When an electric current flows through a wire, it creates a magnetic field around it. Think of the wire as a magnetic “tornado” that spins whenever electrons move.
Two nearby wires each carrying a current produce magnetic fields that influence each other. The magnetic field from wire 1 exerts a force on the moving charges in wire 2, and vice versa. The result is a force between the wires themselves.
Mathematically, the force per unit length on a straight conductor in a magnetic field is:
\$\vec{F} = I\,\vec{L}\times\vec{B}\$
where \$I\$ is the current, \$\vec{L}\$ is the length vector of the conductor, and \$\vec{B}\$ is the magnetic field.
Use the right‑hand rule to find the direction of the magnetic field and the force:
For two parallel wires:
Example: Two wires side by side, both carrying current up. The magnetic field from wire 1 circles wire 2 clockwise. Using the right‑hand rule, the force on wire 2 is downward, pulling it toward wire 1.
| Wire 1 Current | Wire 2 Current | Force Direction | Result |
|---|---|---|---|
| ↑ | ↑ | ↓ (toward each other) | Attractive |
| ↑ | ↓ | ↑ (away from each other) | Repulsive |
Imagine each wire as a magnetic rope that pulls on the other. If both ropes are pulled in the same direction, they tug together (attraction). If they’re pulled in opposite directions, they pull apart (repulsion).
Just like a real tug‑of‑war, the strength of the pull depends on how hard you pull (current magnitude) and how close the ropes are (distance between wires).
Tip 1: Always sketch the wires and use the right‑hand rule step by step. A clear diagram often earns full marks.
Tip 2: Remember the key rule: Same direction → attraction, opposite → repulsion. Write this in your answer to show understanding.
Tip 3: For forces on a segment of wire, use \$F = I L B \sin\theta\$ and specify the angle \$\theta\$ between \$\vec{L}\$ and \$\vec{B}\$.
Good luck, future physicists! 🚀