Chapter 13 — Magnetic Effects of Electric Current
Welcome to HSLC Guru! This page provides a complete English-medium guide to Class 10 Science Chapter 13 — Magnetic Effects of Electric Current, prepared strictly according to the ASSEB (Assam State School Education Board) syllabus. You will find a clear summary, all textbook questions with model answers, additional MCQs, fill in the blanks, true/false questions, and a glossary of important terms. This chapter is one of the highest-scoring units in the HSLC Science exam.
Summary
A magnet is a substance that attracts iron and iron-like materials and aligns itself in a north–south direction when freely suspended. The region around a magnet in which its magnetic force can be detected is called the magnetic field. The magnetic field is represented by magnetic field lines which are imaginary closed curves drawn from the north pole to the south pole outside the magnet and from the south pole to the north pole inside it. Important properties of field lines: they form closed loops, never intersect each other, are crowded near the poles (where the field is strong) and farther apart in weak-field regions. In 1820, Hans Christian Oersted discovered that an electric current flowing through a conductor produces a magnetic field around it. He observed that a magnetic compass placed near a current-carrying wire deflected, proving the close link between electricity and magnetism.
The direction of the magnetic field around a straight current-carrying conductor is given by Maxwell’s right-hand thumb rule: if we hold the conductor in the right hand with the thumb pointing in the direction of current, the curled fingers indicate the direction of the magnetic field lines. The field lines around a straight wire form concentric circles. In a circular loop, every part of the wire produces magnetic field lines in the same direction inside the loop, making the field stronger. A solenoid is a long coil of insulated copper wire wound in the shape of a cylinder; it produces a magnetic field similar to that of a bar magnet, and the field inside it is uniform. By inserting a soft iron core inside a solenoid, we make a powerful electromagnet.
A current-carrying conductor placed in a magnetic field experiences a force. The direction of this force is determined by Fleming’s left-hand rule: stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular—the forefinger points in the direction of magnetic field, the middle finger in the direction of current, and the thumb gives the direction of force or motion. This principle is used in the electric motor, a device that converts electrical energy into mechanical (rotational) energy. The motor consists of a rectangular coil, strong magnets, a split-ring commutator, brushes, and a battery. The split-ring commutator reverses the direction of current in the coil after every half rotation, keeping the rotation continuous.
The reverse phenomenon—producing electricity from magnetism—is called electromagnetic induction, discovered by Michael Faraday in 1831. Whenever the magnetic field through a coil changes, an induced current flows in the coil. The direction of the induced current is given by Fleming’s right-hand rule. The electric generator works on this principle and converts mechanical energy into electrical energy. An AC generator uses slip rings and produces alternating current (which reverses direction periodically; in India 50 Hz), while a DC generator uses a split-ring commutator and produces direct current. In domestic electric circuits, three wires are used: live (red/brown, 220 V), neutral (black/blue, 0 V), and earth wire (green), which is connected to a metal plate buried in the ground for safety. Short circuit occurs when live and neutral wires touch directly, causing a sudden huge current; overloading happens when too many appliances draw current beyond the safe limit. To prevent these, an electric fuse or MCB (Miniature Circuit Breaker) is connected in the live wire; it breaks the circuit when current exceeds the safe value.
Textbook Questions and Answers
1-Mark Questions
Q1. Who discovered the magnetic effect of electric current?
Answer: Hans Christian Oersted, in 1820.
Q2. What is a magnetic field?
Answer: The region around a magnet within which its magnetic influence can be experienced is called a magnetic field.
Q3. What is the SI unit of magnetic field?
Answer: Tesla (T).
Q4. What is the frequency of AC current in India?
Answer: 50 Hertz (Hz).
Q5. Name the rule used to find the direction of magnetic field around a current-carrying straight conductor.
Answer: Maxwell’s right-hand thumb rule.
Q6. What is the colour of the live wire in a domestic circuit?
Answer: Red or brown.
Q7. What is an electromagnet?
Answer: A soft iron core wrapped with a current-carrying solenoid that behaves like a magnet only when current flows is called an electromagnet.
Q8. What does a fuse do?
Answer: A fuse melts and breaks the circuit when the current exceeds a safe value, thereby protecting appliances from damage.
Q9. Who discovered electromagnetic induction?
