A-LEVEL Physics: 5.4.1: Electromotive Force & Internal Resistance (SaveMyExams) Flashcards
What happens when Charge Passes Through a Power Supply (eg a Battery)?
It Gains Electrical Energy.
What is ‘Electromotive Force (EMF)’?
EMF is the Total Amount of Chemical Energy Converted to Electrical Energy Per Coulomb of Charge, from the Source to the Circuit.
EMF = …
Energy Transferred / Charge
ε=E/Q
Symbol of Electromotive Force (EMF):
Epsilon, ε.
EMF is Not Actually a Force, & is Measured in…
Volts (V).
EMF is Equal to…
The Potential Difference Across the Cell when No Current is Flowing Through it. (Before it is Connected to a Circuit)
How Can the EMF of a Source be Measured?
The EMF of a Source Can be Measured by Connecting a High-Resistance Voltmeter in Parallel with the Source.
What is ‘Terminal Potential Difference (Vterminal)’?
Vterminal is the PD that is Actually Supplied to the Circuit by a Source (after Some Voltage is Lost due to Rinternal of the Source).
If there was No ___ ___, the Terminal PD would be Equal to the EMF.
Internal Resistance.
The Vterminal of a Source is Always Less than the EMF of the Source. Why?
Because the Cell has Internal Resistance when Connected to a Circuit.
Explain ‘Internal Resistance’ of a Source: (3)
-When a Source is Connected to a Circuit, Current Begins to Flow.
-When Current Flows Through the Source, their is Some Resistance, Called ‘Internal Resistance’.
-As Resistance Opposes Current, this Reduces the Energy Transferred Per Unit Charge (Voltage) Available to the Rest of the External Circuit. This Difference is Called ‘Lost Volts’, & is Caused by Rinternal.
What is ‘Lost Volts’?
Lost Volts is the Energy Transferred Per Unit Charge (Voltage) that is Lost by the Source, Due to the Internal Resistance.
The Voltage Lost in the Cell Due to Internal Resistance.
Vemf - Vterminal
EMF (ε) = …
ε = V + Ir
EMF = Voltage + Internal Resistance
All Power Supplies have Some Resistance Between their Terminals. What is this Called?
Internal Resistance (Rinternal).
What is ‘Internal Resistance’?
Internal Resistance is the Resistance of the Materials Within a Source.
What Causes the Cell to Become Warm Over Time?
The Cell Becomes Warm Over Time due to Energy being Dissipated from it, Due to the Internal Resistance.
