Current Electricity and Magnetism - Standard 8 - General Science - Balbharati
4. Current Electricity and Magnetism
Exercise
Q. 1 Fill in the blanks with proper words.
(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 gravitational attraction.
(b) In an electric circuit, electrons flow from a point of lower potential to the point of higher potential.
(c) The difference between the electrostatic potential of the positive end and the negative end of an electric cell is the potential difference of the cell.
(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 4.5V.
(e) An electric current flowing in a wire creates magnetism around the wire.
Q. 2 Show how to join 3 dry cells to form a battery (diagram).
Ans. To form a battery (series connection), connect the positive terminal of the first cell to the negative terminal of the second, the positive of the second to the negative of the third. The free negative terminal of the first and the free positive terminal of the third are the battery terminals. Example diagram (text):
(+) |‾‾| (-) ── (+) |‾‾| (-) ── (+) |‾‾| (-) cell 1 cell 2 cell 3
Q. 3 A battery and a bulb are connected and the bulb is not glowing. Which tests will you perform to find out the reason?
Ans. Check the following step by step:
- Check the plug key (switch): Is it closed? If open, close it and see if the bulb glows.
- Check the cell orientation: Ensure the cell(s) are placed with correct polarity in the holder (positive to negative as required by the circuit).
- Check the cell condition: Use a fresh/different cell or measure the cell voltage — a dead/weak cell will not give sufficient potential difference for the bulb to glow.
- Check the bulb: Replace the bulb with a known good bulb or test the bulb’s filament continuity (a fused filament will not glow).
- Check connecting wires and contacts: Ensure all wires and contacts are firmly connected, no loose or broken connections, and no corrosion on terminals.
- Check for short circuits: Make sure there is no unintended short across the battery that bypasses the bulb.
Q. 4 Electric cells having 2 V each are connected to form a battery. What will be the total potential difference in the two cases: series and parallel?
Ans.
(i) Series connection: When n cells each of 2 V are connected in series, their potentials add. Total potential difference = n × 2 V. Example: 3 cells in series → 2 V + 2 V + 2 V = 6 V.
(ii) Parallel connection: When identical cells are connected in parallel (positive terminals joined, negative terminals joined), the total potential difference across the combination remains the same as that of one cell = 2 V, while the capacity (current delivered/ life) increases.
Q. 5 Describe the construction, working and usefulness of a dry cell (with diagram).
Ans.
Construction: A dry cell has an outer zinc container which acts as the negative terminal. Inside there is a moist paste (electrolyte) made of zinc chloride and ammonium chloride. At the centre a carbon (graphite) rod is placed which acts as the positive terminal. A paste of manganese dioxide surrounds the rod. The cell parts are arranged so that chemical reactions occur between the zinc, electrolyte and manganese dioxide. (Textbook diagram shows: zinc cover (negative), electrolyte paste, manganese dioxide, carbon rod (positive).)
Working: Due to chemical reactions in the dry cell, electrical charges are produced on the two terminals (graphite rod and zinc layer). This creates a potential difference between the positive and negative terminals. When the cell is connected in a circuit, electrons flow through the external circuit from the negative terminal to the positive terminal, producing an electric current. The electrolyte supplies ions to complete the internal circuit.
Usefulness / Uses: Dry cells are convenient, can be used in any position, have a long shelf life and are used in small devices such as torches, radios, wall clocks and remote controls. They provide a steady potential difference for portable instruments.
Q. 6 Describe the construction and working of an electric bell (with diagram).
Ans.
Construction: An electric bell contains an electromagnet (a coil of insulated copper wire wound on an iron core), an iron strip with a striker (hammer) attached, a contact screw, a gong (bell), a spring to return the iron strip, and connecting terminals for the electric circuit. The contact screw is positioned so that when the strip is at rest it touches the screw and completes the circuit.
Working: When the plug key is closed, current flows through the coil and the coil becomes an electromagnet. The electromagnet attracts the iron strip, pulling the striker to hit the gong and produce sound. When the strip moves, it breaks contact with the screw and the current stops. The electromagnet then loses its magnetism and the strip returns to its original position by the spring, making contact again. This cycle repeats rapidly and the striker keeps hitting the gong, producing a ringing sound.
Usefulness: Electric bells are used as door bells and alarm signals; they convert electrical energy into mechanical motion and sound using electromagnetism.
