Republic of the Philippines Department of Education Regional Office IX, Zamboanga Peninsula 12 GENERAL PHYSICS 2 2 n d Semester - Module 3 CAPACITANCE AND DIELECTRICS Name of Learner: Grade & Section: Name of School: General Physics 2 – Grade 12 (STEM) Support Material for Independent Learning Engagement (SMILE) Module 3: Capacitance and Dielectrics First Edition, 2021 Republic Act 8293, section176 states that: No copyright shall subsist in any work of the Government of the Philippines. However, prior approval of the government agency or office wherein the work is created shall be necessary for the exploitation of such work for a profit. Such agency or office may, among other things, impose as a condition the payment of royalties. Borrowed materials (i.e., songs, stories, poems, pictures, photos, brand names, trademarks, etc.) included in this module are owned by their respective copyright holders. Every effort has been exerted to locate and seek permission to use these materials from their respective copyright owners. The publisher and authors do not represent nor claim ownership over them. Development Team of the Module Writer: Jeovanny A. Marticion Editor: Zyhrine P. Mayormita Reviewers: Leo Martinno O. Alejo, Zyhrine P. Mayormita Layout Artist: Chris Raymund M. Bermudo Management Team: Virgilio P. Batan Jr. - Schools Division Superintendent Lourma I. Poculan - Asst. Schools Division Superintendent Amelinda D. Montero - Chief Education Supervisor, CID Nur N. Hussien - Chief Education Supervisor, SGOD Ronillo S. Yarag - Education Program Supervisor, LRMS Zyhrine P. Mayormita - Education Program Supervisor, Science Leo Martinno O. Alejo - Project Development Officer II, LRMS Janette A. Zamoras - Public Schools District Supervisor Joselito S. Tizon - Principal, Zamboange del Norte NHS Printed in the Philippines by Department of Education – Region IX– Dipolog City Schools Division Office Address: Telefax: E-mailAddress: Purok Farmers, Olingan, DipologCity Zamboanga del Norte, 7100 (065) 212-6986 and (065)212-5818 dipolog.city@deped.gov.ph What I Need to Know This module will help you learn about capacitors and dielectrics. At the end of this module, you should be able to: 1. Deduce the effects of simple capacitors (e.g., parallel-plate, spherical, cylindrical) on the capacitance, charge, and potential difference when the size, potential difference, or charge is changed (STEM_GP12EMIIId-23) 2. Calculate the equivalent capacitance of a network of capacitors connected in series/parallel (STEM_GP12EM-IIId-24) 3. Determine the total charge, the charge on, and the potential difference across each capacitor in the network given the capacitors connected in series/parallel (STEM_GP12EM-IIId-25) 4. Determine the potential energy stored inside the capacitor given the geometry and the potential difference across the capacitor (STEM_GP12EM-IIId-26) 5. Describe the effects of inserting dielectric materials on the capacitance, charge, and electric field of a capacitor (STEM_GP12EMIIId-29) 6. Solve problems involving capacitors and dielectrics in contexts such as, but not limited to, charged plates, batteries, and camera flash lamps. (STEM_GP12EM-IIId-30) What’s In Devices used for communication, photography, and high-energy accelerators rely on one of the most important electric devices in modern times, capacitors. It is a device composed of conductors separated by an insulator or a vacuum. A potential difference is produced between the conductors (equal magnitude with opposite signs). Aside from the potential difference, an electric field is also formed between the conductors. The energy stored by capacitors is similar to the mechanical energy stored in any physical body. The energy stored is delivered to a circuit when it discharges. The presence of an insulator produces polarization which helps in charge distribution within the material. It also increases the capacitance of a device. 