Electricity and Circuits

Electricity makes our lives easier by running lights, fans and many other devices. In this chapter you will study an electric cell, which has a positive terminal (the metal cap) and a negative terminal (the metal disc), and an electric bulb, whose thin filament glows when current flows through it. A bulb is said to be fused when its filament breaks. You will learn that a bulb glows only when a complete path, called an electric circuit, runs from the positive terminal of the cell through the bulb and back to the negative terminal. The chapter explains how a switch opens or closes a circuit, and why a torch works when its switch is on. Using a simple tester made of a cell, bulb and wires, you will find that materials such as copper, aluminium and iron allow current to pass through them and are called conductors, while rubber, plastic, wood and glass do not and are called insulators. Finally, you will learn safety rules for handling electricity.

Detailed Worksheet: Electricity and Circuits Section A - Definitions (10 marks) 1. What is an electric cell? Name its two terminals. (2 marks) 2. What is an electric circuit? When is it called a closed circuit? (2 marks) 3. What is a switch? Why is it used in a circuit? (2 marks) 4. Define conductors and insulators, with two examples of each. (2 marks) 5. What is a filament? What is meant by a fused bulb? (2 marks) Section B - Calculations and Applications (15 marks) 6. A torch uses 2 cells. A student has 9 cells. (i) How many torches can he fully fit with cells? (ii) How many cells will be left over? (iii) Why must the cells be placed with the positive terminal of one touching the negative terminal of the next? (3 marks) 7. Using a tester, a student checked these objects: iron nail, plastic scale, aluminium foil, rubber eraser, copper wire, wooden pencil (without the lead), steel spoon, glass bangle, coin. (i) List the conductors. (ii) List the insulators. (iii) How many of each did she find? (3 marks) 8. In a circuit, the bulb does not glow. Give three possible reasons and how you would check each one. (3 marks) 9. A cell lasts for about 20 hours of continuous use in a torch. (i) If a torch with 2 such cells is used for 2 hours every day, after how many days will the cells need replacing? (ii) Why should old cells be disposed of carefully? (iii) What should we do to save cell energy? (3 marks) 10. Give the reason for each safety rule: (i) never touch a switch with wet hands (ii) electricians use rubber gloves and screwdrivers with plastic handles (iii) never put metal objects into electric sockets. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of an electric bulb showing the glass case, filament, supporting wires and the two terminals. Trace the path of current through the bulb. (4 marks) 12. Draw a labelled diagram of a simple circuit with a cell, a bulb, a switch and connecting wires. Show the direction of current from the positive terminal. (3 marks) 13. Draw a labelled diagram of a conduction tester made from a cell, a bulb and wires with two free ends. Show how an object is tested. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Paheli connected a bulb to a cell using two wires, but connected both wires to the positive terminal of the cell. (i) Will the bulb glow? (ii) Explain why. (iii) How should she correct the circuit? (iv) Draw the corrected circuit. (5 marks) 15. Our body is a conductor of electricity, and so is impure water. (i) Why is it dangerous to operate electrical appliances with wet hands? (ii) Why is a plastic covering put on copper wires? (iii) Why are electric switches made of plastic? (iv) Why should we never touch an exposed wire? (5 marks) 16. Boojho's torch does not work. (i) Describe how he can check whether the cells are working. (ii) How can he check if the bulb is fused? (iii) Explain the role of the switch in the torch. (iv) Draw a simple diagram of the circuit inside a torch. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw all diagrams neatly in pencil and label every part. Never perform experiments with mains electricity.