Questions
11
Q1
What is a magnetic field? Explain how magnetic field lines can be drawn around a bar magnet using a compass needle. List the properties of magnetic field lines.
A magnetic field is the region surrounding a magnet within which the force of the magnet can be detected. To draw field lines, place a bar magnet on paper and mark its boundary. Place a compass near the north pole — the south pole of the compass points towards it. Mark both ends of the needle, then move the compass so that its south pole occupies the spot previously held by its north pole. Repeat until reaching the south pole, then join the marks with a smooth curve. Properties: (i) field lines emerge from the north pole and merge at the south pole outside the magnet; inside the magnet, they go from south to north, forming closed curves; (ii) the degree of closeness of field lines indicates the relative field strength — they are most crowded near the poles; (iii) no two field lines intersect each other, because at the point of intersection the compass needle would have to point in two directions, which is impossible.
Q2
Describe the experiment of Hans Christian Oersted. What conclusion did he draw from his observation, and why was this discovery significant?
Oersted placed a straight thick copper wire perpendicular to the plane of paper in an electric circuit. He placed a small compass near the wire and passed current through the circuit by inserting a key. He observed that the compass needle deflected. This showed that an electric current flowing through a metallic conductor produces a magnetic effect, establishing that electricity and magnetism are linked. His research later led to technologies such as radio, television, and fibre optics.
Q3
What is the pattern of the magnetic field lines around a straight current-carrying conductor? Describe an activity to demonstrate this pattern, and state how the magnitude of the magnetic field depends on the current and the distance from the wire.
The magnetic field lines around a straight current-carrying conductor form concentric circles centred on the wire. To demonstrate: insert a thick copper wire vertically through the centre of a horizontal cardboard, sprinkle iron filings on the cardboard, and pass current through the wire. On gently tapping the cardboard, the iron filings align in concentric circles around the wire. The direction of the field lines is given by placing a compass on a circle — the north pole of the compass shows the direction. The magnitude of the magnetic field at a given point is directly proportional to the current () and inversely proportional to the distance from the wire (). Reversing the current direction reverses the field direction.
Q4
State the right-hand thumb rule. A current through a horizontal power line flows from east to west. What is the direction of the magnetic field at a point directly below the wire and at a point directly above it?
The right-hand thumb rule states: Imagine holding a current-carrying straight conductor in your right hand such that the thumb points in the direction of the electric current. Then, the fingers wrapping around the conductor indicate the direction of the magnetic field lines. For the power line with current flowing east to west: applying the right-hand thumb rule, the magnetic field turns clockwise in a plane perpendicular to the wire when viewed from the east end, and anti-clockwise when viewed from the west end. Therefore, at a point directly below the wire, the magnetic field is directed from north to south, and at a point directly above the wire, it is from south to north.
Q5
How does the magnetic field pattern of a current-carrying circular loop differ from that of a straight wire? What is a solenoid, and how does its magnetic field compare with that of a bar magnet? Why is the field inside a solenoid uniform?
For a straight wire, the field lines are concentric circles centred on the wire. For a circular loop, every section of the wire produces concentric circles that become larger towards the centre. At the centre of the loop, the arcs of these large circles appear as straight lines, and every section of the wire contributes to the field in the same direction within the loop — so the field lines near the centre are parallel. A solenoid is a coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder. Its magnetic field pattern is similar to that of a bar magnet: one end behaves as a north pole and the other as a south pole, and field lines are closed curves. Inside a solenoid, the field lines are parallel straight lines, indicating that the field is the same at all points — that is, the field is uniform. If there are turns, the field is times that of a single turn because the current in each turn has the same direction and the fields add up.
Q6
What is an electromagnet? How is it made, and how does its magnetic strength compare with that of a solenoid alone? Give two practical reasons why electromagnets are preferred over permanent magnets in many applications.
An electromagnet is a magnet formed by placing a piece of a magnetic material, such as soft iron, inside a solenoid coil through which a current is passed. The strong, uniform magnetic field produced inside the solenoid magnetises the soft iron core, creating a much stronger magnet than the solenoid alone. Electromagnets are preferred because: (i) their magnetic strength can be varied by changing the current through the coil, and (ii) the magnetism can be switched on or off by controlling the current. This makes them ideal for devices like electric bells, cranes for lifting heavy iron loads, and relays.
