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Physics 53 Lynn Fall 2005

DC CIRCUITS EXPERIMENT:  Physics Notes

DC or direct current circuits are ones in which the currents and
voltages at various points in the circuit are constant as a function
of time.  Circuits constructed from resistors and constant voltage
(or current) sources are DC circuits.  By contrast, circuits involving
capacitance and inductance, certain active components, and time-varying
voltage or current sources are definitely not DC circuits.  DC circuits are
simpler to analyze.  However, an electric generator produces an alternating
current and thus many familiar daily examples of electricity are AC
rather than DC in nature.

Voltage or electrical potential difference is measured in Volts. 
Voltage drops (or gains) are potential differences between two points
in a circuit.  For example, we can measure the potential difference
between a point in the circuit on one side of a particular component
and a point on the other side of the component; in this case we say
the voltage is measured across that component.  Voltage measurements
are always made with the voltmeter in parallel with the component in
question.

Here the voltmeter is set
up to measure the potential difference between points and , which we can also refer to as the voltage across R<sub>2.

.

.

V

Current, or charge flow per unit time, is measured in Amperes. 
Current flows through a component in a circuit, and current measurements
are made with the ammeter in series with the component in question.

Here the ammeter is set
up to measure the current through R<sub>2.

A

Kirchhoffs Laws are a great deal for Gustav Kirchhoff.  Generations
of physics students have learned his name, and for what?  Kirchhoffs
loop rule states that the sum of voltage changes around any closed loop
in a circuit must equal zero.  This rule applies to DC circuits
and to the instantaneous voltages changes in an AC circuit.  In essence
it is merely a special case of conservation of energy, and of what we
mean by defining the concept of a potential in any case.  Kirchhoffs
junction rule states that the sum of currents into any junction must
equal the sum of currents out of that junction.  This rule is even
simpler it is a statement of conservation of charge!  Charge
cant be created or vanish into nothingness, so whatever charge/time
flows in must also flow out if the circuit is in a steady state.

Ammeters are designed to have low resistance so as not to stop up
the flow of current they are attempting to measure.  HOWEVER, low
is a relative term.  In Experiment 1 several lab groups measured
that their ammeter had an effective resistance of about 10 Ohms. 
This is indeed low compared to the resistances used in most real-life
circuits.

Resistors come with values quoted in Ohms.  Be careful, though! 
Most resistors are only quality controlled to +/- 5% of these values. 
It is generally possible to find precision resistors with a tolerance
of only +/- 1%, but that is about as good as it gets.  The moral
of the story?  Dont design a circuit that will only work with
a 500.00-Ohm resistor, unless you want to buy five bags of a hundred
and test them all until you find the right one for you.