Conservation Of Energy And Change In Circuits

Question 0

Explain why current flowing into a junction is equal to the current flowing out of a junction.

Answer 0

Whatever charge flows into a junction, flows out of the junction. As current is the rate of flow of charge, it follows that whatever current flows into the junction must be equal to the current flowing out of the junction.

Question 1

State two things that are conserved in circuits.

Answer 1

Energy and charge.

Question 2

State Kirchhoff first law.

Answer 2

The total current entering a junction = the total current leaving it.

Question 3

State Kirchhoffs second law.

Answer 3

The total emf around a series circuit = the sum of the pd across each component.

Question 4

State the five rules for applying Kirchhoffs laws to series circuits.

Answer 4

1. There will be the same current at all points of the circuit.
2. The emf is split equally between the components. ie, a circuit containing three components: emf = V1 + V2 + V3
3. The voltage splits proportionally to the resistance, as V = IR
4. IR(total) = IR1 + IR2 + IR3
5. R(total) = R1 + R2 + R3

Question 5

State the 3 rules for applying Kirchhoffs laws to parallel circuits.

Answer 5

1. The current is split at each junction so I = I1 + I2 + I3
2. There is the same potential difference across all components so: V/(R total) = V/(R1) + V/(R2) + V/(R3)
3. Cancelling the V from the above equation, giving:
   1/Rtotal = 1/(R1) + 1/(R2) + 1/(R3)