HPP Activity 68v1
Capacitors
Charge Inside the Body – A Close Look at Cell Membranes
Our bodies store and use charge to transmit signals across nerves and to tell certain cells what to
do and when to do it.
Heart muscle cells are polarized when at rest. This means that the cells have slightly unequal
concentrations of ions across their cell membranes (See picture below). An excess of positive
sodium ions on the outside of the membrane causes the outside of the membrane to have a
positive charge relative to the inside of the membrane. The inside of the cell is at a potential that
is about 90 mV less that the outside of the cell membrane.
Depiction of A Cell Membrane
This imbalance of charge across a cell membrane is very important. In order to understand how
the human body can transmit signals, one needs to understand the physics of how charges can be
stored and transmitted. Today you will examine a model (called a capacitor) of the cell
membrane.
Exploration
Suppose you have two metal plates, one of which is connected to a ground so it has access to an
infinite supply of any charge it wants. This situation is shown in Figure 17-1. Now you place a
charge q onto the ungrounded plate (with your glass rod, for example).
Activity Guide
 2010 Department of Physics, Doane College
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Figure 1.
GE 1. Putting Charge on a Capacitor
1. Remembering what you learned about induced charges, what charge, if any,
appears on the grounded plate? Why?
2. Sketch the electric field between the two plates. If you put an electron
between the plates and near the grounded plate, what would happen to it?
3. Based on your answer to question 2, is there an electrical potential between
the two plates? Explain.
4. What should happen to the potential difference between the plates as you
add more charge to the ungrounded plate?
Now connect two bulbs and a capacitor in the circuit as shown in Figure 2.
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Figure 2.
GE 2. What Does a Capacitor do?
1. Close the switch in the circuit shown in Figure 17-2 and observe what
happens to the bulbs. Then open the switch, remove the battery, connect the
two wires that were connected to the battery together, then close the switch.
Explain what you observed. Compare with a circuit involving a battery and
bulb. Make a sketch of the circuit for both situations to help with your
explanation.
2. Now open the switch, remove the battery, connect the two wires that were
connected to the battery together, and then close the switch. Explain what you
observed. Compare with a circuit involving a battery and bulb. Make a sketch
of the circuit for both situations to help with your explanation.
3. The class will discuss their findings. Record the important aspects of the
conversation here.
The function of a capacitor is to hold charge. The two plates have equal and opposite charges so
the net charge is zero (Explain why this is so!), but the separation of the charges gives rise to a
potential difference between the two plates. An important fact about capacitors is that the
magnitude of charge on a plate is proportional to the potential difference between them. The
constant of proportionality is called the capacitance C.
Q  C V
This equation can be used to define the capacitance of a pair of conductors. The capacitance C
tells us how much charge can be stored per volt of potential difference.
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 2010 Department of Physics, Doane College
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What determines the size of C for a particular pair of conductors? Let’s consider a simple
arrangement: two parallel plates. You can make such an arrangement very easily. Use aluminum
foil for the metal plates. Keep them separated from each other by inserting paper between them.
The pages of a book work nicely.
GE 3. Capacitance of Parallel Metal Plates.
1. What factors might affect the capacitance of two parallel plates? Think
about all that may apply and record them here. Discuss your ideas with other
groups around you.
2. Choose one of the variables from your list above to test. How do you think
this variable will affect the capacitance?
3. Devise an experiment to test how the variable affects the capacitance.
Describe your experiment here.
4. Set up your experiment and connect your capacitor to the capacitance meter
(ask your instructor for help) and record your data here with a proper data
table.
Your variable goes here! [Units]
Capacitance [F]
5. Graph your data. Try to guess at the functional relationship between C and
the variable you chose to explore.
6. Fit your data and compare the fit with what you predicted?
7. Obtain data for the other variable from another group and list it here.
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8. What is the function that best describes the relationship between spacing,
surface area, and capacitance?
GE 4. The Effect of A Dielectric on Capacitance
How does filling the space with different insulating materials affect the
capacitance? Imagine two parallel plates separated by air. They are charged
up by a battery, which is then removed. This scenario is illustrated below.
First make some predictions about what you think will happen based on what
you have observed so far.
1. Is there an electric field between the plates? Describe why you think so.
2. Is there a potential difference between the plates? Describe why you think
so.
Now a slab, perhaps plastic, is inserted between the plates. Remember that
while charge is not conducted within the slab, it can be polarized.
3. What would you expect to happen to the total electric field in between the
plates once the dielectric slab is inserted?
4. Based on the answer to question 3, predict what would happen to the
potential difference between the plates.
5. Did the charge on the plates change during this process?
6. What will happen to the ratio of charge to potential difference when the
dielectric slab is inserted?
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7. What will happen to the capacitance when the slab is inserted?
8. Obtain two metal plates and a plastic slab. Make a capacitor with the plates
and air in between. Measure the capacitance. You do not need a battery for
this measurement!
9. Now insert the plastic slab and measure the new capacitance.
10. Does it behave as expected?
Activity Guide
 2010 Department of Physics, Doane College