Q1. Consider the circuit shown (the switch is to be regarded as CLOSED so that the circuit is live).


The resistors X, Y and Z are connected in parallel as shown. The total potential difference is 24 V and the total current on the circuit is 11 A.


  1. Find the potential difference across resistor Y


As the resistors are all connected in parallel, they will receive the same potential difference as each other (unlike resistors in series which share the total potential difference). Therefore the potential difference across resistor Y would be 24 V


  1. Find the current passing through resistor Y


In parallel circuits the current is "shared" across all branches, therefore the total current of 11 A will be shared across the three branches. In this case the current across resistor Y would be 11 - (3 + 2) = 6 A.


Q2. Consider this circuit:



Given that 0.6 A flows through the 10 Ω resistor, calculate:


  1. The current flowing through the 20 Ω resistor


We know that this is a parallel circuit and as such the same potential difference will flow through both branches/loops.


The potential difference across the 10 ohm resistor can be calculated by virtue of the fact we have the resistance and the current, therefore simply applying V = IR will give us the potential difference across this 10 ohm resistor.


If V = IR then V = 0.6 x 10 = 6V


Also, because we know that this is a parallel circuit and that all branches of a parallel circuit will receive the same potential difference, we can calculate the current flowing through the 20 resistor by manipulating V = IR as I = V/R = 6/20 = 0.3 A.

  1. The current flowing through the cell


Since we now know that we have 0.6 A flowing through the 10 ohm resistor and 0.3 A flowing through the 20 ohm resistor, the total current in the circuit must be the sum of the two, i.e. 0.3+0.6 = 0.9 A.

  1. The potential difference (voltage) provided by the cell


There is no mathematics involved in answering this part of the question, since in part one we had to calculate the current flowing through the 20 ohm resistor. The same potential difference will flow across all branches in the parallel circuit and we established that the potential difference across the 10 ohm resistor was 6 V because we were given the current 0.6 A and the resistance of 10 ohms. The potential difference/voltage provided by the cell is therefore 6 V


Q3. In the circuit below, lamps P and Q are identical. The reading on the ammeter is 3A. The cell shown is of emf (pd). 6V.



(a) Calculate the current passing through Lamp P


You are told that the reading on the ammeter is 3 A, and that the potential difference supplied by the cell is 6 V. This is a parallel circuit with two identical components therefore the available current will split equally at the junction between the loops, in other words lamp P and lamp Q will both receive 1.5 A.


(b) What would the reading be on a volt meter connected across Lamp P ?


If we connected a voltmeter across lamp P, as this is a parallel circuit and the potential difference is the same as the source potential difference across all of the loops, the reading on this meter would be 6 V.


(c) Calculate the resistance of Lamp P


Now that we know the potential difference across lamp P and the current passing through it, we can calculate the resistance using V = IR rearranged in terms of R = V/I and substituting our known values for V and I:






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