How can this capacitator discharge? (Make: Electronics experiment 9)

Question

I am trying to get through Make: Electronics book and I cannot understand, how does capacitor in following circuit discharge when I press B button.

Wouldn't the current just go in the circle that push button completed, since there is no loop with ground?

Answer 1

The current goes in a circle and that discharges the capacitor.

Capacitors store energy by charging the two plates in opposite "directions". A discharged capacitor has no charge on either plate. When you charge it, one plate gets positively charged and the other plate gets negatively charged.

When you connect the capacitor in a circle, the two plates equalize again, which means the capacitor is discharged.

Answer 2

Yes, you are correct. The meter will give a deflection when A is pressed but none when B is pressed.

The circuit does not look very good.

• The 10k resistor is in-circuit when A is pressed and will limit the current in the circuit to a peak of \$ I = \frac {V}{R} = \frac {9}{10k} = 0.9 \ \text {mA} \$.
• B, however, just shorts out the capacitor as you noticed. This is generally not done as there is no current limiting other than the internal resistance of the capacitor. The high initial current may cause arcing and damage the switch.

I would think that they wired it that way to prevent a dead short on the 9 V supply if A and B are pressed simultaneously.