1 What’s New Activity 1. How Defibrillator Works. Direction: Below is a comic strip that shows analogous examples of how a defibrillator works. Take a look at these visual strips and answer the questions after analyzing the strips. Image Source: https://www.howequipmentworks.com/defibrillator/ 2 (1) In your own words, what is the main function of a defibrillator? (2) What scenario or situation was the defibrillator compared to? Give evidence. (3) What do we need if we want our defibrillator to work effectively? (4) What similar experience do you have which could also be compared to a defibrillator? What Is It Capacitance Capacitors are devices formed from two conductors separated by an insulator. Examples of insulators are plastic, liquid gel, paper, mica, ceramic, or even air. These materials are called dielectrics. Both conductors have an equal magnitude of charges with opposite signs. Thus, the net charge of the device is zero. Their ability to store energy makes it useful when it produces potential differences across the plates. They act like rechargeable batteries. The insulating component of capacitors blocks the flow of direct current. The presence of the electric field found between the plates is directly proportional to the charge Q present in the conductors. Therefore, the potential difference, V ab, is also directly proportional to charge Q. The more charges present, the intensity of the electric field between plates increases, and potential difference increases. However, the ratio between charge and potential 3 difference remains the same. This ratio represents the constant for any capacitor known as the capacitance: where C is the capacitance expressed in or Farad (F) Q is the charge expressed in Coulombs (C); and V is the potential difference between conductors expressed in voltage (V) Capacitors are represented by the symbol: These devices are mainly used in communication such as radio transmitters and receivers. These are also used in regulating the outputs of flash units in our cameras. Figure 1. Practical uses of capacitors Image source: https://tinkersphere.com/wireless-modules-for-raspberry-pi-and-arduino/485-rf-link-transmitter-receivermodule-pair-433mhz-or-315mhz-for-arduino-raspberry-pi-b-b.html The most commonly studied capacitors is composed of two parallel conducting plates. These are separated by a distance which is very small in contrast with their true dimensions. The region in between them has a uniform field and charges are uniformly distributed. This is known as parallel plate capacitors. The electric field in this case can be expressed as: where Q is the charge and 𝟄0 is the electric constant as presented in Module 1 which is equal to or also known as permittivity of space 4 d +Q -Q Va Vb The potential difference (recall this lesson from Module 2: Electric Potential) between plates a and b is expressed as: Manipulating formula divide both sides by V ab divide both sides by d The formula for the capacitance of parallel plate capacitors where A is area in terms of m 2 , d is the distance in terms of meter (m), 𝟄0 is permittivity of space in terms of Example 1: The plates of a parallel plate capacitor are separated at 1.0 mm apart. What is the capacitance in air if the plate is 10 cm 2 ? What is the charge in each plate if a potential difference of 100 V is applied? A What is/are given? A = 10 cm 2 ; d = 1.0 mm; V = 100 V B What is asked? (a) C = ? (b) Q = ? C Are the units No, distance should be converted from mm to m. consistent with the Thus, 1.0 mm = 1.0 x 10 -3 m and formula? ( ) E What strategy must be employed? F Solution ( ) We use the capacitance formula for parallel plate capacitors. Then, we use the capacitance formula to derive charge Q. ( ) ( ) ( )( ) 5 G What is the conclusion? Therefore, the capacitance of the parallel plate capacitor is and the charge of the plate with 100 V is Capacitors in Series and Parallel Connection Suppose the capacitors were arranged as shown in Figure 2 below. Two capacitors were connected between points a and b where a constant potential difference is maintained. In this connection, both capacitors must have the same magnitude of charge Q. The net charge between the right plate of C 1 and the left plate of C2 must be zero. The same net charge must be observed with the remaining plates. Thus, the capacitors are in series connection where the magnitude of charge for each plate is the same. C1 c C2 b a Figure 2. Capacitors in Series Connection The potential difference for each capacitor Combining the potential difference ( ) Divide both sides by Q algebraically, Hence, the equivalent capacitance of the capacitors in series connection. The reciprocal of the equivalent capacitance of a series combination is equal to the sum of the reciprocals of individual capacitors. We may also extend this to any number of capacitors arranged in series connection. 