Voltage
The "push" that makes a current go.
Materials
4 batteries (2 types) + holders, 5 light bulbs + holders, wires
light emitting diode (LED), buzzer, and motor
Multimeter (red box with probes and wires)
Discussion:
> Here are two more ways to connect the batteries, the motor, and the switch.
one of them is a bad idea altogether. What do they do?
These are not good for much; in fact,
D:
E:
> Which of these 2 circuits is a bad idea altogether, and why?
Definition
Voltage is what pushes the current through the wires. It is the electrical equivalent of pressure, or
the height difference in a siphon system. It is measured in volts. When batteries are placed in
series, the voltages add, so that two 1.5 V batteries make a 3 V device.
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What to do
1. There are lots of ways to put several batteries and several light bulbs in series. The two
diagrams just hint at the many possibilities. Using up to 4 batteries and 5 light bulbs, make circuits
of this type. Your goal is to find a rule (or rules) that describes the brightness of the bulbs in any
series combination.
Draw circuit diagrams for 2 of the circuits you tested (that are different from A and B).
2. Give the rule(s) you found.
Definition: A voltmeter is a device for measuring the voltage between two points in a circuit (it is
similar to measuring the height difference between two parts of a water circuit). The device we
have is a "multimeter": it can measure many different things. The multimeters labeled “V” have
already been set up for measuring voltage. Slide the switch to "On" when you want to use the
voltmeter (turn it off when you are not going to use it for a while). You connect the voltmeter wires
to two different places in the circuit (without disconnecting anything); the reading tells the voltage
difference between the two points.
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(To set up the voltmeter, turn the rotating switch on the multimeter so that it points to "20" in the "DCV" sector. This
turns it into a voltmeter capable of measuring up to 20 V. The black wire should be plugged into COM, and the red
wire is plugged into the socket labeled "V
mA.").
3. Use the voltmeter to measure (and record) the voltage of a battery, by
connecting the red and black wires to the two ends of the battery.
There are two kinds of batteries in your kit: an alkaline battery (heavy!)
and a “nickel-metal hydride” battery (light!). Measure (and record) the
voltage of each kind.
4. Measure the voltage between the positive and negative terminals of the
battery while it is lighting a light bulb.
Measure the voltage between the two ends of a wire in the circuit that is lighting a light bulb
Measure the voltage between the two terminals of the light bulb while it is lit.
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5. Build the circuit shown in the diagram.
Measure (and record) the voltage across each battery.
Measure the voltage across the series combination of two batteries.
Measure the voltages between the two sides of each light bulb.
Check #1: Discuss questions 3 - 5 with an instructor.
multimeter set up to measure electrical current.
For the next section, please ask for a 2nd
Measuring current
The multimeter can also be used to measure current in a wire or device. The meters labeled
“current” have already been set up for this use (the switch on the front panel is pointing at "10A",
and the red wire is in a different socket). The meter has to become part of the circuit to measure
the current: you put it in series with the wire you wish to study - to measure a current, the
current must go through the multimeter. The diagrams below show how to use the meter to
measure the current in the wire between the two light bulbs, or in the bottom wire that goes to the
negative terminal of the battery
The current meter acts like a piece of wire in the circuit. Please try to avoid making a short
circuit with the current meter - it could damage it.
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This is the right
way to measure
the battery voltage 
while the light
bulbs are lit
 DON’T DO THIS
6. For the circuit you built for Q. 5, measure the current in each wire – the ones connecting the
light bulbs together, and the wire connecting the two batteries.
7. Measure and record the current in a circuit with one light bulb and one battery.
8. Measure and record the current in a circuit with one light bulb and two batteries.
9. Measure and record the current in a circuit containing a buzzer and one battery. (If you measure
zero or a very small number, ask an instructor for quick help in adjusting the meter to a more
sensitive setting.)
10. Measure and record the current in a circuit with a LED and two batteries.
Check #2: Discuss these results with an instructor. If none is available, skip ahead to read the next
Definition and begin working on your yellow sheet - you can come back to the Making a Battery
activity later, after your check.
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Making A Battery
Materials (in addition to those needed for Voltage activity):




