BIO 100 Green River Community College Fluid Mosaic Model Worksheet

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Bio 100

Green River Community College

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please help me watch the video, note and answer 1 question of the lab experiment


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Capacitance Introduction: The purpose of this experiment is to investigate how the capacitance of a parallel-plate capacitor varies when the plate separation is changed and to qualitatively see the effect of introducing a dielectric material between the plates. A computer model of the system will be developed and the student will observe some of the power of computer modeling. Theory: A capacitor is used to store charge. A capacitor can be made with any two conductors kept insulated from each other. If the conductors are connected to a potential difference, V, as in for example the opposite terminals of a battery, then the two conductors are charged with equal but opposite amount of charge Q, which is then referred to as the “charge in the capacitor.” The actual net charge on the capacitor is zero. The capacitance of the device is defined as the amount of charge Q stored in each conductor after a potential difference V is applied: C = Q/V Rearranging gives: V = Q/C Eq. (1) The simplest form of a capacitor consists of two parallel conducting plates, each with area , separated by a distance d. The charge is uniformly distributed on the surface of the plates. The capacitance of the parallel-plate capacitor is given by: C = κε0A/d Where κ is the dielectric constant of the insulating material between the plates (κ =1 for a vacuum; other values are measured experimentally and can be found in tables), and ε0 is the permittivity constant, of universal value ε0 = 8.85 x 10-12 F/m. The SI unit of capacitance is the Farad (F). The system we use is more complex. In addition to the two moveable parallel plates, the connecting wires and the electrometer also have some capacitance. This capacitance is roughly equal to the capacitance of the moveable plates when the plates are 1 cm apart and cannot be ignored. Including this gives: C = κε0A/d + Csys Eq. (2) where Csys is the capacitance of the rest of the system. Substitution of Equation 2 into Equation 1 yields: V = Q/[ κε0A/d + Csys] Eq. (3) Any material placed between the plates of a capacitor will increase its capacitance by a factor κ called the dielectric constant where: C = κC0 Eq. (4) with C0 being the capacitance when there is a vacuum between the plates of the capacitor. Dielectric materials are non-conductive. Any dielectric material can be used to keep the plates in a capacitor insulated from each other (preventing them from touching and discharging). To three significant figures, κ = 1.00 for air. For all materials, κ > 1. If the charge on a capacitor is kept constant while a dielectric is inserted between the plates, Equations 1 & 4 yield: Q = CV = C0V0 = (C/κ)V0 so V = V0/κ Where V0 is the voltage before inserting the dielectric and V is the voltage after insertion. Since κ > 1 always, we have V < V0 Eq. (5) Setup: Figure 1: Setup Figure 2: Indicator foot 1. Position the movable plate so the leading edge of the indicator foot (see Fig. 3) is at the 0.2cm position, with the plate facing the 0-cm end of the track. The gap between the plates should be 2.0-mm all the way around. 2. If the gap is not 2.0-mm, release the holding screw on the non-moving plate and move it until the gap is 2.0-mm and then tighten the screw back down. 3. Use the adjustment screws on the back of the moveable plate to make the plates parallel. The easiest way to do this is to look directly down from above the plates and adjust the horizontal adjust until the gap looks uniform, then look at the gap from the side and make it even with the center of the plates by adjusting the vertical screw. Check several points around the plates with a ruler to verify the gap is a uniform 2.0-mm. You may need to repeat the process a few times. 4. Attach the twin lead (red & black) connector to the Signal Input jack on the Basic Electrometer. 5. Route the wires as far away from where your hand and your body will be as possible. • The charges in this experiment are all small so static discharge will interfere significantly. • Also, people can be considered conducting plates and have a significant amount of capacitance. • You can interfere just by being close. 