electrons.

4.6 Ohm's Law and Resistance

[Figure: Two circuit diagrams, each with a conductor between points A and B, a battery marked + and - and a current I shown in the wire]

Ohm's law states that at constant physical conditions (that is, at constant temperature), the electrical current flowing through a conductor is directly proportional to the electrical potential difference between the two ends of the conductor.
I is proportional to (V_A - V_B), where V_A - V_B = V
or I is proportional to V
or I = \( \frac{V}{R} \)
Here R is the constant of proportionality, called the electrical resistance of the conductor.

Electrical Resistance: Electrical resistance is the property of a material which obstructs the flow of current or charge through the material.
(i) It depends on the nature of the material.
(ii) It also depends on the temperature.
The SI unit of resistance is ohm, whose symbol is the Greek letter omega (Ω).
Note: Ohm's law is defined at constant temperature because the resistance of a material changes with the change in temperature, and hence affects the flow of current through the conductor.

4.7 Combination of Resistances

Resistances can be combined in two ways. The first combination is called series combination, and the second is called parallel combination.

Series combination (circuit diagram)

Parallel combination (circuit diagram)

Series Combination: The equivalent resistance is given by
R = R₁ + R₂ + R₃

Parallel Combination: The equivalent resistance is given by
\( \frac{1}{R} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} \)

4.8 Heating Effect of Current

The wire gets hot when an electric current passes through it. This is called the heating effect of the electric current. You might have seen an electric room heater or an electric heater used for cooking. All these contain a coil of wire, which is called an element.

Wires made from some special materials melt quickly and break when large electric currents are passed through them. These wires are used for making electric fuses. Fuses are inserted in all electrical circuits in buildings. There is a maximum limit on the current which can safely flow through a circuit. If, by accident, the current exceeds this safe limit, the wires may become overheated and may cause fire. If a proper fuse is there in the circuit, it will blow off and break the circuit. A fuse is thus a safety device which prevents damage to electrical circuits and possible fires.

Fig. 4.5 Fuse used in buildings
Fig. 4.6 Fuses used in electrical appliances

4.9 Magnetic Effect of Electric Current

Take the cardboard tray from inside a discarded matchbox. Wrap an electric wire a few times around the cardboard tray. Place a small compass needle inside it. Now connect the free ends of this wire to an electric cell through a switch, as shown in Figure 4.7.

Fig. 4.7 Effect of current on a compass needle

We know that the needle of a compass is a tiny magnet, which points in the north-south direction. When we bring a magnet close to it, the needle gets deflected. We have also seen that a compass needle gets deflected when current flows in a nearby wire.

So, when electric current passes through a wire, it behaves like a magnet. This is the magnetic effect of the electric current. In fact, an electric current can be used to make magnets.

4.10 Electromagnets

Electromagnets are temporary magnets. When electric current is passed through a coil, it behaves as a magnet. Electromagnets can be made very strong and can lift very heavy loads. Electromagnets are also used to separate magnetic material from junk. Doctors use a tiny electromagnet to take out small pieces of magnetic material that have accidentally fallen in the eye. Many toys also have electromagnets inside them.
We can make our own electromagnet by wrapping a long piece of insulated wire on an iron nail. Finally, connect the ends of the wire to the terminals of a cell, as shown in Figure 4.8.

Fig. 4.8 An electromagnet

4.11 Electric Bell

An electric bell consists of a coil of wire wound on an iron piece. The coil acts as an electromagnet. An iron strip with a hammer at one end is kept close to the electromagnet. There is a contact screw near the iron strip. When the iron strip is in contact with the screw, the current flows through the coil, which becomes an electromagnet. It then pulls the iron strip. In the process, the hammer at the end of the strip strikes the gong of the bell to produce a sound. However, when the electromagnet pulls the iron strip, it also breaks the circuit.
This process is repeated in quick succession. The hammer strikes the gong every time the circuit is completed. This is how the bell rings.

Fig. 4.9 Circuit of an electric bell

EXERCISES

Objective Questions

1. The S.I. unit of electric current is
(a) Maxwell
(b) Ampere
(c) Oersted
(d) Newton

Answer: (b) Ampere

1. The SI unit of electric current is the ampere, written as A.
2. Hence, the correct option is (b).

Teacher's Note:
a) Current is the rate of flow of charge, so its unit is charge per second.
b) Maxwell is a unit of magnetic flux and Oersted is a unit of magnetic field strength, so both are wrong here.