6 Suppose the capacitors were arranged as shown in Figure 3 below. Two capacitors were connected in parallel with points a and b where a constant potential difference is maintained. The right plates are connected together forming equipotential surfaces and the remaining plates formed another. In a parallel arrangement, the potential difference for each capacitor is the same. a C2 C1 Vab b and ( ) ( ) The magnitude of charge for each capacitor Thus, the total charge is the sum of individual charges of the capacitor The common factor is V since the value of potential difference are the same for both capacitors Divide both sides by V Take note: The equ ivalent capacitance of a parallel combination is the su m of individu al capacitance. We may also extend this to any number of capacitors arranged in parallel connections. Example 2: Suppose you have two capacitors 6.0 µF and 3.0 µF connected in series and parallel with points a and b whose potential difference is 18 V, what is the equivalent capacitance, charge, and potential difference for each capacitor. A What is/are given? C 1 = 6.0 µF; C 2 = 3.0 µF; V ab = 18 V B What is asked? Ceq=?; Q and V for each capacitor C Are the units No, C 1 and C 2 must be converted to F consistent with the Thus, C 1 = 6.0 x 10 -6 F and C 2 = 3 x 10 -6 F formula? E What strategy must We use the formula for series and parallel 7 be employed? F combination of capacitors Solution For series combination of C 1 and C 2 , we solve first for the equivalent capacitance. ( () ) Then, we solve for the charge for given the potential difference and computed equivalent capacitance. The computed value of charge will be the charge for each capacitor C 1 and C 2 . Recall that charges for each capacitor in a series combination are the same. ( )( ) Solve for the potential difference in each capacitor C 1 and C 2 using the given magnitude of capacitance and the charge: For parallel combination of C 1 and C 2 , we solve first for the equivalent capacitance. The potential difference for C 1 and C 2 is equal to 18 V. Since parallel capacitors have the same magnitude of potential difference. G Solve for the charge for each capacitor C 1 and C 2. ( )( ) ( )( ) What is the Therefore, in a series connection C 1 and C2 has an conclusion? equivalent capacitance of , a potential difference of 6 V and 1 2 V, respectively and a unfirom charge of . In a parallel connection, C 1 and C2 has an equivalent capacitance of , a potential difference of 18 V and magnitude of charge , respectively. 8 The Energy of a Charged Capacitor A capacitor can be similar to a spring. Suppose a charge was removed from the positive plate and released towards the negative plate. Then, the charge lost potential energy equivalent to V. This is also equal to the work done (W = ΔQV) on a charge due to electric field present between the plates (recall the concepts of electric potential energy from the previous Module 2: Electric Potential). However, the sum of the potential energy is not just QV since potential is directly proportional to the remaining charges in the plate. Hence, transferring these charges to the other plate would decrease the potential. Discharging the plate by transferring all charges to the other plate, the decrease of potential energy is the average potential of the process. This is expressed as: Since , then ( ) and , then ( ) This expression is similar to the potential energy stored in a stretched spring where k is the spring constant and x is the displacement. The constant k is analogous to 1 / C in the potential energy of the capacitor. The work done in stretching the spring is similar to the work done in supplying energy to the capacitor. This shows a direct relationship between stored energy within the capacitor and the electric field between its plates. We could understand this by finding the energy density or the energy per volume. Since , then ( Recall that , then 9 ) where is the permittivity of space, C is the capacitance and E is the electric field. Example 3: Fibrillation is a condition where the heart loses its ability to pump blood effectively leading to heart attacks. Electric shock can be used to correct this behavior. Appropriate shocks can be administered using a 10µF capacitor which was charged with a potential difference of 6000V. (a) How much energy is released in the current pulse? (b) How much charge passes through the patient’s body? What is/are given? What is asked? Are the units consistent with the formula? How will you visualize the problem? C 1 = 10 µF; V ab = 6000 V U = ?; Q = ? No, C 1 must be converted to F Thus, C 1 = 10.0 x 10 -6 F E What strategy must