When electric current flows through a straight conductor, it produces concentric circular magnetic field lines around the wire
Activity 12.4: Compass and Straight Wire
Place a compass needle parallel to a current-carrying copper wire. When current flows, the compass needle deflects, proving the presence of a magnetic field.
- Current from north to south โ needle moves east
- Current from south to north โ needle moves west
- Field direction reverses with current direction
Activity 12.5: Iron Filings Pattern
Sprinkle iron filings around a vertical current-carrying wire to visualize the magnetic field pattern:
- Iron filings form concentric circles
- Circles represent magnetic field lines
- Field strength decreases with distance
- Higher current creates stronger field
Key Observations
๐งญ Direction Dependence
The magnetic field direction depends on the current direction through the conductor.
๐ Distance Effect
Magnetic field strength decreases as distance from the conductor increases.
โก Current Magnitude
Stronger current produces a stronger magnetic field at any given point.
A convenient method to determine the direction of magnetic field around a current-carrying conductor
How to Apply the Rule
Imagine holding a current-carrying straight conductor in your right hand:
- Thumb Direction: Point thumb in direction of current flow
- Finger Direction: Your fingers curl in direction of magnetic field lines
- Field Pattern: Concentric circles around the conductor
Maxwell's Corkscrew Rule
Alternative method: If you drive a corkscrew in the direction of current, the direction of rotation gives the magnetic field direction.
๐ Example 12.1 Solution
Problem: Current flows east to west through horizontal power line.
Solution: Using right-hand thumb rule:
- Field turns clockwise when viewed from east
- Field turns anti-clockwise when viewed from west
- Point below wire: field points south
- Point above wire: field points north
When a straight wire is bent into a circular loop, the magnetic field pattern changes significantly
Activity 12.6: Circular Coil Field
Insert a circular coil through cardboard and observe iron filing patterns:
- Field lines form complex curved patterns
- Field is concentrated at the center
- Multiple turns increase field strength
Field Characteristics
๐ฏ Central Field
At the center of the loop, field lines appear straight and perpendicular to the loop plane.
๐ Cumulative Effect
Each segment of the loop contributes field in the same direction at the center.
๐ Multiple Turns
For n turns, field strength is n times that of a single turn.
Mathematical Relationship
For a circular loop with n turns carrying current I:
B โ n ร I
The field at the center is directly proportional to both the number of turns and the current.
A solenoid is a coil of many circular turns wrapped in a cylindrical shape, creating a strong uniform magnetic field
What is a Solenoid?
A solenoid consists of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder. It behaves like a bar magnet when current flows through it.
Magnetic Field Pattern
๐งฒ Similar to Bar Magnet
The field pattern around a solenoid resembles that of a bar magnet.
๐ฏ Uniform Internal Field
Inside the solenoid, field lines are parallel and uniform.
๐ Magnetic Poles
One end acts as north pole, the other as south pole.
Electromagnet Formation
When a soft iron core is placed inside the solenoid, it becomes magnetized and forms an electromagnet. This greatly increases the magnetic field strength.
- Soft iron core concentrates field lines
- Temporary magnetization only when current flows
- Field strength can be controlled by current
- Used in electric bells, motors, and cranes
Understanding the mathematical relationships in electromagnetic phenomena
Key Relationships
| Configuration | Field Formula | Key Factor | Applications |
|---|---|---|---|
| Straight Wire | B โ I/r | Distance (r) | Power lines |
| Circular Loop | B โ nI | Number of turns (n) | Coils |
| Solenoid | B = ฮผโnI | Turn density (n/l) | Electromagnets |
Factors Affecting Field Strength
โก Current (I)
Doubling current doubles magnetic field strength
๐ Number of Turns (n)
More turns create stronger, more concentrated fields
๐ Distance (r)
Field strength decreases with distance from conductor
๐งฒ Core Material
Iron core can increase field strength by 1000x