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BPhO by topic – circuits

Album of the day…

Make sure you understand the basic concepts and formulae before getting into the questions. If you have any concerns, leave a comment.

Single-loop circuits

EMF with internal resistance

Kirchoff’s Loop Rules

Loop rule: the algebraic sum of the changes in potential encountered in a completed traversal of loop must be zero.

Resistance rules: for a move through a resistance in the direction of the current, the change in potential is -iR; in the opposite direction is +iR.

EMF rule: for a move through an EMF in the direction of the EMF arrow, i.e. from – to +, the change in potential is +E; in the opposite direction is -E.

RC circuit

Charging a capacitor:

Discharging a capacitor:

Instruments – potentiometer and Wheatstone bridge

Three easy non-BPhO circuits questions (w/ answers) to warm up…

Hint 12.4: what can be deduced about this Wheatstone bridge?

Hint 12.5: find the equivalent circuit…

Hint 12.8: when the bridge is obviously not balanced, we use…

I propose the following solutions:

12.4 The key is to spot that the bridge is balanced.
12.5 After drawing the equivalent circuit, you see that it is a balanced bridge.
12.8 When the bridge is not balanced, use Kirchoff’s 2nd law.

BPhO 2016 R1 Section 2 Q2

Do it under timed condition first. If you are stuck, look at the hints 😉

Hint Part (a): nice and easy, potential divider.

Hint Part (b): remember to analyze the circuit… which appliances divide which potential?

Hint Part (c): figure it out yourself, it’s not that difficult…

Hint Part (d): figure out the equivalent circuit. What do you discover?

My solutions are as follows.

BPhO 2012-13 R1 Section 2 Q2

Hint Part (a): (i) consider how the current flows (ii) potential divider (iii) Kirchoff’s loop rule.
Hint Part (b): (i) (ii) (iii) symmetry” suggests that some currents are equal. (iv) (v) consider the points labeled A, B, C and D. Keep track of them as you draw the circuit. Which resistor is in between A and B, B and C etc.?

My solutions:

Sometimes it is better to draw the equivalent circuit.
Sorry for my poor drawing…

BPhO 2015 R1 Section 2 Q4

This is yet another circuit question (w/ hints). Have a go at it and see how many marks you can grab.

BPhO 2014 R1 Section 2 Q2

My solution:

BPhO 2014 R1 Section 2 Q6

My solution:

BPhO 2011 R1 Section 2 Q2

My solutions:

BPhO 2013 R1 Section 2 Q1

My solutions:

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