1. Magnetic Field and Magnetic Field Lines
A magnetic field is the region around a magnet or a current-carrying conductor in which magnetic effects can be experienced. It is represented by magnetic field lines.
- Outside a bar magnet, field lines go from the north pole to the south pole.
- Inside the magnet, they continue from south to north, forming closed curves.
- Field lines never intersect one another.
- Closer field lines represent a stronger magnetic field.
2. Magnetic Field Due to a Current-Carrying Straight Conductor
When electric current flows through a straight conductor, it produces a magnetic field around the conductor. The field lines are concentric circles centred on the wire.
The direction of the magnetic field can be found using the right-hand thumb rule: hold the straight conductor in your right hand with the thumb pointing in the direction of current; the curled fingers show the direction of the magnetic field.
The strength of the field increases when the current increases and decreases as the distance from the conductor increases.
3. Magnetic Field Due to a Circular Current-Carrying Loop
A current-carrying circular loop produces a magnetic field whose lines near the centre are approximately perpendicular to the plane of the loop. The magnetic field becomes stronger when the current increases or the number of turns increases.
The direction of the field can be determined using the right-hand rule. For a coil, the combined effect of all turns makes the field stronger near its centre.
4. Solenoid
A solenoid is a long cylindrical coil containing many closely wound turns of insulated wire. When current flows through it, it produces a magnetic field similar to that of a bar magnet.
- The field inside a long solenoid is strong and approximately uniform.
- The magnetic field outside is relatively weak.
- The polarity of the solenoid depends on the direction of current.
- A soft iron core can greatly strengthen the magnetic field and forms an electromagnet.
5. Force on a Current-Carrying Conductor in a Magnetic Field
A current-carrying conductor placed in an external magnetic field can experience a force. The direction of this force depends on the direction of current and the magnetic field.
The direction is given by Fleming's Left-Hand Rule: stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of current, the thumb gives the direction of force or motion of the conductor.
The force is the working principle behind an electric motor.
6. Electric Motor
An electric motor is a device that converts electrical energy into mechanical energy. It works because a current-carrying coil placed in a magnetic field experiences forces on its sides.
Main Parts
- Rectangular coil/armature
- Strong magnetic field
- Split-ring commutator
- Carbon brushes
- Battery/source
- Axle
Working
Opposite forces act on the two sides of the current-carrying coil, producing rotation. After every half rotation, the split ring reverses the direction of current through the coil. This reverses the forces in the required way and keeps the coil rotating in the same direction.
Commercial motors use an electromagnet, many turns of wire and a soft iron core/armature to increase the strength and power of the motor.
7. Electromagnetic Induction
Electromagnetic induction is the phenomenon in which a changing magnetic field through a coil produces an induced potential difference and, when the circuit is closed, an induced current.
The effect can be observed when a magnet is moved towards or away from a coil connected to a galvanometer. Relative motion between the magnet and coil changes the magnetic field through the coil and causes deflection.
Faster change produces a larger induced effect. Reversing the direction of relative motion reverses the induced current.
8. Fleming's Right-Hand Rule
Fleming's Right-Hand Rule is used to determine the direction of induced current in a conductor moving in a magnetic field.
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, the middle finger gives the direction of induced current.
9. Electric Generator
An electric generator converts mechanical energy into electrical energy using electromagnetic induction.
Principle
When a coil rotates in a magnetic field, the magnetic field through the coil changes, inducing a potential difference and current.
AC Generator
An alternating-current generator uses slip rings so that the external current reverses direction periodically.
DC Generator
A split-ring commutator can be used to obtain current that flows in one direction through the external circuit.
10. Motor vs Generator
| Electric Motor | Electric Generator |
|---|---|
| Electrical energy → mechanical energy. | Mechanical energy → electrical energy. |
| Works using force on a current-carrying conductor. | Works using electromagnetic induction. |
| Uses Fleming's left-hand rule for force direction. | Uses Fleming's right-hand rule for induced-current direction. |
| Split ring acts as a commutator in a simple DC motor. | Slip rings are used for AC generation; a split ring can be used for DC output. |
11. Domestic Electric Circuits – Essential Link
The magnetic effect of current is also important in protective and switching devices. A current-carrying coil can operate an electromagnet, which is used in relays and related electrical devices.
