[DC CIRCUITS: CAPACITORS]STUDIO – Unit 9PHY-2054 College Physics IINOTE:At the end of this activity, there is a briefsection on adding capacitors in series and inparallel. This material is covered in section20.12. Please be sure to review this material.CAPACITORSObjectives1. to understand what a capacitor is.2. to understand how capacitors behave as elements in circuits.3. to understand the definition of capacitance.4. to understand how capacitors behave in series and parallel networks and be ableto calculate the capacitance of series and parallel networks.Let’s think about how to store charge. Charge is stored in a device called aCAPACITOR named for its capacity to store charge. The symbol for a capacitor isshown in next diagram.Capacitor SymbolThe parallel plate capacitor is the commonest type of capacitor that we encounter. In oneof its forms it consists of a set of parallel sheets of metal separated by a thin space whichcan be a vacuum (or air) or can be another material which is referred to as a dielectric. If the capacitor is connected to a battery, charge from one terminal will move to one ofthe plates of the capacitor. Let’s consider the positive side of the device; the sideconnected to the positive side of the battery.2What is the sign of the charge on the plate connected to the positive side of the battery?Explain.As a result of this, what happens at the other plate of the device?? As more and more charge moves into the capacitor the electric field begins to grow and ittakes more and more energy to move charge from one side to the other. Energy, as well ascharge therefore moves into the capacitor and is stored there. This process leaves oneplate of the capacitor with an excess of electrons on it and the other side lacking electronsand hence positive. The magnitude of the charge on each plate is the same. We call it“q” or “Q” and we refer to this as the “charge on the capacitor”.A model for a capacitor is shown for a “parallel plate” capacitor.As you can see, the capacitor consists of two parallel plates (hence the name), each one ofwhich is charged to the same value but oppositely in sign. As a result of this, an electric 3field is created in the space between the plates. The field goes from the positive plate tothe negative plate.We have already calculated the electric field using Gauss’s Law and you should spendsome time making sure that you understand the approach. Create a small cylinder that isparallel to electric field (shown above). This surface starts inside the (+) metal plate andstops inside the space between the plates.Since one end is in the metal, the field at that end of the cylinder is zero. If A is the areaof the Gaussian surface, then the only flux leaving the Gaussian surface is: __________. QUESTION: How much flux enters or leaves the sides of the cylinder? Why?4Area A If the charge density of the metal plate is , then Gauss’s Law says that 0AseF =. From this,what is the electric field in the space between the plates of a parallel plate capacitor? Remember that  is the charge per unit area so the total charge on the plate.Discuss this with your group and make sure that you understand this calculation!Spend a few minutes drawing the diagram and making sure that you understand it. Youmay be asked to present your solution on a white board. Be prepared!You will now explore capacitors. For this part of your explorations you will need thefollowing:Equipment:1 multi-meter2 wires1 battery 1 battery holder (Actually a board that contains a battery holder and other things)1 capacitorA battery is a device that has two terminals separated by some internal noxiouschemicals. The device produces a potential difference between its terminals and it is thispotential difference that we use to power such things as batteries and clocks. Evencomputers depend on batteries. There are many types of batteries around and you canscan the web for some better answers, but there is a reference on the class page that mayhelp. An important thing to remember is that any two points that are exposed to a batterywill always have the “battery voltage” as the difference in potential between the twopoints. This will become very important in the next chapter.5Capacitors come in many sizes and shapes as shown here. The value of the capacitance isusually written directly on the device.Important definition:The charge on each plate of a capacitor is found to be proportional to the potentialdifference applied across the capacitor. The capacitance, C is defined as the ratio of thischarge, Q , to the applied potential difference V:QCVorQ CV==1.1 Consider a capacitor suddenly connected to a battery as in the diagram below. 6a. Briefly discuss in terms of charges and electric fields what happens as a capacitor is“charged”. If charge flows, this a current, a topic we have already discussed. If there is acurrent, is the current constant with time? Is it zero? Does it change? Is it the same indifferent parts of the wire? Can a capacitor be “full” to capacity? (Excuse the pun).Notice that if the battery is connected across the capacitor, then there is a voltage acrossthe capacitor and from the equation above, the capacitor is charged.b. If after a long time the capacitor were disconnected from the battery, would there be aremaining net charge on the capacitor? Explain.c. Would there be a reading on the voltmeter, if you attached a voltmeter across thecapacitor? Explain.7d. If, after the capacitor has been disconnected from the battery, you connected one endof the capacitor to a large piece of metal, what if anything would happen? Explain. e. If, after the capacitor has been disconnected from the battery, and you connected oneend of the capacitor to a large piece of metal, as in part d, would there be a reading on avoltmeter, if you attached a voltmeter across the capacitor? Explain.Let’s make the measurements-A photograph of the meter is shown below. For voltage measurements the black lead willbe the common (ground) and the red lead the positive side that is placed into the secondslot from the right.8The setup for the following is shown in the next photo. The ends of the wires slipinto the spring like contacts. This is the same setup as shown in the schematicdiagram shown above.