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PHY_10_37_V1_TG_ Capacitors.docx
RECOMMENDADTION
on lesson “Capacitors. Capacitance”
This lesson requires students’ pre knowledge about the current flowing in a circuit, circuit components and symbols and the basic understanding of the electricity. Teacher starts the lesson by demonstrating the circuit shown in the LP. Teacher might show only the second part of the circuit (hidden part which consist of the battery and the capacitor will be shown later) where capacitor, switch and the light bulb and show the capacitor discharging by the bulb. After teacher can ask question what makes bulb light for a while. Before demonstrating this teacher should make sure that capacitor is charged by the battery. Aim of this activity is interest students into the topic.
During the main part of the lesson, teacher can give hangouts to student or provide some able students with the internet and ask them investigate their questions by themselves in a small groups. The difficulty of this part of the lesson is that the level of English language might not be enough to understand the text. Teacher can give to students’ keywords in 3 languages and let them use online translator. First groups’ task might be differentiated by extending their question, so students investigate more about the capacitors, especially about their types by the material. If there is an enough time teacher might give these group hangouts from Attachment 1 (next page)
More able students can investigate capacitors in series and parallel connection. Those who are interested in physics more, they can be given an extra task in Attachment 2(next pages).
After each group poster presentation teacher complete their answer and student make notes. Also teacher suggest them solve problems in pairs and compare their answers.
Attachment Types of the capacitors
AIR CAPACITORThere are two sets of parallel plates. One set of plates is fixed and another set of plates is movable. When the knob connected with the capacitor is rotated, the movable set of plates rotates and overlapping area as between fixed and movable plates vary. This causes variation in effective cross-sectional areas of the capacitor. Consequently, the capacitance varies when one rotates the knob attached to the air capacitor. This type of capacitor is generally used to tune the bandwidth of a radio receiver. |
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Plastic Film CapacitorPlastic capacitor can be made also in form of film capacitor. Here, thin strips or films of plastic are kept inside metallic strips. Each metallic strip is connected to side metallic contact layer alternatively; as shown in the figure below. That means, if one metallic strip is connected to left side contact layer, then the very next is connected to right side contact layer. And there are plastic films in between these metallic strips. The terminals of this type of capacitors are also connected to side contact layer and whole assembly is covered with insulated non metallic cover as shown.
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Ceramic CapacitorConstruction of ceramic capacitor is quite simple. Here, one thin ceramic disc is placed between two metal discs and terminals are soldered to the metal discs. Whole assembly is coated with insulated protection coating as shown in the figure below.
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Electrolyte CapacitorVery large value of capacitance can be achieved by this type of capacitor. But working voltage level of this electrolyte capacitor is low and it also suffers from high leakage current. The main disadvantage of this capacitor is that, due to the use of electrolyte, the capacitor is polarized. The polarities are marked against the terminals with + and – sign and the capacitor must be connected to the circuit in proper polarity.
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Silvered Mica CapacitorA silvered mica capacitor is very accurate and reliable capacitor. This type of capacitors has very low tolerance. But on the other hand, cost of this capacitor is quite higher compared to other available capacitors in the market. But this high cost capacitor can easily be compensated by its high quality and performance. A small ceramic disc or cylinder is coated by silver compound. Here, electrical terminal is affixed on the silver coating and the whole assembly is encapsulated in a casing. |
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Attachment 2 Capacitor in series and parallel connection
Capacitors in Parallel Example
Calculate the combined capacitance in micro-Farads (μF) of the following capacitors when they are connected together in a parallel combination:
· a) two capacitors each with a capacitance of 47nF
· b) one capacitor of 470nF connected in parallel to a capacitor of 1μF
a) Total Capacitance,
CT = C1 + C2 = 47nF + 47nF = 94nF or 0.094μF
b) Total Capacitance,
CT = C1 + C2 = 470nF + 1μF
therefore, CT = 470nF + 1000nF = 1470nF or 1.47μF
So, the total or equivalent capacitance, CT of an electrical circuit containing two or more Capacitors in Parallel is the sum of the all the individual capacitance’s added together as the effective area of the plates is increased.
Additional Problems with answers
Additional multiple choice questions: Capacitors (lower level)
42. The magnitude of the charge on the plates of an isolated parallel plate capacitor is doubled. Which one of the following statements is true concerning the capacitance of this parallel-plate system?
(a) The capacitance is decreased to one half of its original value.
(b) The capacitance is increased to twice its original value.
(c) The capacitance remains unchanged.
(d) The capacitance depends on the electric field between the plates.
(e) The capacitance depends on the potential difference across the plates.
43. A parallel plate capacitor with plates of area A and plate separation d is charged so that the potential difference between its plates is V. If the capacitor is then isolated and its plate separation is decreased to d/2, what happens to the potential difference between the plates?