6. It is best to make the fixed plate ground by attaching the black wire’s spade lug to it. Loosen the screw on the back of the fixed plate, slide the spade lug underneath and tighten the screw so the spade lug is flat and secure. 7. Attach the red spade lug to the terminal on the moving plate. The wire must be free to move when the plate moves. 8. Attach the black banana/banana wire, as shown in Figure 1, from the common (com) terminal on the Electrostatic Voltage Source to the ground terminal on the Electrometer. 9. Attach the red banana/spade lead to the +30V terminal and leave the spade end free. 10. Plug in the transformer and apply power to the Electrostatic Voltage Source. 11. Shift the switch on the back to the On position. The green Power On light should glow. 12. Use the supplied adaptor cable to attach from the Signal Output on the Electrometer to the A Analog Input on the 850 Universal Interface. It is important that it be the A input! Basic Variable Capacitor The PASCO experimental Variable Capacitor consists of two metal plates 17.7 cm (7 in) in diameter with a plate area A = 2.46 x 10-2 m2. Procedure A: The Effect of the Plate Separation 1. Set the capacitor plates 0.3 cm apart by setting the movable plate so leading edge of its indicator foot is at the 0.3 cm mark. **Do not move the fixed plate that is screwed to the track or your gap measurement will be off.** 2. Turn on the electrometer and set the range button to the 100 V scale. 3. Remove any charge from the capacitor by momentarily touching both plates at the same time with your hand. 4. Zero the electrometer by pressing the ‘ZERO’ button until the needle goes to zero. 5. In the Capstone software, change “Continuous” to “Keep Mode” at the bottom to the right of the record button. You will now see a Preview button instead of Record. This will be used farther down in the process. Read the following steps before doing them. They need to be performed quickly since the charge will slowly escape from the electrometer, especially if the humidity is high. 6. Momentarily connect a cable from the +30 V outlet in the voltage source to the stud on the back of the movable capacitor plate. This will charge the capacitor. 7. Remove the charging cable. 8. One person should run the computer while one moves the capacitor plate. a. Everyone else should stay back and should try to be in the same position for each reading. Anybody who is close is a significant part of the system and can make the readings change. 9. Slide the movable plate so it is at 8.0 cm (leading edge of the indicator foot). 10. Once the plate is in position, the person moving the plate should move away 50 cm or so and try to be in the same position for each measurement. 11. Click the PREVIEW button at the lower left to begin collecting data. Colored numbers will appear in first row of the table. 12. The person doing the computer should click the Keep Sample (red checkmark in the lower left) button. The number in the first row will turn black and the colored number will move to the second row. 13. The person moving the plate should say the next separation (7 cm) out loud and slowly move the plate, making sure to tell the person at the computer to Keep Sample after the plate has reached the next separation. 14. Move the plate to 7.0 cm and repeat the process until 0.3 cm. 15. Click the STOP button to end the data collection. 16. Examine the graph. If it looks like a smooth curve, you are done. If not, repeat the process until you get a nice looking run. Note: the Data page contains the Air Gap Capacitor Data table with two columns: Separation, Voltage. The separations are: 8.0 cm, 7.0 cm, 6.0 cm, 5.0 cm, 4.0 cm, 3.0 cm, 2.0 cm, 1.5 cm, 1.0 cm, 0.5 cm, 0.3 cm. There is also a graph of Voltage vs Separation. Procedure B: The Effect of a Dielectric between the Plates 1. You will use paper as the dielectric to be inserted between the plates so get your stack of paper, notebook, or pad of paper about 1 cm thick. You should have brought this with you. 2. Position the movable plate of the capacitor at 8 cm. 3. Turn on the electrometer and set the range button to the 100 V scale. 4. Remove any charge from the capacitor by momentarily touching both plates at the same time with your hand. 5. Zero the electrometer by pressing the ‘ZERO’ button. The needle must be at zero. Read the next steps before doing them to prevent charge loss. 6. Momentarily connect a cable from the +30 V outlet in the voltage source to the stud on the back of the movable capacitor plate. This will charge the capacitor. Remove the charging cable. 