1.11 Internal Resistance and Electromotive Force

Question 1

What is the internal resistance?

Answer 1

The resistance within a source of e.m.f

Question 2

What are the two effects of the internal resistance

Answer 2

1. As more current is drawn(from the supply), the voltage across the terminals ( of the supply ) fall
2. Source of e.m.f is always less than 100% efficient as energy is dissipated as heat within it

Question 3

How could you retrieve the value of voltage across a power supply?

Answer 3

Measure the voltage when no current is being drawn from the supply

Question 4

State the equation for e.m.f of a cell

Answer 4

E= I(R+r) . Also may be written as E=V+Ir
E= e.m.f of the cell
I= current
R= load resistance
r= internal resistance
V= voltage across all the terminals

Question 5

What is the terminal potential difference.

Answer 5

The potential difference across the load resistance
I.e V=IR

Question 6

What is the maximum current that can be drawn from a power supply, state the equation

Answer 6

When the cell is short circuited I.e load resistance=0
I= E/R
I= current
E= e.m.f of a cell
R= load resistance

Question 7

EXAMPLE:
When the current drawn form a cell is 1A, the voltage across the cell’s terminals is 0.5V. When the current drawn is 0.5A, the terminal p.d is 1V.
Calculate the e.m.f of the cell and its internal resistance

Answer 7

1. Terminal p.d.= 0.5V, I= 1A, so R= 0.5/1= 0.5ohms and E= 1(0.5+r) = 0.5 + r
2. Terminal p.d.= 1V, I= 0.5A, so R= 1/0.5= 2ohms and E= 0.5(2+r)= 1+0.5r
3. SUBTRACT 2 from 1 to give, 0= 0.5r -0.5
   Hence r= 1 and E= 1.5V

Question 8

EXAMPLE:
When a 12V battery is short circuited, the current drawn is 6A.
What current would u expect to flow when the load resistor = 4ohms

Answer 8

Internal resistance= V/I= 12/6= 2ohms ( battery is short circuited therefore maximum current)
When load=4, total = 4+2=6ohms and I= 12/6= 2A

Question 9

Describe how you would carry out an experiment to find the internal resistance of a cell

Answer 9

Set up a cell, amp meter, variable resistor ( with volt meter in parallel )
Take values for the terminal voltage , V, and current drawn from cell, I, for different load resistances
Results are interpreted by
E=V +Ir
V= E- Ir
Therefore a graph of V against I gives a straight line of gradient -r and y axis intercept of E