Manual

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40. Kirchhoff’s First Law
Build the circuit, press and hold the press switch (61), you will hear
the alarm (78) sound, while the lamp (76) will light dimly. When you
release the press switch (61), the alarm (78) will stop. The lamp is
used as a wire block in this circuit and will light dimly. 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 A1 represents a node. If a positive current
is coming into a node and a negative current is leaving a node, then:
I
alarm
+ I
lamp&press switch
= 0 or I
alarm
= –I
lamp&press switch
This shows that the current through the alarm and the lamp are
the same in this series circuit, but one is owing into node A1 and
one is owing out of node A1.
41. Kirchhoff’s Second Law
Build the circuit, move the magnet (7) towards the reed switch (83), the alarm
(78) will start while the lamp (76) will light dimly. When you move the magnet
(7) away the alarm (78) will stop. Kirchhoff’s second law states: The sum of
the voltages around a closed network is zero. If a drop in voltage is considered
as a negative voltage and a rise in voltage a positive voltage, then the following
equation is a mathematical representation of Kirchhoff’s second law:
V
D1
4
A1
+ V
A1
4
A4
+ V
A4
4
D4
+ V
D4
4
A4
= 0
This shows that the voltage drop across the battery module (91) must
equal the voltage drop across the alarm (78) plus the voltage drop
across the lamp (76) plus the voltage drop across the reed switch (83).