(a) The final potential difference is 4V.
(b) The final potential difference is 2V.
(c) The final potential difference is 0.5V.
(d) The final potential difference is 0.25V.
(e) The final potential difference is V.
44. A parallel plate capacitor with plates of area A and plate separation d is charged so that the potential difference between its plates is V. If the capacitor is then isolated and its plate separation is decreased to d/2, what happens to its capacitance?
(a) The capacitance is twice its original value.
(b) The capacitance is four times its original value.
(c) The capacitance is eight times its original value.
(d) The capacitance is one half of its original value.
(e) The capacitance is unchanged.
45. A parallel plate capacitor is fully charged at a potential V. A dielectric with constant k = 4 is inserted between the plates of the capacitor while the potential difference between the plates remains constant. Which one of the following statements is false concerning this situation?
(a) The energy density remains unchanged.
(b) The capacitance increases by a factor of four.
(c) The stored energy increases by a factor of four.
(d) The charge on the capacitor increases by a factor of four.
(e) The electric field between the plates increases by a factor of four.
46. Which one of the following changes will necessarily increase the capacitance of a capacitor?
(a) decreasing the charge on the plates
(b) increasing the charge on the plates
(c) placing a dielectric between the plates
(d) increasing the potential difference between the plates
(e) decreasing the potential difference between the plates
Additional multiple choice questions: Capacitors (Average level)
The plates of a parallel plate capacitor each have an area of 0.40 m2 and are separated by a distance of 0.02 m. They are charged until the potential difference between the plates is 3000 V. The charged capacitor is then isolated.
56. Determine the magnitude of the electric field between the capacitor plates.
(a) 60 V/m (c) 1.0 ´ 105 V/m (e) 3.0 ´ 105 V/m
(b) 120 V/m (d) 1.5 ´ 105 V/m
57. Determine the value of the capacitance.
(a) 9.0 ´ 10-11 F (c) 3.6 ´ 10-10 F (e) 6.4 ´ 10-10 F
(b) 1.8 ´ 10-10 F (d) 4.8 ´ 10-10 F
58. Determine the magnitude of the charge on either capacitor plate.
(a) 1.8 ´ 10-7 C (c) 4.9 ´ 10-7 C (e) 6.8 ´ 10-7 C
(b) 2.7 ´ 10-7 C (d) 5.4 ´ 10-7 C
59. How much work is required to move a –4.0 mC charge from the negative plate to the positive plate of this system?
(a) –1.2 ´ 10-2 J (c) –2.4 ´ 10-2 J (e) –5.4 ´ 10-2 J
(b) +1.2 ´ 10-2 J (d) +2.4 ´ 10-2 J
60. Suppose that a dielectric sheet is inserted to completely fill the space between the plates and the potential difference between the plates drops to 1000 V. What is the capacitance of the system after the dielectric is inserted?
(a) 1.8 ´ 10-10 F (c) 5.4 ´ 10-10 F (e) 6.8 ´ 10-10 F
(b) 2.7 ´ 10-10 F (d) 6.2 ´ 10-10 F
61. Suppose that a dielectric sheet is inserted to completely fill the space between the plates and the
potential difference between the plates drops to 1000 V. Determine the dielectric constant.
(a) 0.333 (c) 3.0 (e) 2000
(b) 0.666 (d) 6.0
Additional multiple choice questions: Capacitors (Higher level)
Questions 62 through 64 pertain to the situation described below:
The figure below shows four parallel plate capacitors: A, B,
C, and D. Each capacitor carries the same charge q and
has the same plate area A. As suggested by the figure, the plates of
capacitors A and C are separated by a distance d while
those of B and D are separated by a distance 2d.
Capacitors A and B are maintained in vacuum while capacitors C
and D contain dielectrics with constant k = 5.
62. Which list below places the capacitors in order of increasing capacitance?
(a) A, B, C, D (c) A, B, D, C (e) D, C, B, A
(b) B, A, C, D (d) B, A, D, C
63. Which capacitor has the largest potential difference between its plates?
(a) A (c) C (e) A and D are the same
(b) B (d) D and larger than B or C.
64. Which capacitor is storing the greatest amount of electric potential energy?
(a) A (d) D
(b) B (e) Since all four carry the same charge, each will store the
(c) C same amount of energy.
Useful resources:
https://www.brainscape.com/flashcards/capacitors-5193482/packs/7458680
https://www.electronics-tutorials.ws/capacitor/cap_1.html
https://phys.libretexts.org/Bookshelves/University_Physics/Book%3A_University_Physics_(OpenStax)/Map%3A_University_Physics_II_-_Thermodynamics%2C_Electricity%2C_and_Magnetism_(OpenStax)/8%3A_Capacitance/8.1%3A_Capacitors_and_Capacitance
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