Electromotive force (emf) and potential difference (pd)

The Concept of Electromotive Force (emf) and Potential Difference (PD)
Explain the concept of electromotive force (emf) and potential difference (pd) Potential difference (P.d) is the difference in potential between two charged points of conductor. It is measured in volts with the unit V.

Electromotive force

the voltage developed by any source of electrical energy such as a
battery or dynamo. It is generally defined as the electrical potential
for a source in a circui. It is measured in volt with the unit V.

Internal Resistance (r)is
an opposition offered in the batteries or power supplies which have the
effect of reducing the output potential difference as the current
supplied increases.
The voltmeter in the following figure shows the ‘lost volts’.
  • Set the variable resistor in the circuit to 10Ω.
  • Close the switch and note the values of the p.d across the internal resistance and the load resistance.
  • Suppose the result shows a p.d across 10Ω load resistance of 7.5V and a p.d across the
  • 2Ω internal resistance of 1.5V the e.m.f of battery is 9V.
  • The p.d actually available at the battery terminals is called the terminal p.d. If the e.m.f of the battery is Vtpd = Terminal, p.d across the internal resistance = Vlost.Thus E = Vtpd – Vlost.
  • The
    Electromotive force (e.m.f) can be regarded as the total potential
    difference including the potential difference lost across the internal
    resistance of the battery.
  • The SI unit of potential difference or electromotive force is the Volt (Symbol V).
Resistance (R)is an opposition offered by resistor to the flow of an electric current.
From Ohm’s law, R = (V/I)
Resistor is a component that has resistance or is an instrument used to opposes the flow of current.
Variable resistor (rheostat) is a resistor whose resistance can be changed smoothly in order to change the current flowing.
Resistance box consists
of a number of resistors connect in series through thick brass blocks
(any of the resistors may be short-circuited by putting a plug into the
associated socket).
Wire resistorsare
lengths of wire of known resistance. The resistors used in electronic
circuits are made from carbon or metal film and the value of resistance
is shown by colour coded rings.
measured inohms. The resistance of a conductor may be determined by
passing a steady current (I) through it and at the same time recording
the corresponding voltage across it.
The SI Units of Electromotive Force and Potential Difference
State the SI units of electromotive force and potential difference
Electromotive Force (e.m.f.) of
a source is the energy converted from non-electrical to electrical form
when one coulomb of positive charge passes through the source.
SI unit: Volt (V)
E=W/Q, where E = e.m.f., W = work done by source, Q = amount of positive charges
The potential difference between
two points is defined as the energy converted from electrical to other
forms when a current of positive charge passes between the two points.
TheSI unit: Volt (V)
where V = potential difference, W = work done in driving the charge
between the two points, Q = amount of positive charge.
IMPORTANT:There can be e.m.f. without a closed circuit. BUT there cannot be a potential difference without a closed circuit.
Electromotive Force of a Cell and Potential Difference
Measure electromotive force of a cell and potential difference across a conductor
is a device used to compare the e.m.f. (electromotive force) of two
cells, to measure the internal resistance of a cell, and potential
difference across a resistor. It consists of a long wire of uniform
cross-sectional area and of 10 m in length. The material of wire should
have a high resistivity and low temperature coefficient. The wires are
stretched parallel to each other on a wooden board. The wires are joined
in series by using thick copper strips. A metre scale is also attached
on the wooden board.
The potentiometer
works on the principle that when a constant current flows through a
wire of uniform cross sectional area, potential difference between its
two points is directly proportional to the length of the wire between
the two points.
Electromotive force (emf)
is a measurement of the energy that causes current to flow through a
circuit. It is the energy provided by a cell or battery per coulomb of
charge passing through it. It can also be defined as the potential
difference across the terminals of a cell, when no current flows through
it. Electromotive force is also known as voltage, and it is measured in
volts. Electromotive force is not truly a force; rather, it is a
measurement of energy per unit charge.
Measuring potential difference
difference is measured using a device called a voltmeter. Just like
ammeters, some types have a pointer on a dial, but most have a digital
display. However, unlike an ammeter, you must connect the voltmeter in
parallel to measure the potential difference across a component in a

