CAPACITORS UM Physics Demo Lab 07/2013

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EXPLORATION

Capacitors in Circuits

Materials

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Battery Board

Alligator Leads

Capacitor

3V bulb

Multimeter

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1. Take the battery board and the loose 3V bulb. Wire the battery board, the 3V bulb, and capacitor according to the diagram below. The capacitor is will only work in the correct orientation, red is the + (high potential side) lead.

Figure 1: Simple Capacitor-Battery-Bulb Circuit

Observe and write down what happens to the light bulb when you close the switch, remember to be patient and observe the bulb brightness behaves over time:

2. Open the switch again and re-wire the circuit to bypass the battery, as shown below.

Figure 2: Simple Capacitor- Bulb Circuit

Close the switch. Again observe (and write down) what happens to the light bulb:

Obviously, the capacitor has an impact on the behavior of the circuit. Describe what the presence of the capacitor does to the circuit based on your observations. The first circuit could be described as a capacitor charging circuit and the second as a capacitor discharging circuit. When discharging the capacitor, is there current flowing in the circuit when the light bulb first goes out?

APPLICATION

Parallel-Plate Capacitors

Materials

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Aluminum Foil

Transparency Film

Catalog Sheets

Multimeter

Scissors

Heavy Item (Book)

Transparency Marking Pen

Ruler

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1:Build a parallel plate capacitor and measure its capacitance.

Cut the aluminum into two rectangles of about 10 inches by 8.5 inches (narrower than the transparency film). Flatten the aluminum on the table by pressing and smoothing with your hands.

Create an aluminum and transparency film sandwich, making a bottom layer of aluminum, middle layer of transparency film, and a top layer of aluminum. It is important that the top and bottom layers NOT touch (if they touched, it would short out the capacitor).

Put the heavy item on top of the capacitor. This compresses the plates around the dielectric as tightly as possible. Leave some aluminum showing for the top and bottom plate.

Your capacitor should look like the photo below. Notice that the overlapping surface area of aluminum is really only 2/3 of either sheet of aluminum.

Figure 3: Side View of Aluminum Foil Capacitor

2. The multimeter has a capacitor setting {–||–}. Use this setting to measurethe capacitance of this simple parallel plate capacitor by putting a test lead on the top aluminum plate, and the other on the bottom aluminum plate. The units of capacitance are called Farads (1 Farad = 1 Coulomb/Volt), denoted by F (or micro-Farads:μF, nano-Farads: nF, or pico-Farads: pF). Record this capacitance below with the proper unit and prefix as indicated by the multimeter.

Simple Capacitor Capacitance

Capacitance is a capacitor’s ability to store charge at a certainpotential difference (voltage) across the capacitor plates. The capacitance of is determined by the dimensions of the capacitor and the properties of the material placed between the plates (vacuum, air, plastic). You’re going to vary these parameters to see how they affectthe capacitanceof the parallel-plate capacitor.

The diagram below shows the various features you must consider when analyzing the capacitance of a capacitor:

The parameters you’re going to manipulate:

the distance between the plates ()
the overlapping surface area of the plates ()
the type dielectric material between the plates ()

3. Distance between plates

As you add sheets of plastic between the plates, you are adding distance between the plates. Add sheets of transparency filmbetween the plates, and measure the capacitance. Try not to change the overlapping surface area of the plates (i.e. if you had 60% overlapping, try to keep that constant—make alignment marks with the marking pen).

Here is an example of d = 3, you can see that the distance (d) increased, but the dielectric hasn’t changed because we are still using transparency film. The overlapping surface area won’t change if you’re careful to keep it consistent.

Keep using the heavy book to compress the capacitor. Record the values below-don’t forget the proper units for capacitance as indicated by the multimeter:

Layers of film (distance) / Capacitance
1
2
3

Did an increasing distance between the plates (i.e. adding more dielectric material between the plates) increase or decrease the capacitance of the capacitor?

4. Surface Area

Reduce the layers of transparency film to one. Now you are going to vary the overlapping surface area of the capacitor. The original overlap was about 60%. Below is a diagram illustrating a roughly 30% overlap.

We suggest some overlapping ratios below; also select one ratio of your own. Vary the surface area and record your observations.

Overlapping Surface Area / Capacitance
80%
20%

Did increasing (the overlapping surface area of the plates) increase or decrease the capacitance of the capacitor?

5. Dielectric Material

There are many types of dielectrics that will work between capacitor plates: plastic, paper, and air are just a few examples. Each dielectric has a characteristic known as permittivity. Permittivity is the susceptibility of a material to an electric field and is denoted in equations as. The higher the value of, the lowerthe electric field (force/charge) will be in the dielectric material when compared to vacuum for the same distribution of charges on the capacitor plates. For the purposes of this lab, all you need to know is that the permittivity of transparency filmis greater than that of catalog pages. Permittivity is similar to density in that it is independent of volume. It doesn’t matter how thick the dielectric is, the permittivity is constant throughout the material since it is an intrinsic property of the material itself.

We are going to vary the permittivity of the dielectric to see what effect permittivity has on capacitance. To do this accurately, we must be careful to keep the distance between the plates (d) the same. The thickness of 3catalog pagesis about the same as 1 sheet of transparency film.

Measure the capacitance of the basic capacitor with one sheet of transparency film and again with three catalog pages.

Permittivity / Capacitance
High - 1 sheet of transparency film
Low - 3 sheets of catalog pages

Did reducing the permittivity of the dielectric increase or decrease the capacitance of the capacitor?

