The document defines electromotive force (e.m.f) as the work done by a source to drive one coulomb of charge around a complete circuit. It states that the e.m.f of a cell or battery refers to the electrical energy produced for each coulomb that passes through it. However, the potential difference, or voltage, across the external terminals is usually lower than the e.m.f due to the internal resistance of the cell or battery, which causes a drop in potential and some of the energy to be lost as heat. The relationship between e.m.f, potential difference, current, and internal resistance is explained.

2. Define Electromotive Force, E (e.m.f)
The work done by a source
in driving one coulomb of charge around a
complete circuit

3. The voltage label
Indicates its’ e.m.f
1.5 V means 1.5 J
of electrical energy
is produced to the
whole circuit when
1 C of charge flows

4. VS Cell
Cell 1.2 V
V
1.5 V
V
Bulb
Closed circuit – Current
flows
e.m.f = reading of voltmeter that is
connected directly across the cell’s
terminals

5. VS
The electrical energy that is
transformed to other forms
of energy when 1 C of charge
passes through a component in a
closed circuit
Refers to the source of electrical
energy

6. 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

7. e.m.f = 1.5 V
This means the cell
gives 1.5 J of
electrical energy to
each coulomb that
passes through it.

8. e.m.f = 1.5 V
Current that flows
through the circuit
also flows through
the battery.

9. 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.

10. e.m.f = 1.5 V
Practically, the
potential
difference will
always be less
than its’ e.m.f.

11. 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

12. 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 = V
inside cell = Ir

13. Pic Credits
• Flickr / Men, it's a charged situation... / Paul
Watson
• Flickr / Hispania Racing Team - Narain
Karthikeyan / Jarod Carruthers
• Flickr / CRW_7960/ Paul Watson