AP Physics B & C –Two Free-Response Questions (for practice) on Direct Current Circuits

Free response questions help the examiner to assess your standard from different angles and to some extent the evaluation will be subjective. But you can rest assured that the questions are so set that they are as specific as practicable. You must be very careful in presenting your answers in respect of free-response questions. Since the words “determine”, “derive”, “calculate”, “ justify”, “explain”, “what is”, “sketch” and “plot” have precise meanings in the AP Physics examination, you must thoroughly understand their significance so that you can give enough details wherever required and refrain from giving unwanted details where they will be sheer waste of time.

Here are two typical free response questions on direct current circuits. The first question is meant for both AP Physics B and C while the second question is meant for AP Physics C only.

(1) A 10 V battery of negligible internal resistance is connected in series with a 4 Ω resistor R₁, a 6 Ω resistor R₂ and a 2 µF capacitor C as shown in the figure. A series combination of a 6 Ω resistor R₃ and a switch S is connected across the series combination of R₂ and C. The switch S is initially open and the battery has been connected for a long time.

(a) Determine the voltage across the capacitor C and the magnitude of the charge on one of its plates.

(b) The switch S is now closed. Determine the voltage across the capacitor C immediately after closing the switch S.

(c) Calculate the voltage across the capacitor C a long time after closing the switch S.

(d) Calculate the energy stored in the capacitor C a long time after closing the switch S.

(e) On closing the switch S, will the energy stored in the capacitor increase, decrease or remain unchanged? Put a tick mark against the correct option

Increase _____ Decrease _____ Remain unchanged _____

Justify your answer.

The above question carries 15 points.

(2) In the adjoining figure the capacitor C is initially uncharged and the switches S₁ and S₂ are open. The battery has negligible internal resistance.

(a) The switches S₁ and S₂ are closed simultaneously. What will be the voltage across the resistor R₂ immediately after closing the switches S₁ and S₂? Justify your answer.

(b) Determine the current through the resistor R₂ immediately after closing the switches S₁ and S₂.

(c) A long time after closing the switches S₁ and S₂, the voltage across the resistor R₁ was found to be 6 V. Calculate the value of the resistor R₂.

(d) After a long time the switch S₁ is opened but the switch S₂ is retained in the closed condition. Calculate the voltage across the resistor R₂ at the instant when 0.6 seconds have been elapsed after opening switch S₁.

This question carries 15 points.

Try to answer the above questions. I’ll be back shortly with model answers for you.