Stuck on Level 3 Question

[m4tty]

Hi,

Any help on the workings/equasions for the following question would be greatly appreciated.

Two Capacitors 15F & 45F are connected in series to a 230v 50hz supply. Calculate total capacitance, total charge, total energy stored & inductive reactance.

Thanks in advance for any pointers

Cheers

 

[BS3036]

So... is that average or max total charge, and average or max total energy. I don't think those parts of the question are answerable.

Capacitors in series: 1/C = 1/c1 +1/c2

 

[kme]

Concur.

 

[revjames]

I would have to dig out my BTEC notes to have as hope of answering that! Trouble was I did the BTEC 25 years ago...... vaguely remember questions like this. Rambling now like an old man, sorry...... "I remember when this was all fields..."

 

[BS3036]

By the way... Inductive Reactance? of a capacitor? I suppose you have to assume that they meant capacitive reactance. In which case it is 1/2πfC (in ohms).

 

[miggy]

Capacitance in Series

1/C = 1/15F + 1/45F = (3+1)/45 = 4/45 convert to 45/4

Capacitance = 11.25Farads

Charge [Q] = Capacitance x Voltage

= 11.25F x 230V

Charge = 2587.5 Coulombs

Reactance [Xc] = 1 divide by [2x pi x frequency x Capacitance]

= 1 / 2 x 3.142 x 50Hz x 11.25F

= 1 / 314.2 x 11.25

= 1 /3534.75

Xc = 2.83 ohms

TA DA!

 

[revjames]

Well impressed with that Miggy. well done!

 

[BS3036]

Charge [Q] = Capacitance x Voltage

= 11.25F x 230V

Charge = 2587.5 Coulombs

TA DA!

Very nice and all that but the question is still meaningless. You have given the answer for the charge on a capacitor with a direct voltage of 230V applied. In the case of a capacitor connected to an alternating voltage, the charge would be constantly varying from 0 to a greater value than your answer.

I don't think that the person setting the question understood it himself. Fairly typical of training organisations these days.

Happy Christmas everyone! :Y