Explain the concept of internal resistance in a power supply and how it affects the terminal potential difference.

Internal resistance (r) is the opposition to current flow within a power supply itself, causing the terminal potential difference (V) to be less than the EMF (ε) according to the equation: V = ε - Ir

Step-by-Step Solution:
Internal resistance exists due to the physical components within the power supply (e.g., battery chemicals, connecting wires)
As current (I) flows, energy is lost within the power supply as heat due to internal resistance
Terminal p.d. decreases linearly with increasing current: V = ε - Ir
Verify using V-I graph: gradient = -r, y-intercept = ε

Key Mistake to Avoid: Students often forget that internal resistance reduces the terminal p.d., leading to incorrect calculations when the power supply is under load.



Quick Tip: Remember that a real battery's terminal p.d. always drops under load - memorize "EMF minus Ir" as your go-to equation.