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C H A P T E R 20
Electric Circuits
20.10 Kirchhoff's Rules
There are two KIRCHHOFF'S RULES.
1. Junction rule
2. Loop rule
These are useful in circuit analysis.
Kirchhoff's Rules
Junction rule. The sum of the magnitudes of the currents
directed into a junction equals the sum of the magnitudes of the
currents directed out of the junction.
Loop rule. Around any closed circuit loop, the sum of the
potential drops equals the sum of the potential rises.
Junction Rule
Junction rule. The sum of the
magnitudes of the currents directed
into a junction equals the sum of
the magnitudes of the currents
directed out of the junction.
Application of Junction Rule
Q: A galvanometer with a fullscale limit of 0.100 mA is to be
used to measure a current of
60.0 mA. How much current
will pass through the shunt
resistance R?
A: 60.0 – 0.1 = 59.9 mA
Loop Rule
Loop rule. Around any closed circuit loop, the sum of the
potential drops equals the sum of the potential rises.
Application of Loop Rule
The circuit shown below contains two batteries and two
resistors. Determine the current I in the circuit.
Current Measurement
Voltage Measurement
Capacitors in Parallel
Capacitors in Series
RC CIRCUITS
Charging
RC Circuits
Discharging
The physics of heart pacemakers
Heart pacemakers, for instance, incorporate RC circuits to control
the timing of voltage pulses that are delivered to a malfunctioning
heart to regulate its beating cycle.
The physics of safe electrical
grounding
The physics of the physiological
effects of current
Serious and sometimes fatal injuries can result from electrical
shock. The severity of the injury depends on the magnitude of
the current and the parts of the body through which the moving
charges pass. The amount of current that causes a mild tingling
sensation is about 0.001 A. Currents on the order of 0.01−0.02 A
can lead to muscle spasms, in which a person “can’t let go” of
the object causing the shock. Currents of approximately 0.2 A are
potentially fatal because they can make the heart fibrillate, or
beat in an uncontrolled manner. Substantially larger currents stop
the heart completely. However, since the heart often begins
beating normally again after the current ceases, the larger
currents can be less dangerous than the smaller currents that
cause fibrillation.