Q7
Describe an activity to demonstrate that a force acts on a current-carrying conductor placed in a magnetic field. On what factors does the direction of this force depend, and when is the magnitude of the force maximum?
Activity 12.7: Suspend a small aluminium rod AB (about 5 cm) horizontally from a stand using two connecting wires. Place a strong horse-shoe magnet such that the rod lies between its poles — the north pole vertically below and south pole vertically above the rod, so the magnetic field is directed upwards. Connect the rod in series with a battery, a key, and a rheostat. When current flows from B to A, the rod is displaced towards the left. Reversing the current to flow from A to B causes the rod to displace towards the right. This shows that a force acts on a current-carrying conductor in a magnetic field. The direction of the force depends on (i) the direction of current and (ii) the direction of the magnetic field. The magnitude of the force is maximum when the direction of current is at right angles () to the direction of the magnetic field.
Q8
State Fleming's left-hand rule. An electron enters a magnetic field at right angles to it. The magnetic field is directed horizontally to the right and the electron moves vertically downwards. What is the direction of the force acting on the electron?
Fleming's left-hand rule states: Stretch the thumb, forefinger, and middle finger of your left hand such that they are mutually perpendicular to each other. The forefinger points in the direction of the magnetic field, the middle finger points in the direction of the electric current, and the thumb then points in the direction of motion or the force acting on the conductor. For the electron: conventional current is opposite to the electron's motion, so current is vertically upwards. Applying Fleming's left-hand rule — forefinger to the right (field), middle finger upwards (current) — the thumb points into the page. Therefore, the force on the electron is directed into the page.
Q9
Describe the domestic electric circuit system used in Indian households. Explain the colour coding of the three wires, the standard potential difference and frequency, and why household appliances are connected in parallel.
In India, electric power is supplied to homes through overhead poles or underground cables. The supply uses three wires: the live wire (red insulation, positive), the neutral wire (black insulation, negative), and the earth wire (green insulation). The potential difference between the live and neutral wires is , and the AC frequency is . Wires enter through a main fuse into an electricity meter, then through a main switch to the distribution box containing separate fuses for each circuit. Two separate circuits are typically used: a circuit for high-power appliances (geysers, air coolers) and a circuit for low-power appliances (bulbs, fans). Appliances are connected in parallel so that each receives the full supply, can be independently switched on/off, and a fault in one does not affect the others.
Q10
What is the function of an earth wire in a domestic circuit? Why is it necessary to earth metallic appliances such as an electric iron or a refrigerator?
The earth wire provides a low-resistance conducting path for any leakage current from the metallic body of an appliance directly to the ground. In case of accidental contact between the live wire and the metallic body (due to damaged insulation or fault), the current flows through the earth wire to the ground instead of passing through the user's body. This keeps the potential of the appliance's metallic body equal to the earth's potential (zero), thereby preventing severe electric shocks. It is necessary to earth appliances with metallic bodies — such as an electric press, toaster, table fan, and refrigerator — because these appliances can become live and dangerous to touch if a leakage fault occurs.
Q11
Explain the working principle of an electric fuse. What is meant by overloading and short-circuiting? List their causes and suggest precautions to avoid overloading of domestic circuits.
An electric fuse works on the principle of Joule heating (). It consists of a thin wire, typically made of an alloy with a low melting point, connected in series with the live wire. When current exceeds the rated safe value, the excessive heating melts the fuse wire, breaking the circuit and stopping the flow of an unduly high current. Overloading is the increase of current beyond the safe limit due to connecting too many appliances to a single socket, an accidental hike in supply voltage, or short-circuiting. Short-circuiting occurs when the live wire and neutral wire come into direct contact — usually when insulation is damaged or there is a fault in an appliance — causing an abrupt surge in current. Precautions against overloading: (i) avoid connecting too many high-power appliances to a single socket; (ii) use proper fuses of appropriate rating; (iii) ensure wiring is done with good-quality insulated wires and appliances are periodically checked for faults.