EMF, Internal Resistance & Cells — A/L Physics

What must I remember about EMF, Internal Resistance & Cells?

• Electromotive Force (EMF, E): Total chemical/non-electrostatic work done per unit charge in driving electric charge completely around a closed circuit: E = W / Q (units Volt = J/C).
• Internal Resistance (r): Intrinsic opposition offered to current flow by the chemicals, electrodes, and electrolytes inside the cell itself.
• Terminal Potential Difference (V): Voltage across cell terminals under external load: V = E - I r (when delivering current); Lost Volts = I r (potential drop wasted inside cell).
• Cell Operating Regimes: 1) Open Circuit (I = 0): V = E (terminal voltage equals true emf); 2) Delivering current: V = E - I r; 3) Short Circuit (R = 0): Maximum short-circuit current I_sc = E / r, V = 0; 4) Recharging cell: Terminal voltage applied V = E + I r.

Test yourself (1 minute)

1. Maximum Power Transfer Theorem

   A real power source of emf E and internal resistance r transfers MAXIMUM power to an external variable load R when load resistance equals internal resistance: R = r; Maximum Power P_max = E² / (4r); efficiency at maximum power transfer is exactly 50%.

2. Measuring EMF vs Terminal Voltage

   A standard voltmeter draws current, thus always measuring V = E - Ir < E; to measure true EMF E, use a Potentiometer (null-deflection, draws zero current at balance point).

3. Graph of V vs I

   Linear plot of V = -r I + E yields vertical intercept E and negative slope -r.

Practise EMF, Internal Resistance & Cells questions today

Source: NIE G.C.E. (A/L) Physics Resource Book - Unit 7: Current Electricity

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Where does EMF, Internal Resistance & Cells fit in the syllabus?

Unit 7.3 · Year 2 · Term 1