13.3 Electromotive force and internal resistance

Question 1

What is the internal resistance of a source of electricity due to

Answer 1

Opposition to the flow of charge through the source.

Causes electrical energy produced by the source to be dissipated inside the source when charge flows through

Question 2

What is the electromotive force

Answer 2

The electrical energy per unit charge produced by the source.

Question 3

If electrical energy E is given to a charge Q in the source:

Answer 3

ε = E/Q

Question 4

What is the pd across the terminals of the source

Answer 4

The electrical energy per unit charge delivered by the source when it is in a circuit
Less than the emf whenever current passes through the source.

Question 5

What is the difference between the pd across the terminals of a source and the emf caused by?

Answer 5

Caused by the internal resistance of the source

Question 6

The internal resistance of a source is…

Answer 6

…the loss of potential difference per unit current in the source when current passes through the source

Question 7

The cell emf ε =

Answer 7

IR + Ir

In which:
I = Current
r = Internal resistance
R = External resistance

Question 8

What is the lost pd inside the cell equal to…

Answer 8

…the difference between the cell emf and the pd across its terminals.

Question 9

What is the lost pd in energy terms

Answer 9

The energy per coulomb dissipated or wasted inside the cell due to its internal resistance

Question 10

Power supplied by the cell, Iε =

Answer 10

I^2R + I^2r

Question 11

The power supplied by the cell is equal to…

Answer 11

…the power delivered to R+ the power wasted in the cell due to its internal resistance

Question 12

The power delivered to R =

Answer 12

I^2R = (ε^2/(R+r)^2) x R

Question 13

When a source delivers ______ to a load, maximum power is delivered to the load when the load resistance is equal to the ______ ______ of the source.

Answer 13

Power

Internal resistance

Question 14

How can the pd across the terminals of a cell be measured by

Answer 14

Connecting a high-resistance voltmeter directly across the terminals of the cell

Question 15

How can the current in a circuit be changed

Answer 15

By adjusting the variable resistor

Question 16

How does the terminal pd change as current increases

Answer 16

It decreases

Question 17

Why does the terminal pd decrease as current increases

Answer 17

The terminal pd is equal to the cell emf at zero current. This is because the lost pd is zero at zero current

The graph is a straight line with a negative gradient.

Question 18

Rearranging ε=IR+Ir to become…

Answer 18

… V=ε-Ir

Shows that the graph of terminal pd and current is a negative straight line

Question 19

For current I1, the terminal pd V1=

Answer 19

ε-I1r

Question 20

For current I2, the terminal pd V2=

Answer 20

ε-I2r

Question 21

How can r be calculated

Answer 21

r = V1-V2/I2-I1