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AP Unit III B 2

AP Unit III B 2. Capacitors. a) Students should understand the definition and function of capacitance, so they can: (1) Relate stored charge and voltage for a capacitor (2) Relate Voltage, Charge and stored energy for a capacitor.

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AP Unit III B 2

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  1. AP Unit III B 2 Capacitors

  2. a) Students should understand the definition and function of capacitance, so they can: • (1) Relate stored charge and voltage for a capacitor • (2) Relate Voltage, Charge and stored energy for a capacitor. • (3) Recognize situations in which energy stored in a capacitor is converted to other forms.

  3. b) Students should understand the physics of a parallel plate capacitor, so they can: • (1) Describe the electric field inside the capacitor, and relate the strength of this field to the potential difference between the plates and the plate separation • (4) Determine how changes in dimension will affect the value of the capacitance

  4. Capacitance • Physical Capacitance of two parallel plates of area A and distance apart d. Capacitance is measured in Farads (F) • C = A o/d where o is the permittivity of free space (Vacuum Permittivity = 8.85 x 10 -12 C2/Nm2 • Capacitance of a conductor • C =Q/V • 1. Calculate the capacitance of the following capacitor which has an area of 2 m2 and a distance apart of 3 cm. If the potential difference across the capacitor is 2 000 Volts calculate the charge on the plates

  5. Capacitors in combination • Capacitors in series • 1/CT = 1/C1 + 1/C2 + 1/C3 • Capacitors in parallel • CT = C1 + C2 + C3

  6. 1.Calculate the total capacitance in the following circuit and the charge on each capacitor • 2. If the capacitors were in series what would be the total capacitance and the charge on each capacitor • http://www.plus2physics.com/capacitors/capacitors-board_ques-3.gif

  7. Discharging a capacitor • Q = Q0 e-t/CR • when t = CR this is known as the time constant for the circuit and this is the time for the capacitor to drop to 0.37 (1/e)of its original value I = Q/CR dQ/dt = Q/CR 1/q x dQ = dt /CR Integrate between 0 and Q0 Log Q = Log Q0 t/CR Q = Q0 e-t/CR Q • Charging a capacitor • Q = Qf (1 – e-t/CR) • http://www.ieee-virtual-museum.org t

  8. Energy of a Capacitor • Since V = Energy per Coulomb then energy = Coulombs x Average pd V or QVaverage • When charging a capacitor the pd increases from to 0 to V. The Vaverage is V/2. • So the energy (E) is equal to ½ QV • E=1/2 QV • 0r since Q = VC then E= ½ CV2 V Q

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