Magnetic Effects of Electric Current
CBSE Class 10 · Science · Notes, formulas and practice questions
This chapter explains how electric current creates magnetic fields, why a current-carrying conductor experiences a force in a magnetic field, and how these principles are used in electric motors and generators. It also describes domestic electric circuits, fuses and earthing for safe use of electricity.
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The chapter begins with the observation that a current-carrying conductor behaves like a magnet. It produces a magnetic field around it, which can be visualised by drawing magnetic field lines. Field lines are closed curves, never intersect, and are drawn closer together where the field is stronger. The direction of the field around a straight wire is obtained by the right-hand thumb rule, and the same idea is extended to circular loops and to solenoids, where the coil acts like a bar magnet.
The next key idea is that a current-carrying conductor placed in an external magnetic field experiences a mechanical force. The force is largest when the conductor is perpendicular to the field and zero when it is parallel. Fleming's left-hand rule gives the direction of this force by relating the directions of magnetic field, current and motion. This principle is used in an electric motor, which converts electrical energy into mechanical energy with a coil that keeps rotating because a split-ring commutator reverses the current twice in each cycle.
The chapter then deals with the reverse effect, electromagnetic induction. A changing magnetic field linked with a coil or conductor produces an induced current in it. The induced current flows only while the magnetic field is changing, and its direction is obtained with Fleming's right-hand rule. An electric generator works on this principle, converting mechanical energy into electrical energy. The generator is the source of alternating current used in homes.
Finally, domestic wiring ensures the safe delivery of electricity. Circuits have three wires: live, neutral and earth. A fuse is connected in the live wire; it melts and breaks the circuit when the current exceeds a safe limit. Earthing connects the metal body of appliances to the earth, so any leakage current flows safely to ground. Appliance connections are made in parallel so each branch gets the full mains voltage and can be controlled independently.
Key terms
- Magnetic field
- The region around a magnet or a current-carrying conductor in which a magnetic force is experienced. Its strength and direction are shown by magnetic field lines, and its SI unit is the tesla (T).
- Magnetic field lines
- Imaginary closed curves used to represent a magnetic field. Outside a magnet they go from the north pole to the south pole, and inside the magnet from south to north. They never intersect, and their closeness indicates the strength of the field.
- Right-hand thumb rule
- A method to find the direction of the magnetic field around a straight current-carrying wire. Hold the wire in your right hand with your thumb pointing in the direction of conventional current; your curled fingers then point in the direction of the magnetic field around the wire.
- Solenoid and electromagnet
- A solenoid is a long coiled insulated wire that behaves like a bar magnet when current flows through it. When a soft iron core is placed inside the solenoid, it becomes an electromagnet, which is magnetic only while current flows and whose strength can be increased by increasing current or number of turns.
- Fleming's left-hand rule
- A rule for determining the direction of force on a current-carrying conductor placed in a magnetic field. Stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular; if the forefinger points along the field and the middle finger along current, the thumb gives the direction of force or motion.
- Fleming's right-hand rule
- A rule for finding the direction of induced current when a conductor moves in a magnetic field. Stretch the thumb, forefinger and middle finger of the right hand mutually perpendicular; if the thumb points in the direction of motion and the forefinger along the field, the middle finger points in the direction of the induced current.
- Electric motor
- A device that converts electrical energy into mechanical energy. It consists of a current-carrying coil placed in a magnetic field; the magnetic force on the coil rotates it. A split-ring commutator reverses the direction of current every half turn so that the coil keeps rotating in the same direction.
- Electromagnetic induction
- The production of an electric current in a conductor when the magnetic field linked with the conductor changes. An induced current is obtained only while the magnetic field is changing, and its direction is given by Fleming's right-hand rule.
- Electric generator
- A machine that converts mechanical energy into electrical energy using electromagnetic induction. A coil is rotated in a magnetic field; the changing magnetic flux through the coil induces an alternating current that can be drawn through slip rings or a commutator.
Formula sheet
| What | Formula | Notes |
|---|---|---|
| Force on a current-carrying conductor in a magnetic field | F = BIL | B is the magnetic field strength in tesla (T), I is the current in ampere (A), and L is the length of the conductor inside the field in metre (m). This gives the force when the conductor is perpendicular to the magnetic field; if the conductor is parallel to the field, the force is zero. |
Practice questions with answers
1. Define magnetic field and write its SI unit.
The region around a magnet or a current-carrying conductor in which a magnetic force is experienced is called a magnetic field. Its SI unit is the tesla (T).
2. State two properties of magnetic field lines.
Two properties of magnetic field lines are: (1) they never intersect each other; (2) they are closed continuous curves that go from the north pole to the south pole outside a magnet and from south to north inside the magnet. A third common property is that the closeness of lines shows the strength of the magnetic field.
3. State the right-hand thumb rule and use it to find the direction of the magnetic field at a point directly below a wire carrying current from west to east.
The right-hand thumb rule says: grasp the current-carrying wire with your right hand so that the thumb points in the direction of the current; the curled fingers show the direction of the magnetic field. For current from west to east, the thumb points east, and at a point directly below the wire the fingers point north, so the magnetic field there is directed northwards.
4. A circular loop carries current in the anticlockwise direction when viewed from its front. Which face of the loop acts as the north pole? Why?
The front face acts as the north pole because, when current is anticlockwise as seen from the front, the magnetic field lines emerge from the front face of the loop. The face from which the field lines emerge behaves as the north pole.
5. Calculate the force on a 0.5 m long straight conductor carrying 2 A current placed perpendicular to a uniform magnetic field of 0.4 T.
Using F = BIL, F = 0.4 × 2 × 0.5 = 0.4 N. Since the conductor is perpendicular to the magnetic field, the maximum force is produced and is 0.4 N. Its direction is perpendicular to both the field and the conductor, as given by Fleming's left-hand rule.
6. What is a solenoid? How can it be converted into an electromagnet?
A solenoid is a long coil of insulated wire wound in close turns. When current flows through it, it behaves like a bar magnet. If a soft iron core is inserted into the solenoid, the magnetism becomes much stronger; this arrangement, magnetic only while current flows, is called an electromagnet.
7. State Fleming's left-hand rule.
Stretch the thumb, forefinger and middle finger of your left hand so that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of the current, then the thumb points in the direction of the force (or motion) acting on the conductor.
8. On what principle does an electric motor work? What is the role of the split-ring commutator?
An electric motor works on the principle that a current-carrying conductor placed in a magnetic field experiences a force. The split-ring commutator reverses the direction of current in the coil after every half rotation, so the coil keeps rotating in the same direction instead of stopping or reversing.
9. State the principle of electromagnetic induction and name one device based on it.
Electric current is induced in a closed circuit when the magnetic field lines linked with the circuit change. This is called electromagnetic induction. The electric generator (or dynamo) is a device that works on this principle.
10. Why should the metal body of an electric appliance be earthed?
Earthing connects the metal body of an appliance to the earth through a low-resistance wire. If the live wire accidentally touches the metal body, the current flows safely into the ground instead of passing through a person who touches the appliance, thus preventing electric shock. It also causes a large current that can blow the fuse and cut off the supply.
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