Magnetic Effects of Electric Current

In 1820, Hans Christian Oersted noticed that a compass needle was deflected when an electric current flowed through a nearby wire, showing that electricity and magnetism are linked. In this chapter you will learn that a magnet is surrounded by a magnetic field, represented by magnetic field lines that emerge from the north pole, merge at the south pole, never intersect, and are crowded where the field is strong. You will study the magnetic field produced by a current in a straight conductor, whose direction is given by the right-hand thumb rule, in a circular loop and in a solenoid, which behaves like a bar magnet and can be used to make an electromagnet. You will see that a current-carrying conductor placed in a magnetic field experiences a force, whose direction is given by Fleming's left-hand rule, and that this force is the basis of many devices. Finally, you will compare direct and alternating current and study domestic electric circuits: live, neutral and earth wires, the fuse, earthing, short-circuiting and overloading.

Detailed Worksheet: Magnetic Effects of Electric Current Section A - Definitions (10 marks) 1. What is a magnetic field? How is its direction at a point determined? (2 marks) 2. List three properties of magnetic field lines. Why do two field lines never intersect? (2 marks) 3. State the right-hand thumb rule. (2 marks) 4. State Fleming's left-hand rule. (2 marks) 5. What is a solenoid? How is an electromagnet made from it? (2 marks) Section B - Calculations and Applications (15 marks) 6. The magnetic field at a point near a long straight conductor is directly proportional to the current and inversely proportional to the distance from the wire. If the current is doubled and the distance is halved, by what factor does the field change? What if the current is halved and the distance doubled? (3 marks) 7. Alternating current in India has a frequency of 50 Hz. How many times does the current change its direction in one second? What is the time taken for one complete cycle? State one advantage of AC over DC. (3 marks) 8. A household has a 2 kW electric heater, a 1 kW electric iron and five 100 W bulbs. All are switched on together on a 220 V supply. Calculate the total current drawn. Would a 15 A fuse in the main line blow? Show your calculation. (3 marks) 9. An electric oven of 2 kW power rating is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect? Calculate the current drawn and explain. (3 marks) 10. The magnetic field produced by a circular coil is directly proportional to the number of turns. A coil with 50 turns carrying a current produces a certain field at its centre. What happens to the field if the number of turns is made 200 and the current is halved? Explain why the field due to a coil of n turns is n times that of a single turn. (3 marks) Section C - Diagrams (10 marks) 11. Draw the magnetic field lines around a bar magnet, showing the direction of the field with arrows. Mark the regions where the field is strongest. (4 marks) 12. Draw the pattern of magnetic field lines due to a current-carrying straight conductor passing vertically through a horizontal cardboard sheet, showing the direction of the current and the field. (3 marks) 13. Draw the pattern of magnetic field lines of a current-carrying solenoid. Mark the north and south poles and show the direction of current in the turns. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. A positively charged particle (alpha particle) projected towards the west is deflected towards the north by a magnetic field. What is the direction of the magnetic field? Use Fleming's left-hand rule and explain each step. How would the deflection change if the particle were an electron moving in the same direction? (5 marks) 15. Explain the difference between short-circuiting and overloading in domestic circuits. Why is an earth wire connected to the metal body of appliances like an electric iron and a refrigerator? What is the function of an electric fuse, and why is it always connected in the live wire? (5 marks) 16. Describe an activity to show that a current-carrying conductor experiences a force in a magnetic field, using an aluminium rod suspended between the poles of a horseshoe magnet. How does the displacement change when (i) the current is reversed (ii) the magnet is reversed (iii) the current is increased? When is the force maximum? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw all magnetic field patterns neatly with arrows showing directions. Use standard symbols in circuit diagrams.