2. Choose the correct alternative
i) An electric bell has an electromagnet.
ii) Current does not flow in an open circuit.
(a) only (i) is correct
(b) only (ii) is correct
(c) Both (i) and (ii) are correct
(d) Both (i) and (ii) are wrong

Answer: (c) Both (i) and (ii) are correct

1. The coil of an electric bell becomes an electromagnet when current flows, so statement (i) is correct.
2. In an open circuit the path is broken, so no current flows. Statement (ii) is correct.
3. Hence, the correct option is (c).

Teacher's Note:
a) An electric bell works on the electromagnetic effect of current.
b) An open circuit has a break in its path, which stops the flow of current.
c) Both statements describe the same idea: a complete path is needed for current.

3. The energy spent in moving 1 C of charge through 10 V is
(a) 1 J
(b) 10 J
(c) 5 J
(d) 15 J

Answer: (b) 10 J

1. Potential difference V = \( \frac{W}{Q} \), so W = V × Q.
2. W = 10 V × 1 C = 10 J.
3. Hence, the correct option is (b).

Teacher's Note:
a) Potential difference is the work done per unit charge.
b) Multiply, do not divide: work = voltage × charge.

4. The S.I. unit of resistance is
(a) ohm
(b) mho
(c) Newton
(d) Ampere

Answer: (a) ohm

1. Resistance is measured in ohm, whose symbol is Ω.
2. Hence, the correct option is (a).

Teacher's Note:
a) Mho is the unit of conductance, which is the reciprocal of resistance.
b) Newton and ampere measure force and current, not resistance.
c) Keep the spelling of the unit as ohm, not ohms.

5. The value of current flowing through a wire of resistance 20 Ω when it is connected to a battery of 40 volt is
(a) 20 A
(b) 2 A
(c) 10 A
(d) 4 A

Answer: (b) 2 A

1. Given: V = 40 V and R = 20 Ω.
2. By Ohm's law, I = \( \frac{V}{R} \) = \( \frac{40}{20} \).
3. I = 2 A.
4. Hence, the correct option is (b).

Teacher's Note:
a) Ohm's law gives current as voltage divided by resistance.
b) A current of 20 A would need a much smaller resistance.
c) Check the units: volt divided by ohm gives ampere.

6. If 5 A current is flowing through a wire when it is connected to a battery of 20 V, then the value of resistance is
(a) 5 ohm
(b) 10 ohm
(c) 20 ohm
(d) 4 ohm

Answer: (d) 4 ohm

1. Given: I = 5 A and V = 20 V.
2. By Ohm's law, R = \( \frac{V}{I} \) = \( \frac{20}{5} \).
3. R = 4 ohm.
4. Hence, the correct option is (d).

Teacher's Note:
a) Rearrange I = V/R to get R = V/I.
b) Option (a) comes from dividing 20 by 4, which reverses the roles of current and resistance.

7. Three resistances 2 Ω, 6 Ω and 10 Ω are connected in series. The equivalent resistance of the circuit is
(a) \( \frac{30}{23} \) ohm
(b) 6 ohm
(c) 18 ohm
(d) 10 ohm

Answer: (c) 18 ohm

1. In series, R = R₁ + R₂ + R₃.
2. R = 2 + 6 + 10 = 18 ohm.
3. Hence, the correct option is (c).

Teacher's Note:
a) Resistances in series simply add up.
b) The parallel formula gives a value smaller than the smallest resistance, so \( \frac{30}{23} \) ohm cannot be right here.

8. Two resistances 6 Ω and 12 Ω are connected in parallel. The value of equivalent resistance is
(a) 4 ohm
(b) 18 ohm
(c) 12 ohm
(d) 6 ohm

Answer: (a) 4 ohm

1. In parallel, \( \frac{1}{R} = \frac{1}{R_1} + \frac{1}{R_2} \).
2. \( \frac{1}{R} = \frac{1}{6} + \frac{1}{12} = \frac{2}{12} + \frac{1}{12} = \frac{3}{12} \).
3. R = \( \frac{12}{3} \) = 4 ohm.
4. Hence, the correct option is (a).

