Physics · Class 9-10 · CBSE
Magnetic Effects of Current and Electric Motors
Learn magnetic fields around conductors, Right-Hand Thumb Rule, Solenoids, Fleming’s Left-Hand Rule for electric motors, and Faraday’s electromagnetic induction with Fleming’s Right-Hand Rule.
1Magnetic Field and Field Lines
Hans Christian Oersted (1820) discovered that an electric current produces a surroundingMagnetic Field. Magnetic field lines are closed continuous loops emerging from the North pole and merging into the South pole outside a magnet.
Core Properties of Magnetic Field Lines
| Property | Physical Explanation | Significance in Physics |
|---|---|---|
| Direction | Emerge from North, enter South outside; South to North inside | Forms seamless closed continuous loops |
| Crowding / Density | Lines are crowded near poles and spread apart far away | Density represents magnetic field strength (B) |
| Non-Intersecting Rule | Two field lines NEVER intersect each other | If they intersected, a compass needle would point to two directions simultaneously, which is impossible |
| Uniform Field | Equidistant parallel straight lines | Represents uniform magnetic field (e.g. inside a current-carrying solenoid) |
Direction
- Physical Explanation
- Emerge from North, enter South outside; South to North inside
- Significance in Physics
- Forms seamless closed continuous loops
Crowding / Density
- Physical Explanation
- Lines are crowded near poles and spread apart far away
- Significance in Physics
- Density represents magnetic field strength (B)
Non-Intersecting Rule
- Physical Explanation
- Two field lines NEVER intersect each other
- Significance in Physics
- If they intersected, a compass needle would point to two directions simultaneously, which is impossible
Uniform Field
- Physical Explanation
- Equidistant parallel straight lines
- Significance in Physics
- Represents uniform magnetic field (e.g. inside a current-carrying solenoid)
2Rules for Magnetic Field Direction and Solenoids
Mastering Hand Rules in Electromagnetism
| Rule Name | Hand Used | Finger Alignment | Determines What? |
|---|---|---|---|
| Right-Hand Thumb Rule | Right Hand | Thumb = Current direction; Curled fingers = Field lines | Direction of magnetic field around straight wire |
| Fleming’s Left-Hand Rule | Left Hand | Thumb = Motion/Force; Forefinger = Field (B); Middle = Current (I) | Direction of mechanical force on current-carrying conductor (Electric Motor) |
| Fleming’s Right-Hand Rule | Right Hand | Thumb = Motion of conductor; Forefinger = Field; Middle = Induced Current | Direction of induced current produced by electromagnetic induction (Generator) |
Right-Hand Thumb Rule
- Hand Used
- Right Hand
- Finger Alignment
- Thumb = Current direction; Curled fingers = Field lines
- Determines What?
- Direction of magnetic field around straight wire
Fleming’s Left-Hand Rule
- Hand Used
- Left Hand
- Finger Alignment
- Thumb = Motion/Force; Forefinger = Field (B); Middle = Current (I)
- Determines What?
- Direction of mechanical force on current-carrying conductor (Electric Motor)
Fleming’s Right-Hand Rule
- Hand Used
- Right Hand
- Finger Alignment
- Thumb = Motion of conductor; Forefinger = Field; Middle = Induced Current
- Determines What?
- Direction of induced current produced by electromagnetic induction (Generator)
Current-Carrying Solenoid Properties
- 1
1. A coil of many circular turns of insulated copper wire wrapped closely in cylindrical form is a Solenoid.
- 2
2. Field lines inside a solenoid are parallel straight lines, indicating uniform magnetic field throughout.
- 3
3. One end of solenoid behaves as magnetic North pole and the opposite as South pole (identical to a bar magnet).
- 4
4. Placing a soft iron rod inside a solenoid creates an Electromagnet whose magnetic field can be toggled on/off instantly.
3Electric Motor vs Electromagnetic Induction
Electric Motor vs Electromagnetic Induction
| Feature | Electric Motor | Electromagnetic Induction (EMI) |
|---|---|---|
| Underlying Principle | Current-carrying conductor in magnetic field experiences mechanical torque | Changing magnetic flux through a coil induces an electric current |
| Governing Rule | Fleming’s Left-Hand Rule | Faraday’s Law and Fleming’s Right-Hand Rule |
| Energy Conversion | Converts Electrical Energy → Mechanical Energy | Converts Mechanical Energy → Electrical Energy |
| Essential Component | Split-ring Commutator (reverses current direction every half rotation) | Galvanometer to detect micro-ampere induced currents |
Underlying Principle
- Electric Motor
- Current-carrying conductor in magnetic field experiences mechanical torque
- Electromagnetic Induction (EMI)
- Changing magnetic flux through a coil induces an electric current
Governing Rule
- Electric Motor
- Fleming’s Left-Hand Rule
- Electromagnetic Induction (EMI)
- Faraday’s Law and Fleming’s Right-Hand Rule
Energy Conversion
- Electric Motor
- Converts Electrical Energy → Mechanical Energy
- Electromagnetic Induction (EMI)
- Converts Mechanical Energy → Electrical Energy
Essential Component
- Electric Motor
- Split-ring Commutator (reverses current direction every half rotation)
- Electromagnetic Induction (EMI)
- Galvanometer to detect micro-ampere induced currents
4Exam Corner: Tips, Mistakes & Board Questions
High-Yield Board Revision Takeaways
- The function of a split-ring commutator in a DC motor is to reverse the direction of current in the armature coil every half-rotation, ensuring continuous unidirectional rotation.
- Domestic electric supply in India: 220 V alternating current (AC) at frequency 50 Hz (reverses direction 100 times every second).
- Earth wire (green insulation) provides a low-resistance path to ground, protecting users from lethal shocks in metallic appliances.
Exam question · CBSE Class 10 Board
- A.Because field lines are always parallel
- B.Because at point of intersection, a compass would show two directions of magnetic field at once
- C.Because like poles repel
- D.Because magnetic field is a scalar quantity
- The tangent to a magnetic field line gives the direction of magnetic field at that point.
- If two lines intersected, there would be two different tangents and hence two directions of magnetic field at one point, which is physically impossible.
- Correct answer is option B.
Answer: B. Because at point of intersection, a compass would show two directions of magnetic field at once
Remember
- Oersted proved electricity generates magnetic fields; field lines form continuous closed loops.
- Solenoid interior has a uniform magnetic field; soft iron core creates anElectromagnet.
- Fleming’s Left-Hand Rulepowers motors;Fleming’s Right-Hand Rulegoverns induced currents.
- Earth wire protects metallic appliances by routing fault currents safely to the ground.
Check yourself
Question 1
Inside a current-carrying long solenoid, the magnetic field lines are: