Force on a current-carrying conductor：練習題

A wire of length \(L\) carrying a current \(I\) is placed at right angles to a magnetic field of strength \(B\), experiencing a force \(F\). If the current is doubled and the magnetic field strength is halved, what is the new force acting on the wire?

In a simple d.c. motor, what is the primary purpose of the split-ring commutator?

A current-carrying conductor is placed in a uniform magnetic field. Which of the following sets of changes will definitely result in the magnetic force on the conductor remaining in the same direction?

The "catapult effect" describes the force on a wire resulting from the interaction of magnetic fields. If a wire carrying current out of the page is placed in a uniform magnetic field pointing from left to right, where will the magnetic flux density be highest?

A straight horizontal wire is placed in a uniform magnetic field that acts vertically downwards. If a conventional current flows through the wire from East to West, in which direction will the magnetic force act on the wire?

A horizontal wire carries a conventional current directed towards the right of the page. If a uniform magnetic field is applied into the page, state the direction of the magnetic force acting on the wire.

A rectangular current-carrying coil is placed in a uniform magnetic field. If the current through the coil is reversed and its magnitude is doubled, describe the resulting change in the magnetic force acting on one of the sides of the coil perpendicular to the field.

Two long parallel wires carry conventional currents in opposite directions. Using the concepts of magnetic fields and Fleming's Left-Hand Rule, explain whether the wires will attract or repel each other.

A straight horizontal copper wire is placed in the gap between the North and South poles of a strong U-shaped magnet. The wire is connected to a circuit consisting of a d.c. power supply and a rheostat.

(a) When the switch is closed, the wire is observed to move upwards. Explain, in terms of magnetic fields, why a force is produced on the wire. [2]

(b) State the direction of the magnetic field provided by the U-shaped magnet. [1]

(c) Using Fleming's left-hand rule, deduce the direction of the conventional current flow in the wire that causes the upward motion. [1]

(d) The resistance of the rheostat is increased. State and explain the effect this has on the magnitude of the force acting on the wire. [2]

A rectangular coil \( ABCD \) is mounted on an axle and placed horizontally between the poles of a permanent magnet. The coil is connected to a d.c. motor assembly including a split-ring commutator. Side \( AB \) is adjacent to the North pole and side \( CD \) is adjacent to the South pole.

(a) A current flows from \( A \) to \( B \) and \( C \) to \( D \). Describe the direction of the forces acting on sides \( AB \) and \( CD \) and explain how these forces produce a turning effect. [3]

(b) As the coil reaches the vertical position, the brushes lose contact with the split-ring commutator.
(i) State what happens to the current in the coil at this instant.
(ii) Explain why the coil continues to rotate past this position. [2]

(c) Describe the function of the split-ring commutator in maintaining continuous rotation in one direction. [2]

(d) The d.c. supply is replaced with a low-frequency a.c. supply. Describe and explain the resulting motion of the coil. [2]