Magnetic Effects of Electric Current
Electric current produces a magnetic field, and a magnetic field in turn exerts a force on a current-carrying conductor - revealing that electricity and magnetism are two sides of the same phenomenon.
A perennial NCERT-based Prelims source: expect direct or assertion-reason questions on Oersted's discovery, the properties of magnetic field lines, the Right-Hand Thumb Rule, and the uniform field of a solenoid/electromagnet. In Mains it feeds GS-III Science and Technology, since the magnetic effect of current is the working principle behind electromagnets, electric motors and generators, and modern devices like MRI and maglev.
Understand the chapter
The Electricity-Magnetism Link: Oersted's Breakthrough
In 1820 Hans Christian Oersted observed that a compass needle placed near a current-carrying wire got deflected, proving that an electric current produces a magnetic field around it. This single observation established that electricity and magnetism are related phenomena and gave birth to electromagnetism. Crucially, reversing the direction of the current reverses the direction of the deflection, showing the field's direction depends on the current's direction.
- Oersted (1777-1851); discovery in 1820, made accidentally with a compass and current-carrying wire
- Reverse the current and the magnetic field (deflection) also reverses
- The CGS unit of magnetic field strength, the oersted, is named in his honour
- His insight underpinned later technologies like radio, television and fibre optics
Magnetic Field and Field Lines
The region around a magnet or current where its magnetic force can be detected is the magnetic field, a vector quantity having both magnitude and direction. Field lines are the paths traced by iron filings or by the north pole of a compass; by convention they run from north to south outside the magnet and from south to north inside it, so each line is a closed curve. The closeness of the lines indicates the strength of the field.
- Field direction = the direction the north pole of a compass needle points
- Outside the magnet: north to south; inside: south to north; lines are closed loops
- Crowded (closely spaced) lines mean a stronger field
- Two field lines never cross - else the compass would point in two directions at one point
Magnetic Field due to a Current-Carrying Straight Conductor
A straight wire carrying current is surrounded by magnetic field lines in the form of concentric circles lying in a plane perpendicular to the wire. The field grows stronger as the current increases and grows weaker as the distance from the wire increases (the circles get larger and fainter outward). The direction of this field is found using the Right-Hand Thumb Rule.
- Field pattern around a straight wire = concentric circles
- Strength is greater with more current and less with greater distance
- Right-Hand Thumb Rule: thumb along current, curled fingers give the field direction
- Also called Maxwell's Corkscrew Rule
Circular Loop, Solenoid and the Electromagnet
Bending the wire into a loop concentrates the field, and at the centre of the loop the field lines appear as straight lines; a coil of n turns produces a field n times that of a single turn because every turn carries current in the same direction and their fields add up. A solenoid - a closely wound cylindrical coil - produces a uniform field inside and behaves exactly like a bar magnet, with one end a north pole and the other a south pole. Placing a soft-iron core inside a current-carrying solenoid magnetises it into an electromagnet.
- A coil of n turns gives n times the field of one turn (fields add)
- Inside a solenoid the field is uniform - parallel, equally spaced straight lines
- A current-carrying solenoid acts like a bar magnet (two poles)
- Soft-iron core inside the solenoid forms a (temporary) electromagnet
Force on a Current-Carrying Conductor in a Magnetic Field
Because a current creates a field that pushes on a nearby magnet, Andre-Marie Ampere reasoned that the magnet must exert an equal and opposite force on the current-carrying conductor. This force is largest when the current flows perpendicular to the magnetic field, and its direction is given by Fleming's Left-Hand Rule. This force is the working principle of the electric motor.
- Ampere (1775-1836): the magnet exerts an equal and opposite force on the conductor
- Force is maximum when current is perpendicular to the field
- Fleming's Left-Hand Rule: thumb = force/motion, forefinger = field, centre finger = current
- This is the basis of the electric motor
Key terms
- Magnetic field
- The region around a magnet or current where its magnetic force can be detected; a vector with magnitude and direction.
- Magnetic field lines
- Closed continuous curves showing the field's direction (north to south outside) and its strength (by their spacing).
- Right-Hand Thumb Rule
- Grip the wire with the right hand, thumb along the current, and the curled fingers give the field direction; also Maxwell's Corkscrew Rule.
- Solenoid
- A closely wound cylindrical coil of insulated wire that produces a uniform internal field and behaves like a bar magnet.
- Electromagnet
- A temporary magnet formed by passing current through a solenoid wound on a soft-iron core.
