What this chapter is about
This chapter explores the connection between electricity and magnetism. When electric current flows through a conductor, it creates a magnetic field around it. This discovery, made by Hans Christian Oersted in 1820, unified two branches of physics and led to devices we use daily—electric motors, generators, and transformers.
A Class 10 student meets this topic now because it builds on the understanding of current and circuits from the previous chapter on electricity. Here, you learn how moving charges produce magnetism, how magnetic fields exert forces on current-carrying conductors, and how this principle powers motors. You also study electromagnetic induction—how changing magnetic fields produce electric current—which is the working principle of generators.
After studying this chapter, you should be able to describe magnetic field patterns around conductors, apply rules to find field and force directions, explain how electric motors and generators work, and distinguish between AC and DC. You will also understand domestic electric circuits and safety devices like fuses.
Key ideas
- A current-carrying conductor produces a magnetic field around it; the field strength increases with current and decreases with distance from the conductor.
- The magnetic field around a straight conductor forms concentric circles; the right-hand thumb rule gives the field direction: thumb points in current direction, curled fingers show field direction.
- A circular loop carrying current behaves like a small magnet; field lines emerge from one face and enter the other, making the loop have north and south poles.
- A solenoid (coil of many turns) produces a uniform magnetic field inside, similar to a bar magnet; one end acts as north pole, the other as south pole.
- A current-carrying conductor placed in an external magnetic field experiences a force; Fleming's left-hand rule gives force direction: Forefinger for Field, middle finger for Current, thumb for Force (all mutually perpendicular).
- An electric motor converts electrical energy into mechanical energy using the force on a current-carrying coil in a magnetic field; a split-ring commutator reverses current direction every half rotation to maintain continuous rotation.
- Electromagnetic induction: when magnetic field through a coil changes (by moving a magnet or changing current in a nearby coil), an electromotive force (and hence current) is induced in the coil.
- Fleming's right-hand rule gives induced current direction in a generator: thumb for Motion, forefinger for Field, middle finger for induced Current.
Formulas and facts to remember
Magnetic field around a straight conductor: Field lines are concentric circles. Direction given by right-hand thumb rule.
Magnetic field inside a solenoid: B is uniform and along the axis. The end where current flows anticlockwise is the north pole.
Force on a current-carrying conductor: F = B × I × L (when field, current and length are mutually perpendicular). F is force in newtons, B is magnetic field in tesla, I is current in amperes, L is length in metres.
Direction of force: Fleming's left-hand rule — First finger = Field (B), Second finger = Current (I), Thumb = Force (F).
Electromagnetic induction: Induced EMF arises when magnetic flux through a circuit changes. Faster change means larger induced EMF.
Direction of induced current: Fleming's right-hand rule — Thumb = Motion, First finger = Field, Second finger = induced Current.
AC vs DC: Alternating current (AC) reverses direction periodically; direct current (DC) flows in one direction. In India, domestic AC supply is 220 V, 50 Hz.
Domestic circuit components: Live wire, neutral wire, earth wire; fuse or MCB protects against overcurrent; short circuit occurs when live and neutral touch directly.
Worked examples
### Example 1: Applying the right-hand thumb rule
Problem: A long straight wire carries current vertically upward. What is the direction of the magnetic field at a point to the east of the wire?
Solution: Step 1: Point the thumb of your right hand in the direction of the current (upward). Step 2: Curl your fingers around the wire; they show the direction of field lines, which circle the wire. Step 3: At a point to the east of the wire, the curled fingers point towards the north. Answer: The magnetic field at that point is directed northward.
### Example 2: Force on a conductor
Problem: A wire of length 0.25 m carries a current of 4 A and is placed perpendicular to a magnetic field of 0.5 T. Find the force on the wire.
Solution: Step 1: Write the formula: F = B × I × L. Step 2: Substitute values: F = 0.5 T × 4 A × 0.25 m. Step 3: Calculate: F = 0.5 N. Answer: The wire experiences a force of 0.5 N.
### Example 3: Generator principle
Problem: A rectangular coil is rotated in a uniform magnetic field. At what positions of the coil is the induced EMF (a) maximum and (b) zero?
Solution: Step 1: Induced EMF depends on the rate of change of magnetic flux through the coil. Step 2: When the plane of the coil is parallel to the field (coil sides cutting field lines fastest), the rate of flux change is greatest. Answer (a): Induced EMF is maximum when the coil plane is parallel to the magnetic field. Step 3: When the coil plane is perpendicular to the field, the coil sides move parallel to field lines and flux change is momentarily zero. Answer (b): Induced EMF is zero when the coil plane is perpendicular to the magnetic field.
Common mistakes
Confusing the two Fleming's rules → Left-hand rule is for motor (force on conductor), right-hand rule is for generator (induced current).
Using the wrong finger assignments → Remember: in left-hand rule, First finger = Field, seCond finger = Current, thuMb = Motion/Force.
Thinking a stationary magnet near a coil induces current → Induction requires relative motion or a changing magnetic field; no change means no induced EMF.
Believing magnetic field inside a solenoid depends on its length alone → Field strength depends on current, number of turns per unit length, and core material, not just total length.
Forgetting the role of the split-ring commutator in a DC motor → Without it, the coil would oscillate back and forth instead of rotating continuously.
Quick revision
- Current produces a magnetic field; direction by right-hand thumb rule.
- Solenoid behaves like a bar magnet; anticlockwise current face is north pole.
- Force on conductor: F = B × I × L; direction by Fleming's left-hand rule.
- Electric motor uses force on a current-carrying coil; commutator reverses current each half turn.
- Changing magnetic flux induces EMF; direction by Fleming's right-hand rule.
- Domestic supply in India: 220 V AC, 50 Hz; fuse in live wire protects circuit.