Teacher's Note:
a) The equivalent resistance of parallel resistances is always less than the smallest one.
b) For two resistances only, R = \( \frac{R_1 R_2}{R_1 + R_2} \) gives the same answer: \( \frac{72}{18} \) = 4 ohm.

9. Choose the correct alternative
i) in the bulb, filament glows when an electric current passes through it.
ii) If the filament of the bulb is broken, the circuit is called incomplete.
(a) only (i) is correct
(b) only (ii) is correct
(c) Both are correct
(d) Both are wrong

Answer: (c) Both are correct

1. The filament glows when current passes through it, so statement (i) is correct.
2. A broken filament breaks the path of current, so the circuit is incomplete. Statement (ii) is correct.
3. Hence, the correct option is (c).

Teacher's Note:
a) A filament glows because it heats up when current flows through it.
b) A broken filament means an open circuit, so the bulb does not glow.

10. Choose the correct alternative
i) In closed circuit, current flows
ii) In open circuit, current flows
iii) In open circuit, current does not flows
iv) In closed circuit, current does not flows
(a) (i) and (ii) are correct
(b) (ii) and (iv) are correct
(c) (i) and (iii) are correct
(d) There is no term like open or closed circuit

Answer: (c) (i) and (iii) are correct

1. In a closed circuit the path is complete, so current flows. Statement (i) is correct.
2. In an open circuit the path is broken, so current does not flow. Statement (ii) is wrong and statement (iii) is correct.
3. Statement (iv) is wrong, since current flows in a closed circuit.
4. Hence, the correct option is (c).

Teacher's Note:
a) A closed circuit has a complete path for current.
b) Open and closed circuits are real terms used in every circuit topic.
c) Remember that current never flows in an open circuit.

Subjective Questions

1. Define electric current and electric potential.

Answer:
1. Electric current is the rate of flow of electric charge through a conductor. Its SI unit is ampere (A), and it is given by I = \( \frac{Q}{t} \).
2. Electric potential at a point is the amount of work done in bringing a unit positive charge from infinity to that point. Its SI unit is volt (V).

Teacher's Note:
a) Current is about the flow of charge, while potential is about the work done on charge.
b) Students often confuse potential with potential difference; the first is measured from infinity, the second between two points.

2. Draw the diagram of different electric circuit.

Answer:
1. A simple electric circuit has a cell or battery as the source, a switch to make or break the path, and a load such as a bulb, joined in a closed loop by wires.
2. A voltmeter is connected in parallel across the load to measure its potential difference.
3. An ammeter is connected in series with the load to measure the current.
4. The figure below shows the circuit with a load RL, a battery B, a switch S, a voltmeter V and an ammeter A, with points A and B marked.
[Figure: diagram to be added]

Teacher's Note:
a) Use the standard symbols for cell, switch, bulb, voltmeter and ammeter.
b) An ammeter goes in series, while a voltmeter goes in parallel.

3. Explain the benefits of electric fuses.

Answer:
1. A fuse is made of a special wire that melts quickly when a large current passes through it.
2. When the current exceeds the safe limit, the fuse blows off and breaks the circuit, which stops the current.
3. This protects the electrical appliances and wiring from damage.
4. It also prevents overheating of wires, which could otherwise cause a fire.

Teacher's Note:
a) A fuse is a safety device, not a switch to control appliances.
b) The fuse must have the right rating for the circuit, so that it blows at the safe limit.
c) Fire prevention is the most important benefit of a fuse.

4. Explain why the compass needle gets deflected when electric current is switched on through a wire.

Answer:
1. A compass needle is a tiny magnet which points in the north-south direction.
2. When electric current flows through a wire, it produces a magnetic field around the wire, so the wire behaves like a magnet.
3. This magnetic field acts on the compass needle and deflects it from the north-south direction.

Teacher's Note:
a) This is the magnetic effect of electric current.
b) The needle returns to its normal position when the current is switched off.
c) The deflection is greater when the needle is closer to the wire.

5. Write Ohm's law. Hence define resistance.

Answer:
1. Ohm's law states that at constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its ends.
2. Mathematically, I is proportional to V, or I = \( \frac{V}{R} \).
3. Here R is the constant of proportionality, called the electrical resistance of the conductor.
4. Hence, resistance is the property of a conductor which obstructs the flow of current through it. Its SI unit is ohm.

