Current Electricity — Part 2: Cells, Kirchhoff’s rules and bridges: visual lesson, CBSE Class 12 Physics

Lesson

Cells, Kirchhoff’s rules and bridges

Part 2 — what a cell really supplies, how to solve any network of cells and resistors, and how a balanced bridge measures resistance precisely.

In this lesson you will

Relate the terminal voltage of a cell to its emf and internal resistance
Combine cells in series and in parallel
Apply Kirchhoff’s junction and loop rules to multi-loop circuits
Derive the balance condition of a Wheatstone bridge and use a metre bridge

Concept

EMF and internal resistance

The emf $E$ of a cell is the potential difference between its terminals when no current flows (the work done per unit charge by the cell’s chemical action). With current $I$ through internal resistance $r$ :

Discharging: $V < E$ . Charging (current forced in at the + terminal): $V = E + Ir$ .
Short circuit ( $R = 0$ ): $I_\text{max} = E/r$ .

V = E - Ir,\qquad I = \dfrac{E}{R + r}

Diagram

Terminal voltage against current

V = E − Ir (E = 12 V, r = 0.5 Ω)

Intercept on the V-axis = E; slope = −r.

Plotting $V$ against $I$ for one cell with different loads gives a straight line: intercept $E$ , slope $-r$ .

Cookies