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Capacitors

Capacitors. Dielectric ( Insulator ). Units are Farads, F. 0 V. + 5V. Charge cannot flow through the dielectric. Charge is stored on the plates. As charge stored increases the pd across the plates increases. If the working voltage of the capacitor is exceeded the dielectric is destroyed.

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Capacitors

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  1. Capacitors Dielectric ( Insulator ) Units are Farads, F 0 V + 5V Charge cannot flow through the dielectric. Charge is stored on the plates. As charge stored increases the pd across the plates increases. If the working voltage of the capacitor is exceeded the dielectric is destroyed.

  2. Capacitors 2 +5 V Measure pd across the capacitor as a function of time V Use Q = I x t to calculate the charge stored on the plates at specific times 0 V Constant Current (I) source Plot Q v V

  3. Q 0 V Capacitors 3 Gradient of graph = capacitance

  4. Capacitors 4 Energy is transferred when charge is stored on the plates. The external power supply must do work to overcome the electrostatic repulsion of the charges. This energy can be used later for example to ‘light up a camera flash’. The work done charging the capacitor up can be found from calculating the area under the QV graph. Ee = ½.Q.V The ½ factor arise as the pd across the plates is constantly changing and we calculate the average pd [( final + initial pd) / 2]

  5. Capacitors 5 The 2 equations mentioned so far Ee = ½.Q.V andcan be combined to give 2 other equations for working out the energy stored on the plates of a capacitor. Make sure you can do this.

  6. A + + + + A B – – – – Supply p.d. across voltage capacitor Current 0 0 time time Charge/Discharge of a Capacitor. The circuit shown can be used to investigate the charging and discharging of a capacitor. Charge graphs

  7. A + + + + A B – – – – Current Supply p.d. across voltage capacitor 0 time 0 time Charge/Discharge of a Capacitor. Discharge graphs

  8. Small R Current Large R 0 time The larger the capacitance the longer the charging time (V=Q/C larger capacitance requires more charge to raise to the same p.d. as a smaller C.) The larger the resistance the smaller the initial charging current (V=IR), longer it takes to charge the capacitor as Q = It

  9. Small R Current Large R 0 time The area under current/time graph = charge Q If only R changed, all I/t curves will have different starting I (=V/R) but the same area since Q remains the same.

  10. A A B 10·0 mF 5·0 V Capacitors 10 Capacitor example. • The capacitor is initially uncharged. 5kΩ The switch S is moved to ‘B’ and the capacitor begins to charge. What is the p.d. across the capacitor when the p.d. across the resistor is 3·0 V? Pd = 5.0 – 3.0 = 2.0 V

  11. A A B 10·0 mF 5·0 V Capacitors 11 (b) When the capacitor is fully charged, calculate (i) the charge stored on the capacitor (ii) the energy stored in the capacitor 5kΩ When capacitor is fully charged the pd across it equals the supply pd.

  12. Capacitors 12 Calculate the charge stored on the plates of the capacitor when the current flowing is 0.8 mA. Calculate the pd across the resistor using Ohm’s Law V = I x R = 0.8 x 10-3 x 5x 103 V = 4.0 V PD across the capacitor = supply pd – pd across R Vc = 5.0 – 4.0 = 1.0 V Then use C = Q/V Q = C x V Q = 10 x 10-6 x 1.0 Q = 0.01 mC

  13. Capacitors Uses • Possible functions of a capacitor: • Storing energy, storing information • Storing charge, • Blocking d.c. while passing a.c. • Use in tuning circuits ( TV and radio receivers). • Microphones ( lapel microphones worn by TV presenters ) • touch screens

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