1. Alternating Current Capacitors and Inductors are used in a variety of AC circuits.

4. I t The current from a generator or the mains varies sinusoidally

5. AC DC How do you measure the size of the current?

6. I t

7. If the bulbs are the same brightness, the current is effectively the same.

8. The RMS (root mean square) current is the same as the DC current that produces the same power. To calculate it use:

9. 30 Hz AC in a 100μF capacitor using a LabPro

10. AC in a Capacitor PHASE We have seen that the current “through” a capacitor and the voltage across it are out of phase by 90 0

12. Think back to capacitors in DC circuits. When the charge is zero, the capacitor voltage is The current onto the capacitor is zero max

13. When the charge is maximum, the capacitor voltage is The current onto the capacitor is maximum zero

14. Likewise in AC, When the capacitor voltage is zero The current onto the capacitor is max

15. current capacitor voltage phasor diagram

16. Reactance of a Capacitor Reactance of a capacitor is how much it opposes the flow of current.

17. Reactance of a Capacitor Increasing the capacitance decreases the reactance

18. Reactance of a Capacitor Increasing the frequency decreases the reactance high frequencylow frequency

20. RC Circuits What will the voltmeter read? 3 V 4 V V 3 V 4 V 5 V

21. What is the angle between the current and the supply voltage? 3 V 4 V 5 V 53 o current supply voltage

22. What will the capacitor voltage be? 7.1V V 10V 7.1V 10 V What is the angle between the current and the supply voltage?

23. Impedence Impedence is the total of the resiatance and the reactance. It is measured in … OHMS The symbol is Z R XcXc Z

24. What is the impedence of the circuit? 300 Ω 400 Ω 300 Ω 400 Ω 500 Ω

25. What is the impedence of the circuit? 300 Ω 5.3 μF 300 Ω 420 Ω f = 100Hz

26. AC in an Inductor An inductor always opposes ……… a change in current. It produces an EMF that always opposes the changing current.

27. AC in an Inductor

28. current VLVL VRVR VSVS The voltage across the inductor leads the current by 90 o

29. Reactance of an Inductor The reactance of an Inductor is how much it opposes the flow of current.

30. Reactance of an Inductor Increasing the inductance increases the reactance and decreases the current

31. Reactance of an Inductor Increasing the frequency increases the reactance high frequencylow frequency

32. Reason… If the frequency is higher, the current changes more rapidly. This means the flux is changing more rapidly Bigger back emf Smaller current

33. So you can reduce the current either by increasing the inductance of the inductor…… or increasing the frequency of the current.

35. ex RL Circuits What will the voltmeter read? 3 V 4 V V 3 V 4 V 5 V

36. What is the angle between the current and the supply voltage? 3 V 4 V 5 V 53 o current supply voltage

37. What will the inductor voltage be? 7.1V V 10V 7.1V 10 V What is the angle between the current and the supply voltage?

38. What is the impedence of the circuit? 300 Ω 400 Ω 300 Ω 400 Ω 500 Ω

39. What is the impedence of the circuit? 300 Ω 0.48 H 300 Ω 420 Ω f = 100Hz

40. Capacitors v Inductors

41. Reactances and frequency Inductor reactance Capacitor reactance Impedence X f

42. RLC circuits So what happens when we put inductors and capacitors together? Xc XLXL R

43. VCVC VLVL ΣVΣV

44. R XLXL The inductor reactance and capacitor reactance are always opposite. So they can partly (or totally) cancel)

45. Xc R XLXL At low frequency: Capacitor reactance big Inductor reactance small XL- Xc Z (impedence)

46. Xc R XLXL At high frequency: Capacitor reactance small Inductor reactance big XL- Xc Z (impedence)

47. Xc R XLXL At resonant frequency: Capacitor reactance equals Inductor reactance XL= Xc Z (impedence)

48. At resonance…. is the same as impedence equals resistance only

49. Xc R XLXL Z R XLXL Z R XLXL Z Low frequency resonant frequency High frequency

50. impedance is minimum current is max current frequency resonant frequency large impedance small current

51. link to RLC cct http://www.ngsir.netfirms.com/englis hhtm/RLC.htm

52. Applications of AC

53. The frequency of the current is set at the resonant frequency of the RLC circuit. Putting metal in the inductor increases the inductance and reduces the resonant frequency. Current decreases Current drop sets off alarm current frequency resonant frequency