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CIRCUITS
ANALYSIS
In any working circuit the current flows from the high potential (+) to the low potential (-).
The potential is designated as the voltage, V. A successful circuit will allow you to follow the
path of current through the devices.
In reality electrons travel from low potential to high (which is from high potential energy to
low potential energy if you have a negative charge), but we only recently observed that
behavior on the particle scale. Benjamin Franklin established the convention of high
potential to low, and we still use his definition.
Light Bulb
Inside a light bulb, the complete path takes you from the electrical foot, through the
filament, and then out through the threads. The entry and exit points are separated by an
insulating material (usually glass or ceramic) so that the electrons must pass through the
filament and cannot take a short cut (called a short circuit) that bypasses the filament.
Figure 1: Anatomy of a Light Bulb1
1. Glass bulb (or "envelope")
2. Low pressure inert gas
3. Tungsten filament
4. Contact wire (goes to foot)
5. Contact wire (goes to base)
6. Support wires
7. Glass mount/support
8. Base contact wire
9. Screw threads
10. Insulation
11. Electrical foot contact
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Wikipedia 2006
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Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109
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Drawing a Schematic
When a circuit is drawn, it is not common to draw a literal representation of every
component, but to instead use short-hand. The symbols for various components of a circuit
are:

Light Bulb


Resistor
Wire
Switch
Battery
A literal schematic for the flashlight you built would be:
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Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109
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Figure 2: Battery board wired like a flashlight
The short-hand schematic of your circuit is:
Figure 3: Flashlight schematic
The schematic is much simpler and includes all the essentials. The light bulb, and switch are
on the right, and the battery is on the left.
Current and Voltage Measurements
When you purchase a light bulb, the wattage is usually listed. Watts are the units of energy
used per second known as the power. Power is equal to the voltage multiplied by the
current.
P  I V
The current and voltage of a circuit are measured in the fashion shown below.
Voltage: Voltage is the electrical “push” from the battery to produce current. Voltage is
measured across the device, because we want to know the potential difference above and
below the device. This is a direct current (DC) circuit, which means current only flows in
one direction.
Use V on the multimeter for these measurements. To make a connection, simply touch the
probe to an exposed place in the circuit, like a metal prong or the teeth of an alligator lead.
Figure 4: Measuring the
VOLTAGE ACROSS the device
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Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109
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Current: Current is the electrical flow of electrons through the circuit. To measure the
current, one must make the current flow through the multimeter. An alligator clip can be
removed to do this in the lab. The units for current are Amps (A). Note that the meter must
become part of the circuit (be in series with the circuit or device) to measure current,
since the current must flow through the meter to make the measurement.
Figure 5: Measuring the
CURRENT of the circuit.
** WARNING: Do not measure current
across a device, you will make a short
circuit and blow a fuse. **
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109
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