Electricity: Electrical sources and internal resistance

Question 1

What does the term EMF mean, and how do we find it through calculation and graphically?

Answer 1

The Electromotive Force is the potential difference when no current flows.

E = V + Ir
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Look for the point in the graph where current is zero.

Question 2

What is the internal resistance and how do we find it through calculation and graphically?

Answer 2

The internal resistance is the resistance of the power supply.
Ir = E - V
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The gradient of a V against I graph is equal to -r, make it positive to find r.

Question 3

What is the terminal potential difference and how do we find it?

Answer 3

The T.P.D. is the voltage between the cell in a closed circuit, this will be lower than E.M.F due to internal resistance.

use the equation V = E - Ir

Question 4

What is the short circuit current and how do we find it through calculation and graphically?

Answer 4

The current through the cell when it is connected to itsself

This is calculated through I = V/r
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Set V to 0 on the provided graph V against I to find it.

Question 5

What are lost volts and how do we calculate them?

Answer 5

Lost volts are the difference between the EMF and the t.p.d of a cell.
This is the voltage lost within the battery.
This is calculated by E - T.P.D.