Data Sheet

-62-
94. Measuring Voltage
Build the circuit to the left, turn on the switch (62) and you will see the
two LEDs are turned on. Now press the press switch (61) and touch
the reed switch (83) with the magnet (7) and you will see the star LED
(70) goes off, but the heart LED (69) will be brighter.
A Voltmeter is used to measure voltage. Since the LEDs in this circuit
are in series, they each don’t see the full 4.5V from the battery (91).
If you used a voltmeter you would see that when the switch (62) is on
and the press switch (61) is off, then the voltage across the heart LED
(69) is about 1.9V and the voltage across the star LED (70) is about
2.6V. Note that these voltages sum to 4.5V (Kirchhoff’s Voltage Law
in action!).
95. Internal Resistance of the LED Modules
Build the circuit shown on the left, turn on the switch (62), and you will
see the two LEDs are turned on at the same time. Now press the press
switch (61) or touch the reed switch (83) with the magnet (7) and the
star LED (70) will go off, but the heart LED (69) will be brighter.
In project #91 it was discussed that white light requires higher voltage
to turn on the internal LED than red light. Thus, less voltage remains
across the internal resistor in the star LED (70) than in the heart LED
(69). Based on Ohm’s law (I=V/R), in order to get the same current
through the star LED (70) as the heart LED (69), the internal resistor
in the star LED (70) must be smaller than in the heart LED (69). This
is exactly what we saw in project #91 and is why the star LED (70)
module is designed with a smaller internal resistor than the heart LED
(69) module.
2nd level
1st level
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