PRS17 Parallel Wires Consider two parallel current carrying wires. I1 I2 With the currents running in the same direction, the wires are 1. attracted (likes attract?) 2. repelled (likes repel?) 3. pushed in another direction 4. not pushed – no net forcePRS17 Parallel Wires X (1) Wires attracted I1 I2 I1 creates a field into the page at I2. That makes a force to the left. I2 creates a field out of the page at I1. That makes a force to the right.PRS17 Dipole in Field µ The coil above will: 1. rotate clockwise, not move 2. rotate countercw, not move 3. move to the right, no rotation 4. move to the left, no rotation 5. move in another direction, without rotation 6. move and rotate 7. no net force so no rotation or motionPRS17 Dipole in Field (1) Coil will rotate clockwise No net force so no center of mass motion. BUT Magnetic dipoles rotate to align with external field!PRS17 Dipole in Field The current carrying coil above will move 1. upwards 2. downwards 3. stay where it is because the total force is zeroPRS17 Dipole in Field (2) Coil will move down The I ds x B forces shown produce a net downward forcePRS17 Dipole in Field The current-carrying coil above will move 1. upwards 2. downwards 3. stay where it is because the total force is zeroPRS17 Dipole in Field µ S N S N (2) Coil will move down EITHER: The I ds x B forces shown produce a net downward force OR: Think about magnets…PRS17 Free dipoles attract because: 1. The force between dipoles is always attractive independent of orientation. 2. A dipole will always move towards stronger field, independent of orientation. 3. The torque on the dipole aligns it with the local field and the dipole will then move toward stronger field strength.PRS17 (3) Free dipoles attract because the torque on a dipole aligns the dipole with the local field and the dipole then moves toward stronger field strength—that is closer to another dipole. If the dipole were anti-aligned with the local field then it would move toward regions of weaker field
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