Magnetic Effects of Electric Current

Introduction

• Electric current produces magnetic effects, as discovered by Hans Christian Oersted in 1820 when a compass needle deflected near a current-carrying wire.
• This chapter explores the linkage between electricity and magnetism, magnetic fields, field lines, forces on current-carrying conductors, and domestic electric circuits.
• Oersted’s Contribution: Demonstrated electricity-magnetism relationship, leading to technologies like radio, TV, and fiber optics; magnetic field strength unit named "oersted."
• Electromagnetism is foundational to modern technology and medical applications.

12.1 Magnetic Field and Field Lines

• Magnetic Field: Region around a magnet where its force is detectable, influencing objects like compass needles (small bar magnets).
• Compass Needle:
  • North-seeking pole (north pole) points toward geographic north; south-seeking pole (south pole) points south.
  • Deflects near a bar magnet due to magnetic field interaction.
• Magnetic Interactions: Like poles repel; unlike poles attract.
• Activity 12.2:
  • Place a bar magnet on paper, sprinkle iron filings, tap gently (Fig. 12.2).
  • Filings align in patterns, representing magnetic field lines due to the magnet’s force.
• Activity 12.3:
  • Use a compass to trace field lines around a bar magnet (Fig. 12.3).
  • Compass south pole points to magnet’s north pole; move compass step-by-step to map lines (Fig. 12.4).
  • Deflection increases near poles, indicating stronger field.
• Magnetic Field Lines:
  • Closed curves: Emerge from north pole, merge at south pole; inside magnet, south to north (Fig. 12.4).
  • Direction: Path a free north pole would follow; shown by compass needle’s north pole.
  • Strength: Closer lines indicate stronger field (crowded near poles).
  • Non-intersecting: Intersection would imply two directions at one point, impossible for a compass needle.
• Question:
  • Why does a compass needle deflect near a bar magnet? Due to the magnetic field exerting a force on the needle’s poles.
• Magnetic field lines visualize the invisible force field around magnets.

12.2 Magnetic Field Due to a Current-Carrying Conductor

• Principle: Electric current in a conductor produces a magnetic field (demonstrated in Activity 12.1).
• Activity 12.1:
  • Pass current through a copper wire near a compass; needle deflects, indicating a magnetic field (Fig. 12.1).

12.2.1 Magnetic Field Due to a Current through a Straight Conductor

• Activity 12.4:
  • Circuit with copper wire, 1.5 V cells, plug key; place wire over compass (Fig. 12.5).
  • Current north to south: Compass north pole deflects east.
  • Reverse current (south to north): Deflection reverses to west.
  • Conclusion: Magnetic field direction depends on current direction.
• Activity 12.5:
  • Pass current through a vertical copper wire through cardboard; sprinkle iron filings (Fig. 12.6).
  • Filings form concentric circles, representing magnetic field lines.
  • Compass at point P shows field direction; reversing current reverses field direction.
  • Increase current: Deflection increases (stronger field).
  • Move compass away (point Q): Deflection decreases (weaker field).
• Characteristics:
  • Field lines: Concentric circles centered on the wire.
  • Strength: Proportional to current, inversely proportional to distance from wire.
• Current in a straight conductor creates a circular magnetic field, key to electromagnetic applications.

12.2.2 Right-Hand Thumb Rule

• Rule: Hold a current-carrying conductor with right hand, thumb along current direction; fingers curl in the direction of magnetic field lines (Fig. 12.7).
• Alternative: Maxwell’s corkscrew rule—corkscrew driven in current direction rotates in field direction.
• Example 12.1:
  • Power line current: East to west.
  • Below wire: Right-hand rule shows clockwise field (viewed from east).
  • Above wire: Anti-clockwise field (viewed from west).
• The right-hand thumb rule simplifies determining magnetic field direction.

12.2.3 Magnetic Field Due to a Current through a Circular Loop

• Pattern:
  • Current in a circular loop produces magnetic field; field lines are concentric circles, larger further from wire (Fig. 12.8).
  • At loop center, arcs appear as straight lines; all loop sections contribute field in same direction (verified by right-hand rule).
• Strength: For \( n \)-turn coil, field is \( n \) times that of a single turn, as currents add constructively.
• Activity 12.6:
  • Pass current through a multi-turn circular coil through cardboard; sprinkle iron filings (Fig. 12.9).
  • Filings show field pattern, stronger due to multiple turns.
• Circular loops amplify magnetic fields, useful in electromagnets.

12.2.4 Magnetic Field Due to a Current in a Solenoid

• Solenoid: Coil of many insulated copper wire turns wrapped tightly in a cylinder.
• Field Pattern:
  • Similar to bar magnet (Fig. 12.10 vs. Fig. 12.4).
  • One end acts as north pole, other as south pole.
  • Inside: Uniform, parallel straight field lines.
• Application:
  • Electromagnet: Solenoid with soft iron core; current magnetizes core (Fig. 12.11).
• Question:
  • Magnetic field inside a long solenoid is (d) the same at all points (uniform). Answer: (d).
• Solenoids produce strong, uniform magnetic fields, ideal for electromagnets.

