Project 17
Parallel Circuit - Voltage
Build the circuit, set the meter (M5) to the 5V setting, but leave the slide
switch (S1) off. The meter measures the voltage on the batteries. The
lamps (L4) will be off, because the switch is off.
Now turn on the slide switch to light the lamps, and see if the voltage
changes.
How would the voltage change if you replaced your batteries with ones
that are weaker or stronger? (Try it if you have different batteries avail-
able.)
How would the voltage change if you left the switch (and lamps) on for
a long time?
How would the voltage change if you removed one or two lamps? (Try
it.)
How would the voltage change if you replaced one of the lamps with
the white LED (D6)? (Try it.)
Batteries produce electricity using a chemical reaction, but they have a
limited supply of the chemicals, and not all of
them can react at once. If the batteries can-
not produce as much electricity as a circuit
wants, the voltage drops.
Some batteries, called rechargeable batteries (such as the batteries in
your cell phone), allow you to reverse the chemical reaction using an-
other electric source.
5V
Connecting parts in parallel is another way of arranging them in a circuit.
The advantage of it is that if one burns out, the others will still work (re-
move one of the lamps to prove this). The disadvantage is that wiring
the parts together is more complex than with series circuits.
All large circuits are made of combinations of series and parallel circuits.
In this circuit the lamps are the resistances which are limiting the flow of
electricity. Placing resistances in parallel decreases the total resistance.
Advanced users can compute the total resistance as follows:
The voltage is the same across all the resistances in a parallel circuit.
Ohm’s Law says that Voltage equals Current times Resistance, so the
lowest resistances in a parallel circuit will have the most current through
them. Equal resistances will have the same current. In other words:
Current
(through one branch)
=
Resistance
(total in all OTHER parallel branches)
Resistance
(total of resistors in all branches)
x Current
(total applied to the parallel circuit)
1 1 1 1
= + + + . . . .
R
parallel
R1 R2 R3
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