Download 14.2 The Flow of Electric Charge

14.2 The Flow of Electric Charge
How much charge moves when current flows?
1
Measuring voltage
Table 1: Capacitor discharge data
Time
(s)
0
2
a.
Voltage
(V)
Time
(s)
70
10
80
20
90
30
100
40
110
50
120
60
130
Voltage
(V)
How much current flowed?
Use Ohm’s law to write down a formula for the current flowing through an electrical device if you
know the voltage and the resistance.
Investigation 14.2 The Flow of Electric Charge
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b.
Use the relationship you found in question 2a to fill in Table 2 by calculating how much current was
flowing through the resistor.
Table 2: Capacitor discharge data
Time
(s)
Current
(A)
0
10
20
30
40
50
60
3
a.
Time
(s)
Current
(A)
70
80
90
100
110
120
130
Thinking about what you observed
Make a graph of the current versus time for the capacitor. Draw a smooth curve through the points on
your graph.
Investigation 14.2 The Flow of Electric Charge
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b. Describe what your graph shows about the current over time. Explain the reason for the current to
change in this way.
c.
Suppose a light bulb had been used instead of a resistor. What would you have noticed about the light
bulb as time went on? Why?
d. Calculate the power of the resistor at a time of 10 seconds and at a time of 60 seconds. Do your answers
support your explanation in question 3c?
4
a.
Calculating charge from your graph
Write down a formula that allows you to calculate charge if you know the current and the time.
b. If current is in amperes and time is in seconds, what is the unit of charge represented by your formula
from question 4a?
c.
Look at the scale on your graph and determine the time and current represented by one block. Draw one
graph block and label its dimensions.
Investigation 14.2 The Flow of Electric Charge
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d. Use your answers to questions 4a and 4b to calculate the amount of charge represented by one block on
your graph. Show your work.
19
20
1
21
2
6
3
7 10
4
8 11 13 15
5
9 12 14 16 17 18
22
23 24
Count the total
number of grid
blocks under the
curve. Add
partial blocks
together to make
whole blocks.
e.
Count the number of blocks between the curved line on your graph and the x-axis as shown above.
Include fractions of blocks in your count, and estimate as accurately as you can.
f.
In question 4d you figured out the amount of charge represented by one block. Use this information and
the number of total blocks under your graph to calculate the total charge that flowed from the capacitor
and through the circuit.
g.
One coulomb is equal to 6.24151 × 1018 electrons. Calculate the number of electrons that flowed from
the capacitor.
h. Explain how the results for your investigation would differ if you used a 30 resistor instead of a 20
resistor. If you have time you may want to try the experiment with a resistance of 30 (20 and 10
resistors in series) to see if you are correct.
Investigation 14.2 The Flow of Electric Charge
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