Chapter 4 - Current Electricity and Magnetism
1. Write proper words from the following group of words in the blanks.
(magnetism, 4.5V, 3.0V, gravitational attraction, potential difference, potential, higher, lower, 0V)
a. Water in the waterfall flows from a higher level to the lower level because of ________
Ans: gravitational attraction
b. In an electric circuit, electrons flow from a point of ________ potential to the point of ________ potential.
Ans: lower, higher
c. The difference between the electrostatic potential of the positive end and the negative end of an electric cell is the ________ of the cell.
Ans: potential difference
d. Three electric cells of potential difference 1.5 V each have been connected as a battery. The potential difference of the battery will be ________ V.
Ans: 4.5V
e. An electric current flowing in a wire creates ________ around the wire.
Ans: magnetism
2. A battery is to be formed by joining 3 dry cells with connecting wires. Show how you will connect the wires by drawing a diagram.
Ans:
3. In an electric circuit, a battery and a bulb have been connected and the battery consists of two cells of equal potential difference. If the bulb is not glowing, then which tests will you perform in order to find out the reason for the bulb not glowing?
Ans: Tests that can be performed:
(i) Check if the electric bulb and battery are in working condition.
(ii) Check if the connections of the connecting wires between the bulb and battery are proper.
(iii) Check if the two cells are connected in series and are not connected in the reverse direction.
(iv) Check if the battery is supplying enough electric potential for the bulb to glow.
4. Electric cells having 2V potential difference each have been connected in the form of a battery. What will be the total potential difference of the battery in both cases?
(i)
Ans: In this case, three cells with a potential difference of 2 V each are connected in series, so the total potential difference of the battery will be 6 V.
(ii)
Ans: In this case, four cells with a potential difference of 2 V each are connected in series, so the total potential difference of the battery will be 8 V.
5. Describe the construction, working and usefulness of a dry cell, with the help of a diagram.
Ans:
Construction:
(i) A dry cell has an outer protective coating.
(ii) Inside the coating, it has a whitish metal layer made up of zinc.
(iii) This zinc layer acts as the negative terminal of the cell.
(iv) There is another layer inside to separate the zinc layer from the other materials of the cell.
(v) An electrolyte is filled between these two layers and it contains negatively charged and positively charged ions which are the carriers of electricity.
(vi) The electrolyte is a wet pulp made up of zinc chloride (ZnCl₂) and ammonium chloride (NH₄).
(vii) A graphite rod is placed at the centre of the cell which acts as the positive terminal of the cell.
(viii) The graphite rod is surrounded with a paste of manganese dioxide (MnO).
Working:
(i) Due to the chemical reactions of all the chemicals present in the dry cell, an electric charge is produced across the graphite rod and zinc layer. Since electric charge is produced on the two terminals, electric current flows in the circuit.
(ii) A large electric current is not obtained from this cell as, due to the wet pulp used in the cell, the chemical reaction takes place at a very slow rate.
Usefulness:
(i) When compared with electric cells using liquids, the shelf life of dry cells is longer.
(ii) Dry cells are very convenient to use, as they can be used in any direction with respect to the ground. Hence, they can be used in mobile instruments.
(iii) Dry cells are available in 3–4 sizes.
(iv) Dry cells are used in radio sets, wall clocks, torches, etc.
6. Describe the construction and working of an electric bell with the help of a diagram.
Ans:
Construction:
(i) The electric bell consists of an iron piece wound by the copper wire. This coil acts as an electromagnet.
(ii) An iron strip with a striker is fitted near the electromagnet.
(iii) A contact screw is in touch with the strip, and it is connected to the external circuit as shown in the diagram.
Working:
(i) When the screw is in contact with the strip, current flows in the circuit, and the coil acts as a magnet and attracts the iron strip towards it.
(ii) In this way, the striker hits the gong, and the sound is created.
(iii) Simultaneously, the contact screw loses contact with the strip, and the current stops.
(iv) As a result, the electromagnet loses its magnetism, and the iron strip moves back and comes in contact with the contact screw again.
(v) As soon as the electric current is restored, the striker hits the gong again.
(vi) This action keeps repeating itself, and the bell rings.