Electricity

Electricity runs our homes, schools and industries. In this chapter you will learn that electric current is the rate of flow of electric charge, I = Q/t, measured in amperes, and that a continuous closed path called an electric circuit is needed for current to flow. Charge flows only when there is a potential difference between two points; the potential difference is the work done per unit charge, V = W/Q, measured in volts. You will learn the symbols used in circuit diagrams and how to connect an ammeter in series and a voltmeter in parallel. Ohm's law states that the current through a metallic conductor is proportional to the potential difference across it at constant temperature, V = IR. Resistance depends on the length, area of cross-section and material of the conductor, R = rho L/A, where rho is resistivity. You will combine resistors in series and parallel, study the heating effect of current, H = I^2 R t, and its uses in heaters and fuses, and calculate electric power and energy in watts and kilowatt hours.

Detailed Worksheet: Electricity Section A - Definitions (10 marks) 1. Define electric current and state its SI unit. What is one ampere? (2 marks) 2. Define potential difference and state its SI unit. When is the potential difference between two points said to be one volt? (2 marks) 3. State Ohm's law. Draw the shape of the V-I graph for a metallic conductor. (2 marks) 4. Define resistivity. On what factors does the resistance of a conductor depend? (2 marks) 5. State Joule's law of heating. Define electric power and the commercial unit of electrical energy. (2 marks) Section B - Calculations and Applications (15 marks) 6. A current of 0.5 A is drawn by a filament of an electric bulb for 10 minutes. Find the amount of electric charge that flows through the circuit. How many electrons does this correspond to? (Charge on an electron = 1.6 x 10^-19 C) (3 marks) 7. A copper wire has a diameter of 0.5 mm and a resistivity of 1.6 x 10^-8 ohm m. What will be the length of this wire to make its resistance 10 ohm? How much does the resistance change if the diameter is doubled? (Use pi = 3.14) (3 marks) 8. An electric lamp of 20 ohm and a conductor of 4 ohm are connected to a 6 V battery in series. Calculate the total resistance, the current through the circuit and the potential difference across the lamp and the conductor. (3 marks) 9. Three resistors of 5 ohm, 10 ohm and 30 ohm are connected in parallel to a 12 V battery. Find the current through each resistor, the total current and the total resistance. (3 marks) 10. An electric refrigerator rated 400 W operates 8 hours a day. What is the cost of the energy to operate it for 30 days at Rs 3.00 per kWh? (3 marks) Section C - Diagrams (10 marks) 11. Draw a schematic diagram of a circuit consisting of a battery of three cells of 2 V each, a 5 ohm resistor, an 8 ohm resistor and a 12 ohm resistor in series, an ammeter and a plug key. Show how a voltmeter is connected to measure the potential difference across the 12 ohm resistor. Find the reading of the ammeter. (4 marks) 12. Draw the circuit diagram used to verify Ohm's law and sketch the V-I graph expected for a nichrome wire. State what the slope of the graph represents. (3 marks) 13. Draw circuit diagrams showing how three resistors of 6 ohm each can be connected to give a total resistance of (i) 9 ohm (ii) 4 ohm. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Compare the power used in the 2 ohm resistor in each of the following circuits: (i) a 6 V battery in series with 1 ohm and 2 ohm resistors (ii) a 4 V battery in parallel with 12 ohm and 2 ohm resistors. Explain why the results come out as they do. (5 marks) 15. Why are household circuits connected in parallel rather than in series? Give three reasons. A 100 W bulb and a 60 W bulb are connected in parallel to a 220 V supply. Find the total current drawn. (5 marks) 16. An electric iron consumes energy at a rate of 840 W when heating is at the maximum rate and 360 W when the heating is at the minimum. The voltage is 220 V. Calculate the current and the resistance in each case. Why is nichrome, and not copper, used for the heating element? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Show the formula, substitution and units for every numerical. Draw circuit diagrams using standard symbols.