Data Sheet
-23-
20. Parts Connected in Series
Build the circuit, turn on the switch (62) and you will light
up two LEDs at the same time. Notice that both LED
components are dim. This is because they are in series in
this circuit and each LED has an internal resistance that is
adding up together to limit the current through each LED.
21. Kirchhoff’s First Law
Replace the switch (62) with the press switch (61), then press
the press switch (61) and you will light up the two LEDs at the
same time. Release the press switch (61) and they will both
turn off. Kirchhoff’s rst law states: At any node (junction)
in an electrical circuit, the sum of currents owing into that
node is equal to the sum of currents owing out of that node.
Location D1 represents a node. If a positive current is coming
into a node and a negative current is leaving a node, and we
use the letter I to represent current, then:
I
heart
+ I
star
= 0
This shows that the current coming into node D1 from
the heart LED (69) (positive current) is the same as
the current leaving node D1 towards the star LED (70)
(negative current).
22. Reed Switch vs. Mechanical Switch
Replace the switch (62) with the reed switch (83), then
touch the reed switch (83) with the magnet (7) and you
will light up the two LEDs at the same time. Move away the
magnet (7) and they will both turn off. One of the benets
of reed switches over mechanical switches like the press
switch (61) is reliability/lifetime. Mechanical switches can
wear out more quickly as you use them, and some studies
show that reed switches can be used 10,000 times more
often than mechanical switches before they wear out.