Solving Problems with Kirchhoff's Laws (CIE A Level Physics)

For the circuit below, state the readings of ammeters A₁, A₂  and A₃.

Answer:

Step 1: Calculate the current through ammeter A₁

• The 0.270 A and A₁ are connected in series
• The current entering the cell must equal the current out of it at A₁ 
• Therefore A₁ = 0.270 A

Step 2: Calculate the current through ammeter A₂

• From Kirchhoff's first law, the total current into a junction must equal the total current out of it

Step 3: Calculate the current through ammeter A₃

• Applying Kirchhoff's first law again gives:

For the circuit below, state the readings of the voltmeters V₁, V₂  and V₃.

All the lamps and resistors have the same resistance.

Answer:

Step 1: Calculate the potential difference shown by voltmeter V₁

• Kirchhoff's second law states that the sum of the three components is equal to the e.m.f of the supply

Step 2: Calculate the potential difference shown by voltmeter V₂

• Applying Kirchhoff's second law to this loop
• Both lamps have the same resistance R and the same current I through them so they share the energy equally and have the same potential difference

Step 3: Calculate the potential difference shown by voltmeter V₃

• Applying Kirchhoff's second law to this loop