4 brass washers and 4 zinc-plated steel washers
a piece of aluminum foil (5 cm square)
4 paper squares just wider than the washers
small dropper bottle of salty vinegar
11. The simplest kind of battery is called a cell. Some batteries contain many cells in series.
Here is how to make one cell:

Place a zinc/steel washer on the piece of aluminum foil

Place a paper square on top of the washer.

Put one or two drops of the salty vinegar on the paper, over the washer. We want to get the
paper damp but not dripping.

Place a brass washer on the paper.

Push down on the washer. If any liquid oozes out, wipe it off (and use less, next time). The
brass top and the zinc bottom should not have any liquid on them.
12. Measure the voltage between the top washer and the aluminum foil. This is the voltage of the
cell. Which end is positive, and which is negative?
A single cell of this kind (shown on the left) is not strong enough to cause anything to happen with
any of our electrical devices. However, a stack of cells -- a battery -- will be able to do something
interesting. Make three more of these brass/paper/zinc cells. Stack them on the piece of aluminum
foil. This will give a convenient way to provide an electrical connection to the bottom of the stack,
which is going to be rather wobbly.
Build a tower of two of the cells on the aluminum foil, as shown on the right of the diagram.
Always place them so that the brass washer is on top. Putting the two cells together makes a
battery.
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13. Measure the voltage between the aluminum foil and the top washer for your 2-cell battery.
(Your battery probably also will not do anything, but you could try to run the buzzer with it.)
14. Continue to build the battery by adding another cell (zinc/moistened paper/brass). Measure the
voltage, and try to use it to run the buzzer and the light emitting diode.
15. Then, add the last cell, and check it again. (Pay close attention to the LED, and try to touch the
buzzer with your fingertips if you don’t hear anything....)
It is important that the bottom side of the zinc washers stay dry, that the paper is damp, and that
everything is in the right order.
When you get your battery working, compare the brightness of the light-emitting diode to the
brightness using “real” batteries. See if your battery will light a light bulb.
Check #3: Show your battery to an instructor, and discuss the behavior of the different devices
with it.
16. According to what you have seen today, what are the rules that describe how the voltages
across different components of a circuit are related?
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Definition:
Energy is what you have to add to make cold things warmer and slow things faster, or to lift
something up. Energy comes in many forms. We add energy when we charge a battery or
compress a spring. When we pay the gas bill or the electrical bill we are paying for energy.
Power means energy per unit time. To keep many lights on or many machines running at once,
you need high power. Power is measured in watts.
If you multiply the amount of time by the amount of power, you get the energy that was used or
delivered. A thousand watt device running for one hour uses 1 kilowatt-hour, which is about 7
cents worth of energy.
"kilo" is an abbreviation for 1000. A kilometer is 1000 meters, a kilogram is 1000 grams, and a
kilowatt is 1000 watts (but a killer whale is not 1000 whales).
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Each group should hand in one copy of this page at the end of class
Group:
Names of group members present:
17. This diagram shows a possible way to wire a house. The power company is represented by
the batteries. This is not the way houses are usually wired, however. What would be
unsatisfactory about this design?
18. Duke Energy Company serves central North Carolina. It used to be called Duke Power
Company. Most likely this name change was made for reasons that have nothing to do with
physics, but let’s take them at their word: what were they selling before, and what are they selling
now? Is it an energy bill or a power bill that you get at the end of the month?
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Old exam questions
1 In this circuit, which light bulbs are on? Which wires are carrying current?
2. You have a light switch at the bottom of the
stairs and at the top of the stairs, both of which
turn the light on and off. Here are some possible
ways to connect the system. Which do you think
you have in your house? Justify your choice
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