be employed? We use the formula for potential energy and charge. F Solution (a) ( )( ) ( ) A B C D G (B) ( )( ) What is conclusion the Therefore, the energy stored with 6000 V on a capacitor is 180 J while the charge it produced is 0.06 C. Dielectric Constant Suppose a slab of an insulator was inserted between plates of a capacitor. Although this could not conduct electric current, this could also respond to the electric field. The electrons in the molecules of most substances are not uniformly distributed. These molecules are called polar molecules. These molecules have one end as positively charged and the other end as negatively charged. Without an electric field, these polar molecules are randomly arranged. If an electric field is present, the molecules align opposite to the field. In the case of nonpolar molecules, an electric field can disturb their arrangement since they have symmetric charge distributions. The figure below shows the electrons of polar and nonpolar molecules when an electric field is present or not. 10 Figure 2. Polar and nonpolar molecules with and without electric field Image Source: https://www.google.com/search?q=polar+molecules+nonpolar+molecules+dielectric+constant&sxsrf=ALeKk03rOlnKsOSX1RgxVEbKZcaQ MXTwKg:1614824231919&source=lnms&tbm=isch&sa=X&ved=2ahUKEwj2iN-FyZXvAhWaMd4KHROB6kQ_AUoAXoECBAQAw&biw=1280&bih=658#imgrc=EUPaaboX6AqYNM We can also demonstrate this phenomenon using a sensitive electrometer. The figure below shows a dielectric inserted between plates. Readings of its potential difference are shown by the electrometers. When we remove the dielectric, the potential difference returns to its original value. This shows that the charges in the plates did not change. The measure of how effective the substance in reducing the electric field is called dielectric constant. This is represented by symbol K. For the capacitor, it should reduce the electric field and its potential difference. This implies increasing the capacitance since potential difference and capacitance is inversely proportional ( ). Figure 3. The electrometer reads the potential difference before and after the dielectric is inserted between the plates. Image source: https://www.google.com/search?q=effect+of+dielectric+using+electrometer&tbm=isch&ved=2ahUKEwi7 -YLXy5XvAhUODpQKHcOQDEAQ2cCegQIABAA&bih=658&biw=1280#imgrc=fDTK5Fei4vcChM The dielectric can also be expressed as the ratio of the final capacitance C to its initial capacitance C0. 11 It could also be the ratio of initial potential V0 difference to its final potential difference V. On the other hand, This implies that E becomes smaller when the dielectric is present. The decreasing electric field is due to the smaller surface charge density. The induced charge appears on each surface of the dielectric. The induced surface charges are due to the distribution of charges within the dielectric, a property called polarization. Table 1 below shows the dielectric constant K at 20°C. Table 1. Dielectric constant K at 20° Material Vacuum Glass Mica Mylar Neoprene K 1 5-10 3-6 6.70 3.40 Material Teflon Germanium Strontium titanate Water Glycerin K 2.1 16 310 80.4 42.5 Material Benzene Air (1 atm) Air (100 atm) Plexiglass Polyethylene K 2.28 1.00059 1.0548 2.25 3.18 Source: sears, F., Zemansky, M. and Young, H. College Physics 7 t h Edition In deriving the expression for the induced charge, capacitance and potential energy stored in capacitors with dielectric: Solving for the induced charge Rearranging the formula From the previous section, Recall that Algebraic solution Get the common factor ( ) Thus, the expression for the induced charge on a capacitor with dielectric 12 Solving for capacitance Permittivity of dielectric In terms of the permittivity of dielectric, we can express the electric field. The capacitance with dielectric. Since Recall that Thus, the expression for capacitance in terms of permittivity of dielectric, area, and distance of separation. Solving for potential energy In terms of potential energy stored in capacitors, we recall the formula in terms of permittivity of space and electric field. We can express the energy stored with the dielectric constant. Recall that Thus, the expression of potential energy stored in capacitor in terms of electric field and permittivity. Example 5: The parallel plates have an area of 2000 cm 2 and are separated at 1.00 cm apart. The original potential difference between them is 3000 V and it decreased to 1000 V when a sheet of dielectric was inserted. What is the (a) Original capacitance (b) The magnitude of the charge on each plate (c) Capacitance C after dielectric is inserted (d) Dielectric constant K (e) Permittivity of dielectric (f) The magnitude of induced charge (g) Original electric field (h)Electric field after dielectric is inserted A What is/are given? A = 2000 cm 2 ; d = 1.0 cm; V0 = 3000 V; V = 1000 V B What is asked? (a)C0 = ? (b) Q =? (c) C = ? (d) K = ? (e) (f) induced charge (g) E0 =? (h) E= C Are the units No, area should be converted from cm 2 to m 2 consistent with the ( ) formula? 