12.3 Force on a Current-Carrying Conductor in a Magnetic Field

• Principle: Current-carrying conductor in a magnetic field experiences a force (suggested by Ampere), equal and opposite to force on a magnet.
• Activity 12.7:
  • Suspend aluminium rod AB (5 cm) horizontally in a horseshoe magnet’s field (north below, south above) (Fig. 12.12).
  • Pass current from B to A: Rod displaces left.
  • Reverse current: Displaces right.
  • Reverse field (south below, north above): Force direction reverses.
  • Conclusion: Force depends on current and field directions.
• Force Characteristics:
  • Maximum when current and field are perpendicular.
  • Direction perpendicular to both current and field.
• Fleming’s Left-Hand Rule:
  • Stretch left hand’s thumb, forefinger, middle finger mutually perpendicular (Fig. 12.13).
  • Forefinger: Magnetic field direction; Middle finger: Current direction; Thumb: Force direction.
• Example 12.2:
  • Electron moves perpendicular to magnetic field (Fig. 12.14).
  • Current direction opposite to electron motion; apply Fleming’s left-hand rule.
  • Force is into the page. Answer: (d).
• Applications: Electric motors, generators, loudspeakers, microphones, measuring instruments.
• Questions:
  1. Proton in Magnetic Field:
     • Force changes direction of motion, affecting (c) velocity and (d) momentum (mass and speed unchanged). Answer: (c), (d).
  2. Rod AB Displacement (Activity 12.7):
     • (i) Increase current: Larger force, more displacement.
     • (ii) Stronger magnet: Stronger field, more displacement.
     • (iii) Longer rod: More conductor in field, more displacement.
  3. Alpha Particle Deflection:
     • Alpha particle (positive) moves west, deflects north; apply Fleming’s left-hand rule.
     • Magnetic field is upward. Answer: (d).
• Biological Magnetic Fields:
  • Weak magnetic fields from ion currents in nerves (e.g., heart, brain) are ~1 billionth of Earth’s field.
  • Used in Magnetic Resonance Imaging (MRI) for medical diagnosis.
• Forces on conductors in magnetic fields drive many electromechanical devices.

12.4 Domestic Electric Circuits

• Power Supply:
  • Received via mains (overhead poles or underground cables).
  • Live wire (red insulation, positive); Neutral wire (black insulation, negative).
  • Potential difference: 220 V (India).
• Circuit Setup:
  • Wires enter via main fuse and electricity meter, then main switch to house circuits (Fig. 12.15).
  • Two circuits: 15 A (high-power appliances like geysers), 5 A (bulbs, fans).
  • Earth wire (green insulation): Connected to a metal plate in ground for safety.
• Earth Wire Function:
  • Connects metallic appliance bodies to ground, providing low-resistance path for leakage current.
  • Prevents severe shocks by keeping appliance potential at earth’s level.
• Appliance Connection:
  • Connected in parallel across live and neutral wires for equal potential difference.
  • Each has a separate switch for control.
• Electric Fuse:
  • Prevents damage from overloading or short-circuiting (live-neutral contact causing high current).
  • Joule heating melts fuse wire, breaking circuit.
  • Overloading causes: Damaged insulation, faulty appliances, voltage spikes, or too many appliances on one socket.
• Questions:
  1. Safety Measures:
     • Electric fuse and earth wire.
  2. 2 kW Oven in 5 A Circuit:
     • P = 2000 W, V = 220 V; \( I = \frac{P}{V} = \frac{2000}{220} \approx 9.09 \, \text{A} \).
     • Exceeds 5 A rating; fuse will blow to prevent damage.
  3. Overloading Precautions:
     • Use appropriate fuses, avoid multiple high-power appliances on one socket, ensure proper insulation, regular circuit maintenance.
• Domestic circuits are designed for safety and efficiency with fuses and earthing.

Key Questions and Answers

1. Magnetic Field near Long Straight Wire:
   • Field forms concentric circles centered on the wire. Answer: (d).
2. Short Circuit Current:
   • Current increases heavily due to low resistance path. Answer: (c).
3. True/False:
   • (a) True: Field at center of a circular coil is straight and uniform.
   • (b) False: Green insulation is earth wire; live wire is red.
4. Methods to Produce Magnetic Fields:
   • Using a permanent magnet; passing current through a conductor (e.g., wire, coil, solenoid).
5. Maximum Force on Conductor:
   • When current is perpendicular to magnetic field.
6. Electron Beam Deflection:
   • Electron beam moves forward, deflects right; current is backward.
   • Apply Fleming’s left-hand rule: Magnetic field is upward.
7. Direction Rules:
   • (i) Magnetic field around conductor: Right-hand thumb rule.
   • (ii) Force on conductor: Fleming’s left-hand rule.
   • (iii) Induced current: Fleming’s right-hand rule (not covered in text, but relevant).
8. Short Circuit Occurrence:
   • When live and neutral wires contact directly due to damaged insulation or faulty appliances, causing high current.
9. Earth Wire Function:
   • Provides low-resistance path for leakage current to ground, preventing shocks.
   • Necessary for metallic appliances to ensure safety.