1A. (I) to Prevent Short-Circuit from Occurring

Answer to Electromagnetism 3

1a. (i) To prevent short-circuit from occurring. (ii) There will be no electrical connection and the motor will not start when the carbon brushes align exactly in the gap between the two halves of the split-ring commutator. No current flows through the coil; hence no force will be exerted on the two sides of the coil to make it rotates.

1b (i) Downward (ii) When the magnetic field from the current-carrying coil interacts with the external magnetic field from the magnets to give rise to a resultant magnetic field which has region of stronger field and region of weaker field, this causes a resultant force to be exerted on the side of the coil. 1c Increase current flow in the coil Increase no of turns of wire for the coil Wind the turns of wire around a piece of soft iron

2(a) Right

2(b) Greater force to the right, paper cone move further right.

2(c) No. A steady D.C. supply constant current to the coil, this will produces a constant force to the right, hence the coil and cone move to the right and stay there. The air layers will not be vibrated, hence no sound will be produced.

2(d) When a .c. flows through the coil, the changing magnetic field around the coil interacts with the magnetic field of the magnet to give rises to varying forces acting on the coil, thus make it vibrate in accordance with the frequency and magnitude of the current. The coil oscillates in the magnetic field, causing the cone to vibrate. The air surrounding the cone is set into vibration and sound waves are produced.