FOLLOW US ON TWITTER
SHARE THIS PAGE ON FACEBOOK, TWITTER, WHATSAPP ... USING THE BUTTONS ON THE LEFT


YOUR PARTICIPATION FOR THE GROWTH OF PHYSICS REFERENCE BLOG

Saturday, December 14, 2019

A capacitor C is charged using a supply of e.m.f. 8.0 V. It is then discharged through a resistor R.


Question 5
A capacitor C is charged using a supply of e.m.f. 8.0 V. It is then discharged through a resistor R.

The circuit is shown in Fig. 5.1.
Fig. 5.1

The variation with time t of the potential difference V across the resistor R during the
discharge of the capacitor is shown in Fig. 5.2.
Fig. 5.2

(a) During the first 1.0 s of the discharge of the capacitor, 0.13 J of energy is transferred to the resistor R.
Show that the capacitance of the capacitor C is 4500 μF. [3]


(b) Some capacitors, each of capacitance 4500 μF with a maximum working voltage of 6 V, are available.

Draw an arrangement of these capacitors that could provide a total capacitance of 4500 μF for use in the circuit of Fig. 5.1. [2]





Reference: Past Exam Paper – June 2008 Paper 4 Q5





Solution:
(a)
{From the graph,}
at t = 1.0 s, V = 2.5 V

Energy of charged capacitor = ½ CV2  

{Note that this formula is for the energy stored in a charged capacitor (not the energy transferred to the resistor).
Initially (at t = 0), energy of charged capacitor = ½ × C × 82 J
At time t = 1 s, energy of charged capacitor = ½ × C × 2.52 J

During this time, the energy is lost by the capacitor is transferred to the resistor.
Energy transferred = 0.13 J = energy of capacitor at t=0 – energy of capacitor at t=1}

0.13 = ½ × C × (8.02 – 2.52)         
C = 4500 μF                                     


(b)
{When capacitors are connected in series, the equivalent capacitance decreases and when connected in parallel, the equivalent capacitance decreases.

Arranging 2 such capacitors reduces the capacitance to half. So, by connecting another branch of 2 capacitors in parallel, the capacitance doubles, resulting in a capacitance of 400 μF again.

            |--------||------||---------------------|
----------|                                               |-------------------
            |--------||------||---------------------|

}

No comments:

Post a Comment

If it's a past exam question, do not include links to the paper. Only the reference.
Comments will only be published after moderation

Currently Viewing: Physics Reference | A capacitor C is charged using a supply of e.m.f. 8.0 V. It is then discharged through a resistor R.