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Lab 4: Voltage
- Using a multimeter
- Analogies to voltage and current in flowing fluids
- Voltage and current in a series circuit
10) In class, you were shown that different arrangements of a multimeter were required
to measure voltage (left) and current (right) between 2 light bulbs:
C
Using a siphon system analogy to the simple circuit shown above:
a) Name the quantities in the siphon system analogous to voltage and current.
In a siphon system, voltage is analogous to the the height difference
between water levels in the 2 containers, and current is analogous to the
flow rate of the water.
b) Name devices or methods you would use to measure either of these quantities in a
siphon system.
Height is simply measured by using a ruler to measure the distance
between water levels, while to measure flow rate you need to interrupt the
flow with a measuring container and time how long it takes a certain
volume to fill.
c) Using how these devices work in a siphon system as a reference, explain why the
multimeter must be used in different positions to measure voltage and current.
Voltage, like height in a siphon, is simply a measurement between two
reference points in the system, so the circuit (or siphon) need not be
interrupted to measure it. Current, on the other hand, like flow rate,
requires actually inserting a device inside the system to interrupt the flow;
the device must thus become part of the circuit.
11) The following circuit is built using 2 identical batteries and 3 identical light bulbs:
a) If each battery is 1.5 volts, without using or making any measurements, what can you
say about the voltage measured across each light bulb?
The voltage across each light bulb must approximately total that of the
batteries (3 volts total), and since the light bulbs are identical, there should
be about 1 volt across each bulb. (Note this neglects voltage drop in the
wires, which is very small compared to that in the bulbs.)
b) On this diagram, draw a picture showing how you would set up a multimeter to
measure the voltage across the uppermost light bulb.
c) How many more identical batteries would you need to add in series to measure a
voltage of approximately 2 volts across the uppermost light bulb?
To double the voltage across each bulb from 1 volt to 2 volts, you would
need to double the number of batteries from 2 to 4.
12) Joe reads in an online article that running a comb through one’s hair on a dry day
can generate several thousand volts of static electricity. Susan read that more than
20 volts from a wall outlet can be dangerous, but combing one’s hair is obviously
not hazardous, so she argues this can’t be correct. Joe insists it could be correct, if
one considers the source of the electricity.
a) In the analogous siphon systems, how would the fluid source differ if one considers
analogies to a wall outlet and a comb?
A wall outlet would be like a huge (essentially endless) bucket of water that
keeps on flowing - while a comb, though it can hold electricity, can’t hold
much; it would be like a tiny, tiny bucket (maybe just a thimbleful) of water.
b) Considering your answer to a), who is correct, Susan or Joe, and why?
Joe is correct. Thousands of volts of electricity is CERTAINLY dangerous if
the source contains enough energy, but a comb can’t store enough
electrical energy to be dangerous. Everyday static electricity is commonly
generated at several thousand volts - in fact, it must be very high voltage
for a spark to jump through air - but the total amount of energy stored is
usually TINY compared to a typical circuit. Large enough sources of static
electricity, though CAN indeed be dangerous
Lab 5: Voltage, Current and Power
- Relation between power, current and voltage
- Current and power in a series circuit
- Current, voltage and power in practical usage
13) For how long would an LED running at a current of 0.020 amperes from a 9-volt
battery have to run to use the same amount of energy a 60-watt light bulb uses in
60 seconds?
First, find the power of the LED:
power = current ✕ voltage = 0.020 amperes ✕ 9 volts = 0.18 watts
The light bulb uses 60 watts of power; power is the rate at which energy
is used, so the bulb uses energy 60/0.18 = 333.333 times as fast as the
LED, so the LED would have have to run 333 times as long as the light bulb
to use the same amount of energy:
time = 60 seconds ✕ 333.333 = 20000 seconds. (This is about 5½ hours.)
14) A typical lightning bolt is generated by 100,000,000 volts and carries 10,000
amperes of current. What is the power carried by a lightning bolt? How does this
compare to the power generated at Hoover Dam? If Hoover Dam is used as a largescale source of energy, does this mean a lightning bolt could be used as a similar
type of energy source, too? Why or why not?
power = current ✕ voltage = 100,000,000 volts ✕ 10,000 amperes
= 1,000,000,000,000 watts
Hoover Dam generates 2,000,000,000 watts (info from #14 in Lab 5), so a
lightning bolt generates about 500 times the power of Hoover Dam.
However, a lightning bolt only lasts an instant, so the the total energy
delivered over a long period of time is nowhere near what Hoover Dam
supplies, so it would not make for a feasible source of long-term energy.
15) A small plug-in transformer changes the voltage going into your computer from 120
volts (the wall outlet) to 15 volts (for the computer). If the power is the same going in
and out of the transformer, and 1.5 amperes flows in from the wall socket, what
current is delivered to the computer?
power out of wall = current ✕ voltage = 120 volts ✕ 1.5 amperes = 180 watts
Since the power in = power out = 180 watts,
current = power / voltage = 180 watts / 15 volts = 12 amperes delivered