Answer: Michael Faraday in 1831.
Q10. What is the function of a split-ring commutator in a DC motor?
Answer: It reverses the direction of current in the coil after every half rotation, ensuring continuous rotation in the same direction.
2 to 3-Mark Questions
Q1. State three properties of magnetic field lines.
Answer: (i) Magnetic field lines emerge from the north pole and merge at the south pole outside the magnet, forming closed continuous loops. (ii) Two magnetic field lines never intersect each other, because at the point of intersection the compass needle would point in two different directions, which is impossible. (iii) The closeness of the field lines indicates the strength of the magnetic field; crowded lines mean a strong field, while widely spaced lines mean a weak field.
Q2. State Maxwell’s right-hand thumb rule.
Answer: Imagine that you are holding a current-carrying straight conductor in your right hand such that the thumb points in the direction of the conventional current. Then your curled fingers will point in the direction of the magnetic field lines around the conductor. This rule helps determine the direction of the magnetic field due to a straight current-carrying conductor.
Q3. State Fleming’s left-hand rule.
Answer: Stretch the thumb, forefinger and middle finger of the left hand so that they are mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of current, then the thumb gives the direction of the force experienced by the conductor (or the direction of motion). This rule is used in electric motors.
Q4. Differentiate between AC and DC.
Answer: Alternating Current (AC) reverses its direction periodically and its magnitude changes with time, while Direct Current (DC) flows only in one direction with constant magnitude. AC can be transmitted over long distances with low loss, whereas DC cannot. The frequency of AC in India is 50 Hz; DC has zero frequency. AC is produced by an AC generator using slip rings; DC is produced by a DC generator using a split-ring commutator.
Q5. Why is the earth wire used in domestic appliances?
Answer: The earth wire is connected to the metallic body of the appliance and to a metal plate deep inside the earth. If by accident the live wire touches the metal body of the appliance, the leakage current flows directly to the earth through the earth wire instead of passing through the user’s body. This protects the user from severe electric shocks. The green-coloured wire is used as the earth wire.
Q6. What is short circuit and overloading?
Answer: Short circuit happens when the insulation of the live and neutral wires gets damaged and the two wires come in direct contact. The resistance of the circuit becomes nearly zero, and a sudden very high current flows, which can cause sparks and fire. Overloading occurs when too many high-power appliances are connected to the same socket so that the current drawn exceeds the rated value of the wire, causing the wire to heat up dangerously.
5 to 6-Mark Questions
Q1. Describe the construction and working of an electric motor with a labelled diagram.
Answer: An electric motor is a device which converts electrical energy into mechanical energy.
Construction: It consists of (i) a rectangular coil ABCD made of insulated copper wire, (ii) two strong horseshoe-shaped permanent magnets (north pole N and south pole S) which provide a uniform magnetic field, (iii) a split-ring commutator made of two halves P and Q of a copper ring, (iv) two carbon brushes X and Y which keep the split rings in contact with the battery, and (v) a battery connected to the brushes.
Working: When current is passed through the coil, the side AB carries current in one direction (say from A to B) and the side CD carries it in the opposite direction. By Fleming’s left-hand rule, AB experiences an upward force while CD experiences a downward force. These two equal and opposite forces form a couple, which rotates the coil. After half a rotation, the split rings change their contact with the brushes; this reverses the direction of current in the coil. Thus the side which was earlier going up now goes down, and the rotation continues in the same direction. The coil keeps rotating as long as current flows.
Q2. Explain the principle, construction and working of an AC generator.
Answer: Principle: An AC generator works on the principle of electromagnetic induction. When a coil rotates in a magnetic field, an induced current is produced in the coil.
Construction: It consists of a rectangular coil ABCD placed between the poles of a strong horseshoe magnet. The two ends of the coil are connected to two separate slip rings R1 and R2, which rotate with the coil. Two carbon brushes B1 and B2 press against the slip rings and are connected to the external circuit.
Working: When the coil is rotated by an external mechanical source (water, steam or fuel), the side AB moves up and CD moves down. By Fleming’s right-hand rule, an induced current flows from A to B in side AB and from C to D in side CD. After half a rotation, AB moves down and CD moves up, so the direction of induced current in the coil reverses. Hence the current in the external circuit also reverses direction after every half cycle, producing alternating current. In India this happens 50 times per second (50 Hz).