In household safety, a fuse or suitable circuit breaker protects circuits from excessive current. The detailed domestic wiring arrangement is studied with electricity and electrical safety.
12. Quick Revision & Important Rules
Gives magnetic field direction around a current-carrying straight conductor.
Gives force direction on a current-carrying conductor in a magnetic field.
Gives induced-current direction in a moving conductor.
Electrical → mechanical energy.
Mechanical → electrical energy.
Produces a bar-magnet-like field; soft iron core strengthens it.
13. Quick Revision – One Look
- Current produces a magnetic field.
- Straight conductor → concentric circular field lines.
- Right-hand thumb rule → magnetic field direction.
- Solenoid → bar-magnet-like field; soft iron core strengthens it.
- Fleming's left-hand rule → force/motion direction.
- Motor → electrical energy to mechanical energy.
- Electromagnetic induction → changing magnetic field produces induced potential difference/current.
- Fleming's right-hand rule → induced-current direction.
- Generator → mechanical energy to electrical energy.
14. 30 Practice Questions
- Define a magnetic field.
- What do magnetic field lines represent?
- State two properties of magnetic field lines.
- Why do magnetic field lines never intersect?
- Describe the magnetic field around a straight current-carrying conductor.
- State the right-hand thumb rule.
- How does increasing current affect the magnetic field around a straight conductor?
- How does the magnetic field change with distance from a straight conductor?
- Describe the magnetic field produced by a current-carrying circular loop.
- What is a solenoid?
- Describe the magnetic field inside a long solenoid.
- How can the strength of a solenoid be increased?
- What is an electromagnet?
- State the principle of an electric motor.
- State Fleming's left-hand rule.
- Name the main parts of a simple electric motor.
- What is the function of the split ring in an electric motor?
- Why does a motor coil continue rotating in the same direction?
- What is electromagnetic induction?
- Describe an experiment showing electromagnetic induction using a coil and magnet.
- What happens to induced current when the direction of relative motion is reversed?
- State Fleming's right-hand rule.
- State the principle of an electric generator.
- How does an AC generator produce alternating current?
- What is the role of slip rings in an AC generator?
- How does a DC generator obtain unidirectional current in the external circuit?
- Differentiate between an electric motor and an electric generator.
- Differentiate between Fleming's left-hand and right-hand rules.
- Explain why a soft iron core strengthens an electromagnet.
- Draw and label a simple electric motor and explain its working.
15. 20 CBSE PYQ / PYQ-Based Questions
Exam-oriented questions based on the chapter and CBSE-style competency practice. They are not presented as verbatim copies of official papers.
- Draw magnetic field lines around a bar magnet and state two properties of the lines.
- State the right-hand thumb rule and use it to determine the field direction around a straight current-carrying conductor.
- How does the magnetic field due to a current-carrying conductor change when the current is increased?
- Why is the magnetic field inside a long solenoid nearly uniform?
- How can a solenoid be converted into an electromagnet?
- State Fleming's left-hand rule.
- Explain the force on a current-carrying conductor placed in a magnetic field.
- State the principle of an electric motor.
- Explain the role of the split ring in a simple electric motor.
- Why does the direction of current in the motor coil have to be reversed after every half rotation?
- Name three ways in which the power of a commercial electric motor can be increased.
- Define electromagnetic induction.
- Describe an experiment to demonstrate electromagnetic induction.
- State Fleming's right-hand rule.
- State the principle of an electric generator.
- Explain how an AC generator produces current.
- Differentiate between slip rings and a split-ring commutator.
- Compare the energy transformations in a motor and generator.
- Why does a galvanometer deflect when a magnet is moved relative to a coil?
- How can the induced current be increased in an electromagnetic-induction experiment?
16. Interactive MCQ Quiz – 20 Questions
Select one option for each question and click Check Score.