Example 1 · level 1
A cell of emf 12 V and internal resistance 2 Ω is connected to a 7 Ω resistor. Find the current (in A, correct to 2 decimal places).
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- I = 12 ÷ 9 = 1.33 A.
Answer: 1.33 A
CLASS 11–12 PHYSICS · CURRENT ELECTRICITY
Current electricity: emf, internal resistance, Wheatstone bridge
I = E/(R + r); terminal voltage V = E − Ir. Wheatstone bridge is balanced when P/Q = R/S (no current through the galvanometer).
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Worked examples
Example 2 · level 2
A battery of emf 3 V and internal resistance 2 Ω supplies a 4 Ω load. Find the terminal voltage (in V, correct to 2 decimal places).
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- I = 3/(4 + 2) = 0.5 A.
- V = 3 − 0.5 × 2 = 2 V (equivalently IR).
Answer: 2 V
Example 3 · level 3
In a balanced Wheatstone bridge, P = 20 Ω, Q = 40 Ω and R = 5 Ω. Find the unknown resistance S (in ohm, correct to 2 decimal places), where the balance condition is P/Q = R/S.
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- S = 40 × 5 ÷ 20 = 10 Ω.
Answer: 10 ohm