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# 7.4 electromotive force and internal resistance

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Malaysia SPM syllabus Physics Chapter 7 Part 4: Electromotive force and internal resistance

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### 7.4 electromotive force and internal resistance

1. 1. Define Electromotive Force, E (e.m.f) The work done by a sourcein driving one coulomb of charge around a complete circuit
2. 2. The voltage labelIndicates its’ e.m.f 1.5 V means 1.5 Jof electrical energyis produced to thewhole circuit when1 C of charge flows
3. 3. VS Cell Cell 1.2 V V 1.5 V V Bulb Closed circuit – Current flows e.m.f = reading of voltmeter that isconnected directly across the cell’s terminals
4. 4. VS The electrical energy that is transformed to other forms of energy when 1 C of charge passes through a component in a closed circuitRefers to the source of electrical energy
5. 5. So why the drop in potential difference? Cell Cell 1.2 V V 1.5 V V Bulb Caused by internal resistance of the cell
6. 6. e.m.f = 1.5 V This means the cell gives 1.5 J of electrical energy to each coulomb that passes through it.
7. 7. e.m.f = 1.5 V Current that flows through the circuit also flows through the battery.
8. 8. e.m.f = 1.5 V Some of the energy per charge the battery provides will be used to overcome the internal resistance of the battery.
9. 9. e.m.f = 1.5 V Practically, the potential difference will always be less than its’ e.m.f.
10. 10. Internal Resistance, r “Resistance against the moving charge due to the electrolyte in the source or cell” Driving the charge against the internal resistance requires work As the charge flows through the cell, energy is lost in the form of heat, causing a drop in potential difference
11. 11. Relationship between E, V, I and r E.m.f of Cell = E r Dry Cell Potential Drop in difference to E potential external difference R circuit = Vinside cell = Ir
12. 12. Pic Credits• Flickr / Men, its a charged situation... / Paul Watson• Flickr / Hispania Racing Team - Narain Karthikeyan / Jarod Carruthers• Flickr / CRW_7960/ Paul Watson