- Uniform magnetic field
- A field with the same magnitude and direction at all points, shown by equally spaced parallel lines, as inside a solenoid.
- Fleming's Left-Hand Rule
- Thumb, forefinger and centre finger held mutually perpendicular give the directions of force, field and current respectively.
- Oersted
- The CGS unit of magnetic field strength, named after Hans Christian Oersted.
Must-know facts exam-ready
- Hans Christian Oersted (1777-1851) discovered the magnetic effect of electric current in 1820.
- Reversing the direction of current reverses the direction of the magnetic field.
- Magnetic field is a vector quantity - it has both magnitude and direction.
- Outside a magnet, field lines run north to south; inside, they run south to north, forming closed continuous loops.
- No two magnetic field lines ever intersect.
- A straight current-carrying wire is surrounded by concentric circular field lines; the field is stronger with more current and weaker farther away.
- The Right-Hand Thumb Rule is also called Maxwell's Corkscrew Rule.
- A circular coil of n turns produces a field n times that of a single turn, because the turns' fields add up.
- The magnetic field inside a long current-carrying solenoid is uniform, and the solenoid behaves like a bar magnet.
- An electromagnet uses a soft-iron core (not steel) inside a current-carrying solenoid.
- Ampere (1775-1836) proposed that a magnet exerts an equal and opposite force on a current-carrying conductor.
- The CGS unit oersted honours Oersted; the SI unit of magnetic field is the tesla (T).
Timeline
- 1820Hans Christian Oersted accidentally finds that an electric current deflects a nearby compass needle, establishing the link between electricity and magnetism.
Memory tricks remember it for good
Traps to avoid
- Field lines run north to south only OUTSIDE the magnet; INSIDE they run south to north - forgetting this breaks the 'closed loop' idea.
- 'Crowded lines = stronger field' refers to spacing, not lines carrying force; and field lines never cross.
- Right-Hand Thumb Rule (field around a wire) is NOT Fleming's Left-Hand Rule (force on a conductor) - and neither is Fleming's Right-Hand Rule (induced current in a generator).
- An electromagnet's core is SOFT IRON (temporary magnetism); STEEL is used for permanent magnets - a classic swap.
- Inside a solenoid the field is UNIFORM (same at all points) - it is neither zero nor does it decrease toward the ends (a favourite MCQ distractor).
- Oersted (current produces magnetism) is the converse of induction (magnetism produces current); also don't confuse the unit oersted with the SI unit tesla.
Exam focus
🧠 Prelims angles
- Oersted's experiment, the year 1820, and his role in linking electricity and magnetism.
- Properties of magnetic field lines (closed curves, never intersect, density indicates strength).
- Right-Hand Thumb Rule / Maxwell's Corkscrew Rule for the field around a straight conductor.
- Solenoid producing a uniform internal field and acting as a bar magnet; electromagnet uses a soft-iron core.
- Field depends directly on current and number of turns (n turns give n times the field), and inversely on distance.
- Fleming's Left-Hand Rule and Ampere's equal-and-opposite force on a current-carrying conductor.
✍️ Mains angles GS-III
- How does the magnetic effect of electric current underpin modern technology?Trace Oersted's link to electromagnets, electric motors (force on a conductor) and applications such as MRI, maglev and electric machinery.
- Assess the contributions of Oersted and Ampere to the foundation of electromagnetism.Oersted (1820) showed current produces a field; Ampere proposed the reciprocal force - together unifying electricity and magnetism.
- Electromagnets versus permanent magnets - significance for industry and technology.Soft-iron-core electromagnets are controllable and temporary (cranes, relays, motors); contrast with the permanence and uses of steel magnets.
Last-minute revision tick as you recall
- Current produces magnetism (Oersted, 1820); reverse the current and the field reverses.
- Magnetic field is a vector; lines are closed, north-to-south outside, south-to-north inside, and never cross.
- Closely spaced field lines mean a stronger field.
- Straight wire gives concentric circular field lines; field rises with current, falls with distance; direction by Right-Hand Thumb Rule.
- A coil of n turns gives n times the field because all turns share the current direction.
- A solenoid has a uniform internal field and behaves like a bar magnet.
- Electromagnet = soft-iron core inside a current-carrying solenoid (temporary magnet).
- Force on a current-carrying conductor (Ampere); direction by Fleming's Left-Hand Rule - the basis of the electric motor.
Distilled from NCERT Class 10 · Science (Class 10) for UPSC. Always cross-check facts with the original NCERT.