Q3. Compare the magnetic field produced by a current-carrying straight conductor, a circular loop and a solenoid.
Answer: (i) Straight conductor: When current flows through a straight wire, the magnetic field lines around it are concentric circles whose plane is perpendicular to the wire. The strength of the field increases with current and decreases with distance from the wire. The direction is given by the right-hand thumb rule.
(ii) Circular loop: Every small part of the loop contributes magnetic field lines in the same direction inside the loop. Hence the field at the centre is much stronger than that around a straight wire. The field strength increases with the number of turns and current.
(iii) Solenoid: A solenoid is a long coil of many circular turns. The magnetic field pattern produced is exactly like that of a bar magnet, with one end behaving as the north pole and the other as the south pole. The field inside the solenoid is strong and uniform, with parallel field lines. By placing a soft iron rod inside, the field becomes much stronger and the device acts as an electromagnet.
Q4. Explain electromagnetic induction with the help of an experiment, and state Fleming’s right-hand rule.
Answer: Electromagnetic induction is the phenomenon of producing an induced current in a coil whenever the magnetic field through it changes.
Experiment: Take a coil of insulated copper wire connected to a sensitive galvanometer. When a bar magnet is pushed with its north pole into the coil, the galvanometer needle deflects in one direction, showing that a current has been induced. When the magnet is held stationary inside the coil, the needle returns to zero, showing no current. When the magnet is pulled out, the needle deflects in the opposite direction. The faster the motion of the magnet, the larger the deflection. This proves that an induced current is set up only when there is a change in the magnetic field.
Fleming’s right-hand rule: Stretch the thumb, forefinger and middle finger of the right hand mutually perpendicular. If the forefinger points in the direction of the magnetic field and the thumb in the direction of motion of the conductor, then the middle finger gives the direction of the induced current. This rule is used in generators.
Q5. Describe a domestic electric circuit. Why is the fuse always connected in the live wire?
Answer: Electric power from the mains is supplied to a house through three wires: the live wire (red/brown insulation, at 220 V), the neutral wire (black/blue insulation, at 0 V) and the earth wire (green insulation, connected to a copper plate buried deep in the earth). The live and neutral wires enter through a main fuse, then through an electricity meter and a main switch into the distribution box. From the distribution box, two separate circuits are taken—one for high-power appliances (15 A line for geyser, AC) and another for low-power appliances (5 A line for bulbs and fans). All the appliances are connected in parallel so that each gets the full voltage of 220 V and works independently. A switch is always placed in the live wire of every appliance.
The fuse is connected in the live wire (and not the neutral) because if the fuse blows, the appliance gets disconnected from the high-voltage supply. If the fuse were connected in the neutral wire, the appliance would still remain connected to the live wire even after the fuse blew, and the user could get a shock by touching it.
Additional Multiple Choice Questions (MCQs)
Q1. The SI unit of magnetic field is:
(a) Newton (b) Tesla (c) Weber (d) Henry
Answer: (b) Tesla.
Q2. The magnetic effect of electric current was first discovered by:
(a) Faraday (b) Maxwell (c) Oersted (d) Fleming
Answer: (c) Oersted.
Q3. A solenoid behaves like a:
(a) Straight conductor (b) Bar magnet (c) Galvanometer (d) Capacitor
Answer: (b) Bar magnet.
Q4. Fleming’s left-hand rule is used to find the direction of:
(a) Induced current (b) Magnetic field (c) Force on a current-carrying conductor (d) Electric current
Answer: (c) Force on a current-carrying conductor.
Q5. The frequency of alternating current used in India is:
(a) 60 Hz (b) 100 Hz (c) 50 Hz (d) 220 Hz
Answer: (c) 50 Hz.
Q6. The colour of the earth wire in a domestic circuit is:
(a) Red (b) Black (c) Green (d) Blue
Answer: (c) Green.
Q7. An electric motor converts:
(a) Mechanical to electrical energy (b) Electrical to mechanical energy (c) Heat to electrical energy (d) Sound to electrical energy
Answer: (b) Electrical to mechanical energy.
Q8. Inside a bar magnet, the magnetic field lines move:
(a) From N to S (b) From S to N (c) Do not exist (d) Are random
Answer: (b) From S to N.
