Magnetic Field Lines
Patterns Around Magnets
Magnetic field lines visually represent the direction and strength of a magnetic field.
ExampleMagnetic field lines always point from the north pole to the south pole outside a magnet, and from south to north inside the magnet, forming closed loops.
- Consider a bar magnet.
- If you sprinkle iron filings around it, the filings align along the magnetic field lines, revealing a pattern that is densest near the poles (where the field is strongest) and spreads out as you move away.
Patterns Around Wires
- A straight current-carrying wire generates a magnetic field with lines forming concentric circles around the wire.
- The right-hand rule helps determine the direction:
- Point your thumb in the direction of the current.
- Your fingers will curl in the direction of the magnetic field lines.
If the current flows upward, the magnetic field lines will circle the wire in a counterclockwise direction.
Patterns Around Solenoids
- A solenoid is a coil of wire that produces a magnetic field similar to a bar magnet when current flows through it.
- Inside the solenoid, the field lines are parallel and uniform, indicating a strong and constant magnetic field.
- Outside, the lines resemble those of a bar magnet, exiting from one end (the north pole) and entering the other (the south pole).
To find the direction of the magnetic field in a solenoid, use the right-hand grip rule, curl your fingers in the direction of the current, and your thumb will point toward the solenoid’s north pole.
Force on a Moving Charge
- When a charged particle moves through a magnetic field, it experiences a force called the magnetic force.
- The magnitude of this force is given by the formula: $$F = qvB \sin \theta$$ where:
- $F$ is the magnetic force.
- $q$ is the charge of the particle.
- $v$ is the velocity of the particle.
- $B$ is the magnetic flux density (strength of the magnetic field).
- $\theta$ is the angle between the velocity vector and the magnetic field vector.
