2014 Q2 Electricity

Hi there,

Why does I3 = I1 + I2? This is the 2014, Q2
Here’s my working and the actual answer:

Thank you

Kia ora @Shank21,

Great question! We went over this one in the webinar the other evening, but I realise you’re short on time before your exam so will give you a direct answer.

The I1, I2, I3 relationship is largely determined by which section of the circuit you designate as 1,2, or 3.

In the exam question I would have used the switch numbers to identify which wire or section I was identifying as I1, I2, and I3. So the top wire in the diagram has S1 and I will identify the current through that wire as I1.

I2 will be through the middle wire, and I3 will be through the bottom wire.

As both supplies (11V and 10V) have positive terminals on the left, I will assume the current in each of those branches (I1 and I2) goes from right to left.

When these currents join at the junction to the left of S2, they will combine and form the current I3. This means the current in I3 is the combination of I1 and I2, which can be expressed as I3=I1 + I2

If you identified your currents differently, you could still get full credit.