PASCO MG-8600 Lenz's Law Demonstrator - Manual (Page 2)

magnet is such that it repels the falling magnet, again,
slowing its motion. Therefore, Lenz's Law predicts that the
motion of the magnet will be slowed due to the induced
fields. Experiment confirms this prediction.
Lenz's Law provides a simple way to determine the direc-
tions of induced currents and magnetic fields. However, the
currents and fields can also be determined using more basic
laws of electromagnetics.
Extending Lenz's Law
You can extend the length of your tube in 1.5 meter incre-
ments with the PASCO 1.5 Meter Extension Tube with
Coupler (Model MG-8601). The extension tube and coupler
attach to the Lenz's Law Tube as shown in Figure 3.
.
Figure 3 Attaching the Extension Tube
Extension tube
Coupler
Lenz's law tube
The direction of the induced currents in the tube can be
determined using Faraday's law of induction,
ε
= - dØ/dt;
where dØ/dt is the rate of change of the magnetic flux
through a selected cross section of the tube, and
ε
is the
induced emf around that cross section. The right hand rule
can be used to determine the direction of the current. If the
magnetic flux through the tube is increasing, point your
thumb in the direction opposite the magnetic field (because
of the minus sign). If the magnetic flux through the tube is
decreasing, point your thumb in the direction of the magnetic
field. In each case, your fingers will curl in the direction that
the current flows.
At cross section A, the magnetic field from the magnet is
pointing down, but the field, and therefore the flux, is
decreasing as the magnet descends. The current therefore
flows clockwise (looking down from the top) in that cross
section. At cross section B, the flux due to the field is
constant, because the field near the middle of the magnet is
constant. There are therefore no induced currents in cross
section B. In cross section C, the field points down and the
flux is increasing as the magnet descends. The current
therefore flows counterclockwise.
The circulating currents at cross sections A and C produce
magnetic fields. The magnitude and directions of these fields
can be determined using Ampere's Law ( B dl = µ
0
I) and a
different right hand rule. Curl the fingers of your right hand
in the direction of current flow. Your thumb will point in the
direction of the magnetic field. Using this rule, you can show
that the predicted directions of the induced fields are the
same as predicted by Lenz's Law.
Figure 2 Diagram of the Falling Magnet
Velocity of magnet
B
A
C
S
N
N
S
I
induced
I
induced
S
N
(b) (a)
Limited Warranty
PASCO scientific warrants this product to be free from
defects in materials and workmanship for a period of one
year from the date of shipment to the customer. PASCO
will repair or replace, at its option, any part of the product
which is deemed to be defective in material or workman-
ship. This warranty does not cover damage to the product
caused by abuse or improper use. Determination of whether
a product failure is the result of a manufacturing defect or
improper use by the customer shall be made solely by
PASCO scientific. Responsibility for the return of equip-
ment for warranty repair belongs to the customer. Equip-
ment must be properly packed to prevent damage and
shipped postage or freight prepaid. (Damage caused by
improper packing of the equipment for return shipment will
not be covered by the warranty.) Shipping costs for
returning the equipment, after repair, will be paid by
PASCO scientific.
Equipment Return
Should this product have to be returned to PASCO scientific,
for whatever reason, notify PASCO scientific by letter or
phone BEFORE returning the product. Upon notification,
the return authorization and shipping instructions will be
promptly issued.
NOTE: NO EQUIPMENT WILL BE ACCEPTED
FOR RETURN WITHOUT AN AUTHORIZATION.
When returning equipment for repair, the units must be
packed properly. Carriers will not accept responsibility for
damage caused by improper packing.
To be certain the unit will not be damaged in shipment,
observe the following rules:
1.
The carton must be strong enough for the item shipped.
2. There should be at least two inches of packing material
between any point on the apparatus and the inside of the
carton.
3. Make certain that the packing material can not shift in
the box, or become compressed, thus letting the
instrument come in contact with the edge of the box.