A circuit diagram showing a voltmeter in parallel with a lamp
two components are connected in parallel, you cannot follow the circuit
through both components from one side to the other without lifting your
finger or going back over the path you have already taken.
Simple cellis the cell consists of copper and zinc cathode with dilute sulphuric acid as Electrolyte.
The Mode of Action of a Dry Cell (Leclanche)
Describe the mode of action of a dry cell (Leclanche)
Action of simple cells;
  1. At cathode:
    The zinc plate dissolves in the sulphuric acid solution and liberates
    electrons into the external circuit. The metal discus had to be of
    different material Volta used copper and zinc discs sand witched by
    cloth soaked in salt water. The combined device was called a voltaic
    pile. Volta also obtained the same effects by using copper and zinc
    plates dipped in dilute sulphunic acid. Volta called these devices,
    arranged in series. The “ Crown of cups” zn – 2e – Zn 2+ The 2n 2+ ions go into solution.
  2. At anode: Positively charged hydrogen lons (H+) are attracted towards the negatively charged copper plate2H + + 2e —– H2.The
    chemical reaction in the cell creates a potential different between
    plates, causing electrons to flow when the two plates are joined with a
    wire. The electrons flow is maintained by the chemical change that
    occurs when the zinc dissolves in the sulphuric acid. Since simple cell,
    which is able to adrive an electric current through a circuit, is said
    to be a source of electromotive force (e.m.f)
Voltage of Combination of Cells in Series and Parallel
Determine voltage of combination of cells in series and parallel
Connecting in Series
connecting your batteries in Series you are doubling the voltage while
maintaining the same capacity rating (amp hours). This might be used in a
scooter, Power Wheels kids vehicle, or other applications. Just use a
jumper wire between the negative of the first battery and the positive
of the second battery. Run your negative wire off of the open connector
from the first battery and your positive off of the open connector on
your second battery.

Connecting in series (double voltage, same capacity [ah]
Connecting in parallel
connecting cells in parallel, you are doubling the capacity (amp hours)
of the battery while maintaining the voltage of one of the individual
batteries. This would be used in applications such as laptop batteries,
some scooters, some ups backups, etc. Use a jumper wire between the
positives of both batteries and another jumper wire between the
negatives of both batteries. Connect your positive and negative wires to
the same battery to run to your application.

