Grade 12 - Electricity and Magnetism (Capacitance)

PRESCRIBED LEARNING OUTCOMES

• build a timing circuit that uses capacitance
• define capacitance
• describe the role of a capacitor in simple circuits
• use an exponential equation to represent the charging and discharging cycle of a capacitor
• use the formula t = RC to determine the time constant of a capacitor in an RC circuit
• use an oscilloscope to measure the time constant of a charging and discharging capacitor
• identify several common applications of capacitors in circuitry
• calculate the equivalent capacitance in series and parallel combinations

SUGGESTED INSTRUCTIONAL STRATEGIES

• Challenge students to build variable-period strobe circuits and demonstrate them to the class. Lead a discussion on where such circuits might be used (e.g., timing of a car, disco lights).
• Bring in a camera flash and have students explain how it can produce a large voltage from a 1.5 V cell.
• Ask students to investigate both series and parallel capacitance circuits and identify the characteristics of each. Have them determine when it is advantageous to have capacitance in series or in parallel and identify some applications.
• Have students explore exponential relationships and relate them to capacitance.
• Provide real-life problems involving voltage, time constant, resistance, and capacitance in RC circuits. Ask students to develop solutions to the problems and present them to the class.
• Instruct each student to hook up a voltmeter in parallel with a slowly charging and discharging capacitor, then estimate the time constant of the circuit and compare this to the actual measured time constant.
• Lead a discussion on the uses of capacitors in circuits (e.g., filters, timers). Ask students to research the use of capacitors in some area of technology and report findings to the class.
• Have each student use an oscilloscope to observe the effect of a capacitor in parallel with a diode in a rectifier connected across an AC source. Ask students to describe the effect and explain why it occurs.
• Provide students in groups with simple circuit diagrams and the needed materials to construct xenon flashes. Have them assemble, calibrate, and use the flashes. As an extension, students could design their own light shows.

SUGGESTED ASSESSMENT STRATEGIES

• As students describe the role of capacitors in circuits, check their understanding by asking the following questions:
  • What are the limitations on the voltage in the circuit?
  • How could the capacitance be increased?
  • What effect would increasing or decreasing the capacitor have?
  • Where would such a capacitance circuit be used?
  Note the extent to which students' responses indicate an understanding of the role, principles involved, and possible applications of capacitors.
• After students have constructed timing circuits, have them complete sentence stems such as:
  • I understand that the limitations of my design are _________________ .
  • An application of this design would be _________________ .
  • One improvement would be _________________ .
  Note the extent to which students can critically evaluate their own designs.
• Have students describe applications of capacitors in everyday life (e.g., in automotive circuits, photography flash units, cardiac defibrillation equipment, electronics circuits). Note the extent to which students:
  • identify the advantages of the capacitor in each application
  • explain the limitations of the capacitor in each application
  • predict what would happen if the capacitor failed to function
  • select and accurately perform appropriate calculations
• As students use equations to explore situations that involve capacitance, check for understanding of:
  • significance of the exponential function
  • relevant variables
  • implications of the calculated values (e.g., in the capacitance required in a particular application)

RECOMMENDED LEARNING RESOURCES

Applications of Physics 11 and 12 develops an understanding of the concepts of physics through the design, construction, and use of a variety of devices. Although the resources listed below provide a range of teaching and learning opportunities in physics, none of them provide a comprehensive resources package that completely supports the philosophy and approach of the British Columbia curriculum in the applications of physics. Please see the Appendix B Introduction for further information.

Print Materials

• Advanced Level Physics, Seventh Edition
• Conceptual Physics, Third Edition
• Heinemann Advanced Science: Physics
• Physics: Principles and Problems, American Edition
• Physics: Principles and Problems, Canadian Edition

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Last Modified: April 1, 1998.
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