PASCO SE-8657 MOTOR ACCESSORY - Manual (Page 20)

16
Motor Accessory 012-06247A
¨
➅ Adjust the gap of the Variable Gap Magnet so that there is approximately 1 mm of clearance
between the pole pieces and the armature when it is rotated by hand.
➆ Position the armature so that it is at right angles to the N-S orientation of the Variable Gap
Magnet. Then rotate it by hand 90 degrees until the end of the armature marked with tape is
near the north pole of the magnet. The magnetic field of the magnet may be envisioned as
arrows passing out of the north pole piece and into the south pole piece.
(a) What happens to the amount of this magnetic field that passes through the loops of the
coils during your 90-degree rotation above? If the amount changed, did it increase or
decrease?
(b) What does Faraday’s induction law say about this situation?
➇
Continue rotating the armature another 90 degrees.
(a) What happens to the amount of this magnetic field that passes through the loops of the coils
during your 90-degree rotation above? If the amount changed, did it increase or decrease?
(b) What does Faraday’s induction law say about this situation?
(c) How would the induced emf be different during the rotation of step 7, compared to step 8?
➈
The forces due to Lenz’s’ law in this equipment are much less than other effects and are not
readily noticeable. Nonetheless, the reasoning involving Lenz’s law allows you to predict the
direction of current. Consider the 180 degree rotation you performed above:
(a) To oppose the motion during the first 90 degrees of rotation, what pole (N or S) would the
taped end of the armature need to be?
(b) To oppose the motion during the second 90 degrees of rotation, what pole (N or S) would the
taped end of the armature need to be?
➉
In order to cause the armature to act as you stated in step 9 above, what direction would the
induced current need to move?
➤ To answer this, you will need the “right hand rule”, which can be used to predict the
direction of the magnetic field of a coil. Grasp the coil with the fingers wrapped around
the coil in the direction of the current. The thumb will point in the direction of the field.
(i.e., toward the north pole of the coil.) Current direction here is described as being from
the positive to the negative (conventional current). Note that this is opposite of the
direction of electron movement.
(a) Must conventional current enter the coil, or leave the coil, from the upper brush, in order
to make the armature act as you described in 9 (a) above?