Electric Current and its Effects

An electric circuit is shown simply by a circuit diagram using standard symbols for a cell, a battery, a switch in the ON and OFF positions, a bulb and connecting wires. In the symbol of a cell, the longer line is the positive terminal and the shorter, thicker line is the negative terminal. Cells joined so that the positive terminal of one is connected to the negative terminal of the next form a battery. A bulb glows only when the circuit is closed, letting current flow through it. Electric current has two important effects. The heating effect makes a wire hot when current passes through it and is used in heaters, irons, geysers and bulb filaments. It is also the basis of the fuse, which melts and breaks the circuit when excessive current flows; MCBs now replace fuses. The magnetic effect, first observed by Hans Christian Oersted, makes a current-carrying wire deflect a compass needle. A coil wound on an iron core becomes an electromagnet, used in electric bells and cranes.

Detailed Worksheet: Electric Current and its Effects Section A - Definitions (10 marks) 1. What is a circuit diagram? Why are symbols used to draw it? (2 marks) 2. What is a battery? How are cells connected to make a battery? (2 marks) 3. What is the heating effect of electric current? Give two appliances based on it. (2 marks) 4. What is an electric fuse? How does it protect a circuit? (2 marks) 5. What is an electromagnet? How is it different from a permanent magnet? (2 marks) Section B - Calculations and Applications (15 marks) 6. A torch uses two cells of 1.5 V each joined correctly in series. (i) What is the total voltage? (ii) How must the cells be placed? (iii) What happens if one cell is reversed? (3 marks) 7. Why does the filament of a bulb glow but the connecting wires do not get very hot? Name the material used for filaments and why. (3 marks) 8. Explain why CFLs and LEDs are preferred over incandescent bulbs. What wasteful effect occurs in an ordinary bulb? (3 marks) 9. An electromagnet picks up 4 pins when wound with 20 turns and 9 pins with 40 turns, using the same cell. (i) What happens to its strength as the number of turns increases? (ii) Suggest one more way to increase its strength. (iii) What happens when the current is switched off? (3 marks) 10. What is an MCB? Why are MCBs replacing fuses? What does the ISI mark on electrical appliances indicate? (3 marks) Section C - Diagrams (10 marks) 11. Draw the symbols of (i) an electric cell (ii) a battery of two cells (iii) a switch in ON position (iv) a switch in OFF position (v) a bulb (vi) a wire. Then draw a circuit with a battery of two cells, a switch and a bulb. (4 marks) 12. Draw a labelled diagram of an electric bell and show the electromagnet, armature, hammer, gong and contact screw. (3 marks) 13. Draw a labelled diagram showing a compass needle placed below a current-carrying wire, as in Oersted's experiment. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain the working of an electric bell step by step. (i) What happens when the switch is pressed? (ii) Why does the hammer strike the gong? (iii) Why does the circuit break? (iv) Why does the bell keep ringing as long as the switch is pressed? (5 marks) 15. Paheli connected a bulb, switch and cells, but the bulb did not glow. (i) List four possible reasons. (ii) How would you check each? (iii) Why is a fuse placed in household circuits? (iv) Why should one never use an ordinary wire in place of a fuse wire? (5 marks) 16. A crane in a scrapyard uses an electromagnet. (i) Why is an electromagnet used rather than a permanent magnet? (ii) How does it separate iron from other scrap? (iii) What happens to the load when the current is cut off? (iv) Give two more uses of electromagnets. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Use standard symbols in all circuit diagrams and never experiment with mains electricity.