physics notes on DC circuits lab - HMC Physics

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Oct 7, 2013 (4 years and 1 month ago)

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Physics 53 – Lynn – Fall 2006
DC CIRCUITS EXPERIMENT: Physics Notes

Read Sections 1.1-1.3 of the Physics 53 Lab Manual. The following notes supplement those sections but are
not a substitute for them.

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, electric generators and motors involve 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, consider a single component (a resistor, a
battery, a meter, etc.) in a circuit. We can measure the potential difference between one side of the
component and the other side of the component; in this case we say the voltage is measured across that
component. By contrast, there is no such thing as the “voltage through the component.” Voltage
measurements are always made with the voltmeter in parallel with the component in question.


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.


Kirchhoff’s Laws were a great deal for Gustav Kirchhoff. Generations of physics students have learned
his name, and for what, exactly? Kirchhoff’s 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 the first place. Kirchhoff’s 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 can’t be created or vanish into
nothingness, so whatever charge per 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 you may find that your ammeter (or
some other nominally non-resistive component) has an effective resistance that is large enough to be seen
in your results. Even so, it will certainly be “low” compared to the resistances used in most real-life
circuits; these tend to be well into the hundreds of Ohms, if not kilo-Ohms and higher.

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? Don’t 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.
V
α

β

.

.

Here the voltmeter is set up to measure
the potential difference between points α and β, which we can also refer to as
the voltage across R
2
.
A
Here the ammeter is set up to measure
the current through R
2
.