Based on your observations, discuss in your group which of the following equations best describes how the capacitance of a parallel plate capacitor depends on the parameters you have varied:

is capacitance

is area (the overlapping surface area)

is the distance between the plates

is the permittivity of the material

Circle the equation that best fits the observations you made from the options below. Consider that components in the numerator (top) of the right side make the capacitance larger as they increase, and components on the denominator (bottom) make the capacitance smaller as they increase:

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Equation 1

Equation 3

Equation 2

Equation 4

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Explain how the equation you chose is consistent with your observations.

Challenge Work:

Explain howthe multimeter measures the capacitance of a capacitor when connected across the capacitor leads.

Both capacitors and electrochemical cells store can be used to store charge and energy. Explain the fundamental difference between these two different devices.

Everyday Applications

The energy to fire flash bulbs and Xenon flash tubes in camera flash units is stored in a capacitor which is recharged by the flash unit battery after each flash.
Large capacitors are now used instead of electrochemical cells to provide back-up electrical power to the memory in electronic devices such as computers and VCRs.
Large capacitors are now used instead of electrochemical cells and batteries to power many toys, including flying model airplanes.
A capacitor in combination with an inductor (a coil of wire) produces a resonant circuit which is sensitive to alternating electrical current at a specific frequency. This is how basic radio circuits are tuned to respond to a specific frequency (radio station).

Summary:

Capacitors store electric charge by separating charge of opposite sign on the plates of the capacitor.
The separated charges on a capacitor also store electric potential energy since a potential difference must exist across the plates of the charged capacitor on which the charge resides.
Capacitance is defined as (charge)/(potential difference): C = Q/V.
Capacitance is a measure of efficiency—a large capacitor is efficient because it stores a large amount of charge at a low potential difference which means that relatively little work was required to move the charge onto the plates of the capacitor working against electric forces. By contrast, a small capacitor requires a large potential difference to store the same amount of charge as a larger capacitor and much more work must be done against electric forces to move the charge onto the plates. A small capacitor is therefore less efficient at storing charge.
The energy stored in a capacitor can be calculated in terms of the capacitance and potential difference across the plates as

E = ½ CV2.

The capacitanceof a parallel-plate capacitor is proportional to the area of the plates and the permittivity of the dielectric material between the plates and inversely proportional to the distance of separation between the plates: C = ε A/d.

CAPACITORS

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Storing Electrical Energy

Electrochemical cells and batteries are electrical energy storage devices: chemical energy is stored in the battery ready to be converted to electrical energy. We use it up at our leisure until it’s gone, then we replace or recharge the battery. The chemicals in a non-rechargeable cell have a fixed capacity to deliver energy to a circuit and then they must be replaced.

There is another type of device that stores energy called a capacitor. Capacitors are used in many electronic circuits. One common application that many of you may know about is a camera flash unit. Capacitors can discharge more quickly than batteries producing a very fast “flash”, ideally suited for photography. The diagrams below take you step by step through how a capacitor is charged and how it delivers its energy to a circuit.

This is a dischargedcapacitor in equilibrium. The basic features of the capacitor are two conductive plates separated by a thin non-conductive dielectric (such as air, paper, or plastic). The dielectric in these graphics is simply the space between the plates that has no conducting material in it. Each plate of an uncharged capacitor is electrically neutral.

This is a capacitor that is charging. A battery has been connected across the capacitor. The energy from the battery displaces the charges in the capacitor from equilibrium and causes them to flow as a current. They do this even though there is a gap in the circuit between the capacitor plates! This current flow results in a charge imbalance on the plates. One plate is accumulating positive charges on its surface; the other is accumulating negative charges. The charges cannot leap across the dielectric gap, but they are attracted to the opposite-sign charges accumulating on the opposite plate.

As the plates accumulate charges of opposite sign, a difference in electric potential (voltage) develops across the plates. The attraction of the opposing charges

gives rise to this potential, and the accumulated charges acquire and store electric potential energy by virtue of this potential difference.

After some time, the capacitor becomes fully charged. When the capacitor is charging, the opposing charges accumulate on the plates and create a potential difference or voltage across the plates. This voltage increases until it is the same as the voltage of the battery (e.g. 3V or 1.5V). The current does not flow anymore because the plates are charged to the same potential difference as the battery and there is no potential difference available between the battery and the plates to drive any further current. Charge is stored on the surface of the plates because the opposing charges on the plates attract each other across the gap between the plates.

If we remove the battery and connect a wire across the capacitor (shorting the capacitor) the capacitor discharges. Previously in the disconnected circuit, the charges were attracted to each other across the gap, but unable to travel back to the opposing plate to recombine to charge neutrality. With the wire connected they can travel to the opposite plate along the wire and the plates can return to charge neutrality with a potential difference of zero across the plates—back to the uncharged state.

The first two experiments you did with the battery, light-bulb and capacitor circuits were exactly the charging/discharging process for a capacitor. Here’s what happened:

Initial Condition: The capacitor was uncharged—both plates were uncharged (neutral) and the potential difference across the plates was zero.

Step 1: With the light-bulb in series, you charged the capacitor from the battery. At first current was flowing, so the light-bulb lit up. When the capacitor was fully charged it no longer took charge from the battery, so the current stopped and the light-bulb went out.

Step 2: After the capacitor was charged, you removed the battery from the circuit. With the light-bulb still in series with the capacitor, the switch was closed. The opposing charges in the capacitor flowed back to the oppositely charged plate, thereby creating a current and once again lighting the light-bulb. Once the capacitor returned to charge neutrality on the plates, the current ceased and the light went out again.

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