Magnetic Effects of Electric Current
NCERT Chapter 12 • Field Lines, Electromagnets, and Domestic Circuits
NCERT 2025–26
Electromagnetism
Physics
We know that electricity and magnetism are linked. Hans Christian Oersted (1820) discovered that a compass needle gets deflected when placed near a current-carrying wire. In this chapter, we explore magnetic fields, electromagnets, and domestic circuits.
1. Magnetic Field and Field Lines
The region surrounding a magnet, in which the force of the magnet can be detected, is said to have a Magnetic Field. It is a quantity that has both magnitude and direction.
Properties of Magnetic Field Lines:
- They emerge from the North pole and merge at the South pole (outside the magnet).
- Inside the magnet, the direction is from South to North. Thus, they are closed curves.
- The relative strength of the field is shown by the degree of closeness of the lines. Crowded lines = Stronger field.
- No two field lines cross each other. If they did, the compass would point in two directions at once, which is impossible.
Q1. Why does a compass needle get deflected when brought near a bar magnet?
A compass needle is a small magnet. The magnetic field of the bar magnet exerts a force on the compass needle, causing it to deflect.
Q2. Draw magnetic field lines around a bar magnet.
(Refer to the diagram above. Lines should curve from N to S outside the magnet.)
Q3. List the properties of magnetic field lines.
1. They originate from the North pole and end at the South pole (outside).
2. They are continuous closed loops.
3. They do not intersect each other.
4. Crowded lines indicate a stronger magnetic field.
2. They are continuous closed loops.
3. They do not intersect each other.
4. Crowded lines indicate a stronger magnetic field.
Q4. Why don’t two magnetic field lines intersect each other?
If they did, it would mean that at the point of intersection, the compass needle would point towards two directions simultaneously, which is impossible.
2. Magnetic Field due to a Current-Carrying Conductor
A. Straight Conductor
The magnetic field lines around a straight current-carrying wire are concentric circles centered on the wire. The direction is given by the Right-Hand Thumb Rule.
Right-Hand Thumb Rule:
Imagine holding the current-carrying conductor in your right hand such that the thumb points towards the direction of current. Then your fingers will wrap around the conductor in the direction of the field lines.
B. Circular Loop
The magnetic field lines are concentric circles near the wire but become straighter near the center. At the center of the circular loop, the arcs appear as straight lines.
3. Magnetic Field due to a Solenoid
A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a Solenoid.
- The field pattern is similar to a bar magnet.
- Inside the solenoid, field lines are parallel straight lines, indicating a uniform magnetic field.
- Electromagnet: A strong magnetic field inside a solenoid can serve to magnetize a piece of soft iron placed inside it. The magnet so formed is called an electromagnet.
Q1. Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand rule to find out the direction of the magnetic field inside and outside the loop.
Using the Right-Hand Thumb Rule:
Inside the loop: The magnetic field lines go into the table.
Outside the loop: The magnetic field lines come out of the table.
Inside the loop: The magnetic field lines go into the table.
Outside the loop: The magnetic field lines come out of the table.
Q2. The magnetic field in a given region is uniform. Draw a diagram to represent it.
A set of equidistant, parallel straight lines represents a uniform magnetic field.
Q3. Choose the correct option. The magnetic field inside a long straight solenoid-carrying current
(a) is zero.
(b) decreases as we move towards its end.
(c) increases as we move towards its end.
(d) is the same at all points.
(a) is zero.
(b) decreases as we move towards its end.
(c) increases as we move towards its end.
(d) is the same at all points.
(d) is the same at all points (Uniform field).
4. Force on a Current-Carrying Conductor in a Magnetic Field
An electric current flowing through a conductor exerts a force on a magnet. Conversely, a magnet exerts an equal and opposite force on the current-carrying conductor.
The displacement (force) is largest when the direction of current is at right angles to the direction of the magnetic field.
Fleming’s Left-Hand Rule:
Stretch the thumb, forefinger, and middle finger of your left hand mutually perpendicular.
- Forefinger: Magnetic Field
- Middle Finger: Current
- Thumb: Force (Motion)
Q1. Which of the following property of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer.)
(a) mass (b) speed (c) velocity (d) momentum
(a) mass (b) speed (c) velocity (d) momentum
(c) Velocity and (d) Momentum.
The magnetic force acts perpendicular to motion, changing direction but not speed. Since direction changes, velocity and momentum change.
The magnetic force acts perpendicular to motion, changing direction but not speed. Since direction changes, velocity and momentum change.
Q2. In Activity 12.7, how do we think the displacement of rod AB will be affected if (i) current in rod AB is increased; (ii) a stronger horse-shoe magnet is used; and (iii) length of the rod AB is increased?
The force on the conductor is given by 
. Thus, the displacement increases if:
(i) Current (I) increases.