Connecting in parallel (double voltage, same capacity [ah]
The Cell Defects
Identify the cell defects
Simple Have two main defects which cause the current to diminish quickly when the cell is being used.
Two defects of simple cell are:
  1. Polarisation
  2. Local Action
is the defect occurs in simple cell caused by the formation of hydrogen
bubbles around the copper plate.These bubbles insulate the copper plate
and prevent other positive hydrogen ions from receiving electrons from
the copper plate to become neutral.
hydrogen ions accumulating at the copper plate repels other Hydrogen
ions ( This defect is called Back e.m.f and opposes or weakens the main
e.m.f of the cell).
How to minimize polarisation
  • Polarisation
    can be minimised by using suitable oxidising agents, called
    depolarisers, to remove the hydrogen. An example of depolarisers for
    hydrogen is potassium dichromate. The dichromate oxidizes the hydrogen
    to form water.
Local Action
action is the process by which a cell is used up when no external
current is flowing.Commercial zinc normally contains atoms of Iron,
lead, carbon, etc called impurities.
commercially zinc is used in a simple cell, bubbles of hydrogen are
seen escaping from the zinc plate (evidence of local Action).The
Impurities on the surface of the Zinc act as a second plate of a cell.
As a result, Zinc dissolves in the acid even when the cell is not in use
(This process wastes the zinc)
How to minimize local action
The problem of local action can be overcome by using amalgamated zinc plate (zinc coated with mercury).
Pure zinc in mercury form zinc amalgam.This is done by rubbing some mercury on the zinc plate.
The leclanche cell:Is
the cell uses an aqueous solution of ammonium chloride (sal ammoniac)
as the electrolyte, amalgamated zinc rod as cathode. The carbon rod as
anode fixed in a porous pot, containing a powdered mixture of carbon and
manganese (iv) oxide.
The carbon makes the mixture more conducting, and the manganese (IV) oxide (manganese dioxide,MnO2) acts as a depolarizer.
The Dry cell
a modified leclanche cell in which the main electrolyte is a paste of
starch and ammonium chloride. The action of the cell is similar to that
of the wet leclanche cell.
paste is prevented from drying by sealing the top of cell with some
insulating materials. This type of cell gives a larger current and have a
shorter recovery (demoralising time) than the ‘wet’ type;
it is useful for a greater variety of applications.However, Local
action cannot be eliminated completely in these cells, so that the cells
have a storage (or shelf) life ranging from a few months to up to
several years if stored in a cool place
leclanche cell is called a primary cell and this type of cell current
is produced from a non recoverable or irreversible chemical reaction.
all the zinc has been dissolved in the simple cell it can never be
recovered to its original form by passing a current thought the cell in
the opposite direction.
Secondary cell
is the cell which can be recharged after it hasrun down (used). This is
done by passing a d.c current from a dynamo or similar device through
the cell in the opposite direction to that in which the cell usually
supplies current in an external circuit.
Also called storage cells or accumulator some common accumulators are:
  1. Lead Acid accumulators
  2. Nickel – Cadmium accumulators
  3. Alkaline and chloride accumulators
main advantage of this type of cell is that it has a very low internal
resistance and can therefore give a large current with very little drop
in the terminal potential difference.
The Mode of Action of Lead-acid accumulator
Describe the mode of action of lead-acid accumulator
lead – acid accumulator cell consists of two plates of lead immersed in
sulphuric acid.The acid is in a plastic container.Two or more cells may
be connected to forms battery
The positive terminal is lead (iv) oxide and the negative terminal is lead.
Wood / rubber/ separator/Insulator
Cathode electrode: Lead plate (–)
Anode Electrode:lead (iv) oxide (+)
Before the accumulator is use it has to be charged
The Charging and Discharging Phenomenon of an Accumulator
Explain the charging and discharging phenomenon of an accumulator
is done by connecting across its plates as source of direct current. If
the current of 2A may be allowed to flow across the terminal of the
cell. The positive terminals of the cell and the source of current
(dynamo) must be connected together similar the negative terminal must
be connected together.
the cell is now fully charged the p.d across its terminals on open
circuit is 2v, and the cell is ready for use.It can light says a2v
electric lamp connected across its terminals. The currents will flow in
the reverse direction to the charging current.
Discharging Accumulator
  • Is the process of using charges stored in the lead – Acid Accumulator.
  • When the accumulator is discharged after long use both plate become coated with lead sulphate.
  • The relative density (R.D) of Acid also become less and the P.d of cell falls.
  • Recharging will restore the plate of lead (iv) oxide and lead, and the relative density of acid will rise to 1.25
  • The
    accumulator must not allowed to discharge below the stated values of
    p.d and Relative density, or it will not be possible to recover it on
  • The capacity of an accumulator is the amount of
    current in amperes that the cell can send thought a circuit is measured
    in ampere – Hour (Ah)
  • The charging rate an Accumulator is a
    current in amperes, numerically equal to one – tenth (1/10) of the
    capacity required in Recharging.
  • Lead-acid accumulators have high e.m.f (2v per cell) and allow internal Resistance.
  • Accumulators are best cared for by a regular check of the level of the sulphuric acid.
  • Any loss due to evaporation must be replenished with distilled water only.
  • No acid should be added unless there has been same spillage from the cell.
  • Accumulators must be recharged regularly using the charging current recommended by the manufactures.
  • They should not left in discharged condition for a long time.
  • When not in use they should be recharged at least once every month.
  • An accumulator should never be short-circuited.
  • Shorting
    the cell may cause swelling and buckling of the plates due to excessive
    heat developed in the cell, leading to permanent damage (A cell in this
    condition is said to be sulphated).
Cells and Accumulators in Daily Life
Use cells and accumulators in daily life
Uses of electric cells
  • Electric
    cells are very useful when no mains supply of electricity is available
    or when connecting to a main supply of electricity would be
  • Portable radios, torches, calculators and watches are example of devices that use primary cells.
  • It
    is possible to buy rechargeable batteries for these devices.These are
    secondary cells to start the engine and to run all the electrical
  • This cell is recharged by the alternator when the car is in use.


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