Data Sheet

-45-
59. Internal Resistance of Heart LED
Build the circuit shown on the left and put a 4-wire (4) across the points
A and B. If you have a voltmeter, you can measure the voltage across the
heart LED (69) and 100W resistor (41) (W is the symbol representing
Ohms) and see that about 3.3V is across the heart LED (69) and only
about 1.2V is across the 100W resistor (41). The heart LED (69) module
consists of an actual LED and a resistor in series to protect the actual
LED from ever seeing too much current that could burn it out.
Actual LEDs have very little resistance but do require a certain “turn on”
voltage to light, which is color dependent. Red light is one of the easier
colors to light and requires only about 1.8V to turn on the LED. So this
means that the internal resistor will see about 1.5V of the 3.3V across
the heart LED (69). Since the heart LED (69) and 100W resistor (41) are
in series, the same current is running through each. Since the internal
resistance of the heart LED (69) and the 100W resistor (41) are both
seeing the same current, and we calculated they both are seeing about
1.2-1.5V across each of them, Ohm’s law (R = V/I) tells us that the
internal resistance of the heart LED (69) must be very close to 100W.
60. Revisiting Fleming’s Left Hand Rule
Build the circuit shown on the left, touch the reed switch (83) with the
magnet (7) and you will see the motor (95) spin and the lamp (76) is
turned on. Move away the magnet (7), then turn on the switch (62). The
lamp (76) will be on again and if you press the press switch (61) now the
motor (95) will spin in the opposite direction.
Fleming’s left hand rule was mentioned in project #46. The rule states:
When current ows through a conducting wire, and an external magnetic
eld is applied across that ow, the conducting wire experiences a force
perpendicular both to that eld and to the direction of the current ow (i.e.
they are mutually perpendicular). You can use your left hand to implement
this rule by pointing your index and middle ngers perpendicular to each
other and pointing your thumb up in the air. If your index nger points in
the direction of the magnetic eld (internal to the motor) and your middle
nger points in the direction of the current, then your thumb will point in
the direction of the force. You can see that if the current is reversed (like
in this project), then the force is in the opposite direction which is why
the motor spins in the opposite direction.
WARNING: Moving parts. Do not
touch the motor during operation. Do
not lean over the motor.
!