7. Click on the PREVIEW button at the bottom of the screen. 8. One student holds the stack of paper directly above the gap between the capacitor plates so that the long side of the paper is vertical. Note: Hold the paper with one hand and keep the other hand on the metal connector attached to the signal input of the Electrometer so that there is no static charge on the student holding the paper. 9. Press the Keep Sample button to record the voltage when the paper is not between the plates. 10. Lower the paper between the two plates until it touches the base. Do not let the paper touch either plate! Keep your hand as far above the plates as possible. 11. Press the Keep Sample button to record the voltage when the paper is between the plates. 12. Pull the paper back above the plates and repeat steps 8 and 9 several times. 13. Click the STOP button to stop monitoring the data. 14. If the final voltage with the paper out is much different from the initial paper out value, you probably touched the plates and should repeat the experiment. Analysis A: V = Q/[ κε0A/d + Csys] Eq. (3) Examination of Equation 3 from Theory A show that if Csys = 0, then V is directly proportional to d and the Voltage vs Separation graph on the Data page should be a straight line. This is clearly not the case. To verify Equation 3 for the case where Csys is not zero, we need to know Q and Csys. We determine these by fitting the math model (Equation 3) to the data. First we note that κε0A = (1.00)*( 8.85 x 10-12 F/m)( 2.46 x 10-2 m2) = 2.18x10-13Fm = 2.18x10-11F cm. So the parallel plate capacitance when d = 1 cm is C1.0 = 2.18x10-11 F. Note that this value is entered in line 2 of the Calculator. When d is small (0.3 cm) the first term in the denominator dominates and Q ~ V0.3(κε0A)/d = (30 V)*( 2.18x10-11F cm)/(0.3 cm) = 2.2x10-9 C. This value is entered as an initial guess for the value of Q in line 1 of the calculator. Q is constant so when d becomes large, Csys dominates in the denominator and we have: Csys ~ Q/V8 ~ 2.2x10-9 C/80 V = 2.7x10-11 F Where V8 is the voltage when d = 8 cm. This is taken as the initial guess for Csys (=C1) on line 3 of the calculator. Note that Csys is about equal to C1.0 at 1.0 cm. At 0.3 cm, C0.3 = 7x10-11 F so C0.3 ~ 3 Csys and the approximation above is decent but not great. At 8 cm C8 = 2.7x10-12 F = Csys /10, so the approximation is good, but not perfect. 1. Look at your graph. 2. Use the Run Select button on the graph toolbar to select your best run. 3. Enter the following equations into the calculator. Calculations 1 2 3 4 Q=3.0*10^(-9) κε₀A=2.18*10^-11 C₁=3.6*10^-11 V model=[Q (C)]/([κε₀A (F cm)]/[Separation (cm)]+[C₁ (F)]) Units C F cm F V 4. Adjust the values for Q on line 1 of the Calculator and for C1 on line 3 to make the model match the experimental curve as well as possible. 5. Click on the Calculation in line 4 to select it, then look below the table of calculations to find where the equation is written out like you would write it on paper – circled in red in the image below. 6. Click the equation and drag it to your graph. This should give you a best fit curve based off of your data and the calculations. Capacitance Procedure A: Separation (cm) 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.5 1.0 0.5 0.3 Voltage (V) 86 85 86 83 81 75 69 63 59 46 38 Procedure B: Paper Position Out In Out In Out In Out In Out Voltage (V) 24 18 23 17 22 17 22 17 21 Capacitance 1. What happened to the voltage as the plates got closer together ( decreasing)? 2. What were your best fit values for the charge Q and Csys? 3. How well did your model fit the data? Try to explain any discrepancy. Hint: what approximations are made when deriving the parallel plate capacitance (C = κε0A/d) from Gauss’ Law? 4. Briefly discuss the value of computer modeling. 5. Examine the Paper Dielectric Data table under Procedure B on the data sheet. Does the data agree with Equation 5 under Theory above? 6. What does a dielectric do?
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Fluid Mosaic Model
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Fluid Mosaic Model


The objectives of the video were: to know the function of the cell membrane, identify the
critical components of the plasma membrane, and understand the meaning of fluid
mosaic.



The primary role of the plasma membrane is controll...


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