(ii) Magnetic field (B) is stronger.
(iii) Length (l) is increased.
. Thus, the displacement increases if:(i) Current (I) increases.
(ii) Magnetic field (B) is stronger.
(iii) Length (l) is increased.
Q3. A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is
(a) towards south (b) towards east (c) downward (d) upward
(a) towards south (b) towards east (c) downward (d) upward
(d) Upward.
Current (alpha particle motion) is West. Force (deflection) is North. Using Fleming’s Left-Hand Rule, the Field points Upward.
Current (alpha particle motion) is West. Force (deflection) is North. Using Fleming’s Left-Hand Rule, the Field points Upward.
5. Domestic Electric Circuits
In our homes, we receive electric power (Mains) at 220 V.
| Wire Color | Name | Function |
|---|---|---|
| Red | Live Wire | Carries current from the source (220V). |
| Black | Neutral Wire | Completes the circuit (0V). |
| Green | Earth Wire | Safety measure. Connected to a metal plate deep in the earth. Ensures leakage current flows to earth preventing shock. |
Safety Devices:
- Fuse: Prevents damage due to overloading or short-circuiting by melting and breaking the circuit.
- Short-Circuiting: Occurs when live and neutral wires come into direct contact.
- Overloading: Occurs when too many appliances are connected to a single socket.
Q1. Name two safety measures commonly used in electric circuits and appliances.
1. Electric Fuse.
2. Earthing (Earth wire).
2. Earthing (Earth wire).
Q2. 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? Explain.
Current drawn .
The circuit rating is only 5 A. Since 9.09 A > 5 A, the fuse will melt and break the circuit (or the circuit will overload).
.The circuit rating is only 5 A. Since 9.09 A > 5 A, the fuse will melt and break the circuit (or the circuit will overload).
Q3. What precaution should be taken to avoid the overloading of domestic electric circuits?
1. Do not connect too many appliances to a single socket.
2. Do not use high-power appliances (AC, heater) on the same circuit simultaneously.
3. Use appropriate fuses.
2. Do not use high-power appliances (AC, heater) on the same circuit simultaneously.
3. Use appropriate fuses.
6. Chapter Exercises
Practice these NCERT exercise questions to master the chapter:
Q1. Which of the following correctly describes the magnetic field near a long straight wire?
(a) Straight lines perpendicular to wire
(b) Straight lines parallel to wire
(c) Radial lines
(d) Concentric circles centred on the wire
(a) Straight lines perpendicular to wire
(b) Straight lines parallel to wire
(c) Radial lines
(d) Concentric circles centred on the wire
(d) The field consists of concentric circles centred on the wire.
Q2. At the time of short circuit, the current in the circuit
(a) reduces substantially
(b) does not change
(c) increases heavily
(d) vary continuously
(a) reduces substantially
(b) does not change
(c) increases heavily
(d) vary continuously
(c) increases heavily.
Q3. State whether the following statements are true or false.
(a) The field at the centre of a long circular coil carrying current will be parallel straight lines.
(b) A wire with a green insulation is usually the live wire of an electric supply.
(a) The field at the centre of a long circular coil carrying current will be parallel straight lines.
(b) A wire with a green insulation is usually the live wire of an electric supply.
(a) True.
(b) False. Green is Earth wire; Live wire is Red.
(b) False. Green is Earth wire; Live wire is Red.
Q4. List two methods of producing magnetic fields.
1. Using a permanent magnet (Bar magnet).
2. Using a current-carrying conductor (Straight wire or Solenoid).
2. Using a current-carrying conductor (Straight wire or Solenoid).
Q5. When is the force experienced by a current-carrying conductor placed in a magnetic field largest?
When the direction of the current is at right angles (perpendicular) to the direction of the magnetic field.
Q6. Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?
1. Electron direction: Back to Front. Current direction: Front to Back (Opposite to electron).
2. Force/Deflection: Right side.
3. Using Fleming’s Left Hand Rule (Middle finger = Front to Back, Thumb = Right), the Forefinger (Field) points Vertically Downwards.
2. Force/Deflection: Right side.
3. Using Fleming’s Left Hand Rule (Middle finger = Front to Back, Thumb = Right), the Forefinger (Field) points Vertically Downwards.
Q7. State the rule to determine the direction of a (i) magnetic field produced around a straight conductor-carrying current, (ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it.
(i) Right-Hand Thumb Rule.
(ii) Fleming’s Left-Hand Rule.
(ii) Fleming’s Left-Hand Rule.
Q8. When does an electric short circuit occur?
It occurs when the live wire and the neutral wire come into direct contact, usually due to damaged insulation or a fault in the appliance.
Q9. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
The earth wire provides a low-resistance conducting path for the current to the ground. It is necessary to prevent electric shock in case of leakage of current to the metallic body of an appliance.