Teacher's Note:
a) The law holds only at constant temperature, because resistance changes with temperature.
b) Resistance depends on the nature of the material as well as on its temperature.

6. Explain the magnetic effect of current with a neat diagram.

Answer:
1. A wire is wrapped a few times around a cardboard tray, and a small compass needle is placed inside it.
2. The free ends of the wire are connected to a cell through a switch.
3. When the switch is closed, current flows through the wire and the compass needle is deflected.
4. This shows that the current-carrying wire behaves like a magnet, which is the magnetic effect of electric current.
[Figure: diagram to be added]

Teacher's Note:
a) The diagram must show the wire wound around the tray, the compass needle inside, the switch and the cell.
b) The needle deflects only while the current flows.
c) The effect is the basis of the electromagnet and the electric bell.

7. Explain how we can make our own electromagnet. Support your answer with a diagram.

Answer:
1. Take a long piece of insulated wire and wrap it around an iron nail.
2. Connect the two free ends of the wire to the two terminals of a cell.
3. When current flows, the nail becomes magnetic and can pick up small pins or iron pieces.
4. When the current is switched off, the nail loses its magnetism.
[Figure: diagram to be added]

Teacher's Note:
a) The insulation on the wire stops the turns from short-circuiting each other.
b) More turns of wire make the electromagnet stronger.

8. Explain the construction and working of an electric bell.

Answer:
1. Construction: An electric bell has a coil of wire wound on an iron piece, which acts as an electromagnet. An iron strip with a hammer at one end is placed close to the electromagnet, and a contact screw is fixed near the iron strip.
2. Working: When the iron strip touches the contact screw, current flows through the coil, and the coil becomes an electromagnet.
3. The electromagnet pulls the iron strip, the hammer strikes the gong to produce a sound, and the pull also breaks the circuit at the contact screw.
4. With the circuit broken, the magnetism is lost and the strip springs back. The circuit is completed again, and the process repeats in quick succession, so the bell keeps ringing.

Teacher's Note:
a) The bell works on the magnetic effect of current and the making and breaking of the circuit.
b) The hammer strikes the gong each time the circuit is completed.
c) A diagram with labels for the coil, iron strip, hammer, gong and contact screw helps in the exam.

9. Find the equivalent resistance and total current in the circuit given below.

[Figure: (a) Three resistors of 2 Ω, 4 Ω and 6 Ω joined one after another in a single row, connected across a 24 V battery. (b) Three resistors of 9 Ω, 6 Ω and 4 Ω joined side by side between two common points, with an 18 V battery connected across the same two points]

Answer:

(a) Series circuit

1. Resistances in series: R = R₁ + R₂ + R₃ = 2 + 4 + 6 = 12 ohm.
2. Total current: I = \( \frac{V}{R} \) = \( \frac{24}{12} \) = 2 A.

(b) Parallel circuit

1. \( \frac{1}{R} = \frac{1}{9} + \frac{1}{6} + \frac{1}{4} \) = \( \frac{4}{36} + \frac{6}{36} + \frac{9}{36} \) = \( \frac{19}{36} \).
2. Equivalent resistance R = \( \frac{36}{19} \) ohm, which is about 1.89 ohm.
3. Total current: I = \( \frac{V}{R} \) = \( \frac{18}{\frac{36}{19}} \) = 18 × \( \frac{19}{36} \) = 9.5 A.

Teacher's Note:
a) Series resistances are added directly, which gives 12 ohm here.
b) For parallel resistances, add the reciprocals first, then take the reciprocal of the sum.
c) The equivalent resistance in parallel is smaller than the smallest branch, that is, smaller than 4 ohm.

10. An electrician is carrying out some repairs in your house. He wants to replace a fuse by a piece of wire. Would you agree? Give reasons for your response.

Answer:
1. No, I would not agree.
2. A fuse protects the circuit by melting and breaking it when the current exceeds the safe limit.
3. A piece of ordinary wire does not melt at the safe limit, so the current can rise to dangerous levels.
4. The overheated wires may cause a fire and damage the appliances connected to the circuit.

Teacher's Note:
a) A fuse is a safety device and must never be replaced by an ordinary wire.
b) Overheating is the reason fires start in faulty circuits.
c) Always replace a blown fuse with one of the correct rating.

Answers

In the above figure, various electrical components have been shown.