Download 05VoltageCurrentPower

Voltage, Current, and Power
Volts, Amperes, and Watts
Materials
2 batteries
3 light bulbs and sockets
LED
buzzer
wires
multimeter
Discussion: We will begin class by discussing the questions from last time:
>. What would be the water analogy to the rule you gave for combining batteries and light bulbs in
series?
Suppose you wanted to siphon water out of a fish tank quickly: what does the rule tell
you?
.
> 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?
>Rivers are dammed to make lakes. Sometimes the dam is used to operate a hydroelectric plant
that turns water power into electrical power. What are the properties of a dam that represent the
storage of a lot of energy? What are the properties that will produce lots of power?
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1. Use the analogy between electrical circuits and water systems to answer this:
What are the properties of an electrical circuit which will allow the battery to provide lots of
power?
2. How are power, current, and voltage related? Start by considering how changing current or
voltage changes the power, and then give a mathematical relationship that is consistent with it.
Check: discuss your relationship with an instructor. If they are busy, start on the next section:
interpreting the sunplot.
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Sun Plot
Using the sun’s shadow to determine how it moves across the sky.
On a separate page is the sun plot that was made on Thursday, August 26, 2010.
It shows the
position of the tip end of the shadow of a gnomon at various times. The gnomon is a cone that is
4.1 cm tall; the center of the cone is marked by the + sign on the plot.
Make measurements and observations of the sun plot to answer the following questions:
3 Sketch a smooth curve that goes through the data points. Use this curve to estimate when the
shadow of the gnomon was the shortest (it's possible that we don't have an observation at exactly
this time). Was the sun ever directly overhead? Was the shortest shadow at noon? Why not?
4. Draw in a line from the center of the gnomon to the place on the curve where the shortest
shadow ended. This line points very nearly north-south.
5. Measure the length of the shortest shadow on the paper and
make a drawing showing a side view of the gnomon and the
shadow (like the drawing at right). Measure the angle to
determine how close to "straight up" the sun got.
Check: discuss your measurement with an instructor.
6. Here is a circle, representing the earth. The sun is far off to
the left. At noon, Lexington is at the position marked (the
latitude of Lexington is 38 degrees). Draw a line indicating the
direction to the sun August 26.
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Definition:
To have electrical power you have to have both a current (Amperes) and a voltage (Volts). The
sizes of these units were chosen so that the product gives the power in Watts
Power = Current x Voltage
1 Watt = 1 Ampere x 1 Volt
On the electrical bill energy is measured in “kilowatt-hours.” One KWH means that 1000 Watts
was used for 1 hour (or 2000 Watts for ½ hour, etc). (note “kilo” means “1000”)
The reason that energy is interesting is that the total amount of energy in the universe never
changes. When energy moves from one object to another, the amount that one gains is exactly
balanced by the amount that the other loses. Then the power produced by the battery should be
equal to the power emitted by the other elements in the circuit.
7. Last time you measured the current and voltage when a light bulb is connected to a battery. Use
your results to determine how much power is being delivered to the light bulb.
8. Use your results from last time to determine how much power is being delivered to the light
bulb when there are two batteries in series.
9. Here is a greatly simplified circuit diagram for a table lamp.
The power company is represented by a 120 V battery (household
circuits have voltage 120 V). What is the current in a 60 Watt light
bulb?
10. In Kentucky Electricity costs 7 cents per kWH. If an air conditioner draws 15 Amperes when
operated on 120 V, how much will it cost to run it for an hour?
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11. Assume that in these two circuits the current is 0.5 Amperes, so the light bulbs are equally
bright. In each circuit, how much power is each battery providing? How much power is being
delivered to the light bulb?
Please note that this example explains why voltages add when you put batteries in series.
Check: Discuss questions 8 through 11 with an instructor
12. Predict what will happen when the switch closes.
Then build the circuit to check your prediction. Explain the result, especially if it did anything
different than what you expected.
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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:
Please discuss these questions:
13. In the circuit of Q11, there are more light bulbs lit when the switch is open, but the light bulbs
are brighter when it is closed. For which case is the power supplied by the battery greater?
(Write out an explanation of your answer).
14. When all the generators are running at Hoover Dam, it produces
2,000,000,000 Watts. This power is then sent elsewhere by power lines.
The diagram at right is a simplified version of the situation, where the
battery represents the generators and the light bulb represents the city that
uses the power. If the “battery” is driving the circuit at 120 V (household voltage), what is the
current in the power line?
They use transformers to change the voltage, so that the Hoover Dam power station acts like a
100,000 Volt battery. Now what is the current in the power lines?
Please note the homework assignment (on a separate page)
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Homework assignment:
Name:
Electrical appliances usually have a little placard on the back or bottom giving their electrical
ratings, specifying voltage (V) and either current (A) or wattage (W) or possibly both. (It also tells
the frequency (Hz or cycles); this is of no interest to us at the moment).
Here are some examples:
This label on a space heater gives
both current and wattage information.
The label on the bottom of a toaster.
The current value is not given.
Please find four devices with placards like this and record what they say. If information is
missing, calculate what should be there. The space heater and the toaster have already been
reported on the table.
Device
Voltage
Current
Power
Space heater
120 V
12.5 A
1500 W
Toaster
120 V
6.7 A
800 W
Please hand in your table at the beginning of class next time (Wednesday, September 8).
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