Q9. Two magnetic field lines:
(a) Always intersect (b) Never intersect (c) Sometimes intersect (d) Are parallel only
Answer: (b) Never intersect.
Q10. The device used to produce direct current is:
(a) AC generator (b) Transformer (c) DC generator (d) Galvanometer
Answer: (c) DC generator.
Q11. The strength of the magnetic field inside a solenoid:
(a) Increases at the ends (b) Is uniform throughout (c) Is zero at the centre (d) Becomes negative
Answer: (b) Is uniform throughout.
Q12. Which of the following is not a method of increasing the magnetic field of a solenoid?
(a) Increasing the number of turns (b) Inserting an iron core (c) Increasing the current (d) Increasing the length of the wire only
Answer: (d) Increasing the length of the wire only.
Q13. The earth wire of a domestic circuit is connected to:
(a) The live wire (b) A metal plate buried deep in the ground (c) The neutral wire (d) The fuse
Answer: (b) A metal plate buried deep in the ground.
Fill in the Blanks
Q1. A current-carrying conductor produces a __________ field around it.
Answer: magnetic.
Q2. The direction of the magnetic field in a current-carrying solenoid is given by the __________ rule.
Answer: right-hand thumb (clock face).
Q3. An electric generator works on the principle of __________ induction.
Answer: electromagnetic.
Q4. The fuse wire is made of an alloy of tin and __________.
Answer: lead.
Q5. The potential difference between live and neutral wires in India is __________ volts.
Answer: 220.
True / False
Q1. Magnetic field lines start from the south pole and end at the north pole outside the magnet. — False (they go from N to S outside).
Q2. A solenoid carrying current behaves like a bar magnet. — True.
Q3. Fleming’s right-hand rule is used in electric motors. — False (it is used in generators).
Q4. A short circuit occurs when current flows through an extremely low-resistance path. — True.
Q5. AC current cannot be transmitted over long distances. — False (AC is preferred for long distance transmission).
Q6. An electromagnet retains its magnetism even after the current is switched off. — False (it loses magnetism when current stops).
Q7. The unit of magnetic flux density (magnetic field) is Tesla. — True.
Glossary of Important Terms
| Term | Meaning |
|---|---|
| Magnet | A substance that attracts iron and points in the N–S direction when freely suspended. |
| Magnetic field | The region around a magnet in which its magnetic effect can be felt; SI unit Tesla. |
| Magnetic field lines | Imaginary closed curves which represent the direction and strength of a magnetic field. |
| Oersted’s experiment | 1820 experiment showing that a current-carrying wire deflects a nearby compass needle. |
| Right-hand thumb rule | Rule giving the direction of magnetic field around a straight current-carrying wire. |
| Solenoid | A long cylindrical coil of insulated wire that behaves like a bar magnet when current flows. |
| Electromagnet | A temporary magnet made of a soft iron core wound by a current-carrying coil. |
| Fleming’s left-hand rule | Gives the direction of force on a current-carrying conductor in a magnetic field; used in motors. |
| Electric motor | Device that converts electrical energy into mechanical (rotational) energy. |
| Split-ring commutator | A device that reverses current direction in the coil of a DC motor every half rotation. |
| Electromagnetic induction | Production of induced current in a coil due to a changing magnetic field (Faraday, 1831). |
| Fleming’s right-hand rule | Gives the direction of induced current in a generator. |
| Electric generator | Device that converts mechanical energy into electrical energy. |
| Alternating current (AC) | Current whose direction reverses periodically; in India 50 Hz. |
| Direct current (DC) | Current that flows in only one direction with constant magnitude. |
| Live wire | Red/brown wire at 220 V that carries current to the appliance. |
| Neutral wire | Black/blue wire at 0 V that completes the circuit. |
| Earth wire | Green wire connected to ground; protects the user from electric shock. |
| Short circuit | Direct contact of live and neutral wires causing a sudden very high current. |
| Overloading | Use of too many appliances simultaneously, drawing current beyond the safe limit. |
| Fuse | A safety device made of low-melting-point alloy that melts and breaks the circuit during overload. |
| MCB | Miniature Circuit Breaker—a switch that automatically trips off during high current. |