13 E F What strategy must be employed? Solution Distance should be converted from cm to m. Thus, 1.0 cm = 1.0 x 10 -2 m We use the previous formaul presented under dielectric constant section (a) ( ) ( ) (b) ( )( ) (c) (d) (e) ( ) (f) ( ) ( ) ( ) (g) (h) What’s More Activity 2: Qualitative Problems. Direction: Answer the following questions. (1) A student claims that capacitors in parallel combinations have greater equivalent capacitance compared to the magnitude of either the capacitor and less than either the capacitors when connected in series combination. How will you agree or disagree with the claim? (2) A capacitor was made of aluminum foil strips which were separated by Mylar films. The capacitor was subjected to too much voltage and produced holes in Mylar films. The capacitance was still found to have the same magnitude but the voltage was lesser. Why do you think this happened? (3) Is there any kind of material that when inserted between parallel plate capacitors would reduce its capacitance? 14 What I Have Learned Activity 3: Quantitative Problem. Direction: Write your answers on a separate sheet of paper. You may also consult your Physics teacher. (1) Compute the equivalent capacitance of the network. (2)A 300 µF capacitor separated by a distance of 4.0 mm was charged to a potential difference of 250 V. What is the energy density in its region in terms of Joules/m 3 ? (3)Two parallel plates have equal and opposite charges. When space was evacuated, the electric field was 2.80 x 105 V/m. When it was filled with a dielectric material, the electric field was 1.20 x 105 V/m. What is the dielectric constant? What is the material? Criteria Physics Approach 3 Approach is appropriate and complete 2 Approach contains minor errors Procedure Mathematical and logical procedures are clear, complete and connected Diagrams and symbols used are appropriate and complete Description 0 Solution doesn’t indicate an approach Mathematical and logical procedures are missing/contain errors 1 Some of the concepts and principles are missing or inappropriate Most of the mathematical and logical procedures Parts of the diagrams and symbols contain errors Most of the parts of the diagrams and symbols are not useful The entire visualization is wrong or did not include visualization. 15 All procedures are incomplete and contain errors What I Can Do Activity 4. Building Concept Map. Direction: Create a concept map from what you have learned in this module. You can use words, terms, phrases, or formulas in connecting these concepts. Refer to the scoring guide below: Legible (easy to read) Accurate (concepts were used accurately) Complete (sufficient number of relevant concepts and relationships) Sophisticated (finding meaningful connections between relevant concepts) No (0-1) Many inaccuracies (0-2) Yes (2) A few inaccuracies (3-4) No inaccuracies (5) Limited use of concepts (0-2) Some use of concepts (3-4) Sufficient number of concepts (5) Little or none (0-1) Few meaningful connections made (2-4) Some meaningful connections made (5-7) Meaningful and original insights demonstrated (8) Mueller’s Classroom Concept Rubric Assessment Direction: Read each item and write the letter of your choice in the space provided. 