4.5.3 Magnetic Effect of a Current
Magnetic Field Around a Current-Carrying Wire
When an electric current passes through a wire, a magnetic field is set up around it. The field lines form concentric circles around the wire.
- The direction of the magnetic field depends on the direction of the current.
- Reversing the current reverses the direction of the field lines.
- Strength of the field decreases as distance from the wire increases.
Ampere’s Right-Hand Grip Rule
- Point the thumb of your right hand in the direction of the conventional current.
- Then, the way your fingers curl shows the direction of the magnetic field lines around the wire.
- Also called the Right-Hand Screw Rule, Coffee-Mug Rule, or Corkscrew Rule.
Magnetic Field of a Solenoid
A solenoid carrying current produces a magnetic field similar to that of a bar magnet.
- Field is stronger inside the solenoid and weaker outside.
- Field lines are nearly parallel and evenly spaced inside → uniform field.
- Field lines spread outward at the ends; the end where lines emerge is the north pole, and where they enter is the south pole.
- Strength of the magnetic field decreases with distance from the solenoid.
Increasing the Strength of the Magnetic Field
- Increase the current through the coil.
- Increase the number of turns in the solenoid.
- Insert a soft iron core inside the solenoid.
Direction of the Field in a Solenoid
Use the Right-Hand Rule — curl fingers around the solenoid in the direction of the current through the windings; the thumb points towards the solenoid’s north pole.
Applications of the Magnetic Effect of Current
- Used in electric motors.
- Used to create electromagnets for lifting scrap metal or in relays.
- Used in cathode ray tubes and magnetic resonance (MRI) scanners.
- Used in instruments where magnetic deflection is needed (e.g., galvanometers).
Summary of Key Points
| Concept | Explanation |
|---|---|
| Magnetic field around wire | Concentric circles around the wire; direction given by right-hand grip rule. |
| Magnetic field in solenoid | Similar to bar magnet; uniform inside, weak outside. |
| Increase field strength | Increase current, turns, or add soft iron core. |
| Poles of solenoid | Side where lines emerge → North; where lines enter → South. |
Exam Pointers
- Use “Ampere’s Right-Hand Grip Rule” to describe field direction around a wire.
- State that solenoid fields are uniform inside and like a bar magnet’s field.
- Remember: reversing the current reverses the field direction and pole positions.
- Be able to name applications such as relays, electromagnets, and MRI scanners.
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