1.If the value you computed has an SI unit of , the quantity could be a.potential difference b.dielectric constant c.electric field d.capacitance 2.A charge was removed from one of the plates, the capacitance of the capacitors a.decreases b.stays the same c.increases d.it is halved 3.The energy of a charged capacitors could be found in a.plates b.potential difference c.charges d.electric field 4.The Farad is not equivalent to which of the following units: a. b. 16 c. d. 5.When a slab of insulating material is placed between the plates of a charged capacitor, the electric field becomes a.less b.similar c.greater d.depends on the situation 6.A capacitor has a charge of 0..02 C when connected with a 0 V battery. The capacitance is a.1 µF b.4 µF c.2 µF d.40 µF 7. A 50 µF has a potential difference of 8 V. What is its charge? a.4 x 10 -3 C b.6.25 x 10 - 5 C c.4 x 10 -4 C d.6.25 x 10 -6 C 8.A parallel plate capacitor has an energy of 2.5 J. It must be placed in a potential difference of a.150 V b.350 V c.500 V d.0.25 MV 9.Two 50 µF capacitor are connected in series. The equivalent capacitance of the combination is a.25 µF b.100 µF c.50 µF d.200 µF 10. Two 50 µF capacitor are connected in parallel. The equivalent capacitance of the combination is a.25 µF b.100 µF c.50 µF d.200 µF 11.Which of the following portrays the electric field of a parallel plate capacitor? 17 12. The capacitance of a parallel-plate capacitor depends on I.plate area II. distance of separation III.voltage a.II only b.III only c.I and II only d.II and III only 13. A parallel-plate capacitor has a capacitance C. What is the new capacitance if a dielectric with K = 4 is inserted between its plates? a. b. c.C d.4 C 14.What is the new capacitance if the plate separation is doubled? 18 a. b. c.C d.2 C 15.What can dielectric do for the capacitors? a.decrease the value of C b.decrease the intensity of electric field c.increase the value of C d.increase the intensity of electric field Additional Activities Activity 5. Social Context. Direction: The community is a rich repository of learning opportunities for sources of magnetic forces and fields. Choose one from the following suggested activities in understanding the importance and utilization of electric potential in our daily lives: 1. 2. Conduct simulations on capacitors and dielectric. From this, write a short reflection. Scan the QR code to gain access to the simulations. Making your capacitor. You will need the following materials: aluminum foil, paper, scissors, 2 AA batteries, 2 office fasteners, 2 paper clips, tape, and a multimeter Step 1 . Cut out the strip of wax paper about 3 inches wide. Cut a piece of aluminum foil about inches wide. Step 2. Cut two squares from the aluminum foil strip. Cut the paper so it is about 0.5-inch wider than the aluminum foil at the top and bottom. Cut the paper so it is 4 times that of the width of the aluminum foil. Step 3. Bend the other end of the paper clip until it becomes straight leaving the other part in its original form. Step 4. Place the aluminum foil near the end of the paper. Use a little piece of tape to secure the paper clip. Tape the curved end of the paper clip in the middle of the aluminum foil and make sure the straight portion of the clip is placed outside the wax paper. Step 5. Fold the wax paper so the aluminum foils are covered on each side with paper. 19 Step 6. Place the second foil directly over the same location as the other foil square. Use tape on top to secure it in place. Do step 4 but this time the paper clip must be on the other side opposite the first paper clip. Step 7. Fold the remaining length of paper over the top of the foil square so it is not exposed. You can have the option of rolling the sandwiched foil and paper. Step 8. Attach your home-made capacitor to your battery. Step 9. After some time, measure the amount of voltage present by dialing the multimeter to microvolts. Make sure the probes are properly inserted in the negative and positive ports. Step 10. Check if the multimeter has a reading of voltage stored. Step 1 1 . Prepare a reflection paper on the experience you had while making the capacitor. You may do some revisions or changes and share your observations after applying the revisions or changes. . Capture the QR code and visit this site if you are on the right track. 20 Answer Key General Physics 2 Module 3 References Printed Resources Sears, F., Zemansky, M. and Young, H. (1992). College Physics 7 t h Edition. Addison-Wesley Publishing Company Zitzewits, Haase and Harper (2013). PHYSICS Principles and Problems. The MAcGraw-Hill Companies, Inc. Online References Digital Vector Studios (2021). How to Make a Capacitor. Retrieved last March 2, 2021, from https://sciencewithkids.com/Experiments/Energy-Electricity-Experiments/how-tomake-a-capacitor.html How Equipment Works (n.d.). How defibrillators work explained simply. Retrieved last March 1 , 2021, from https://www.howequipmentworks.com/defibrillator/ Physics Prep. (2020). Virtual Activity: Capacitor Lab. Retrieved last March 3, 2021, from https://www.physics-prep.com/index.php/virtual-activity-capacitor-lab-2 21 22