THERMAL CURRENT ELECTRICITY

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) Potemial difference (Pd) isthe difference in potential between two charged points of conductor It is measured in volts with the unit V Electromotive force (e.m4) isthe voltage developed by any source of electrical energy such as a battery or dynamo. It is generally defined asthe electrical potential fora source ina circu. Its measured in volt with the unit V

Infernal Resistance (nis an opposition offered in the batteries or power supplies which have the effect oF reducing the output potential difference as the current supplied increases.

\ ot The voltmeter in the following figure shows the “lost volts

• Set the variable resistor in the eircuit © 100,
• Close the switeh and note the values of the pd across the intemal resistance and the load

resistance.

• Suppose the result shows a pd across 100 Toad resistance of 7 SV and a pd across the
• 20 intemal resistance of 1 5V the em fot battery is 9V
• The pd actually available at the battery terminals is called the terminal p.d. Ifthe em.f of

• The Blectromotive force (¢:m.f) can be regarded as the total potential difference including

the potential difference lost across the intemal resistance of the battery.

• The $1 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/A) Resistoris 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 ofthe resistors may be short-cteuited by putting a plug into the associated socket), ler * ? ge _ a \"g oo acon 22° Wire resistorsarc lengths of wie 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 Resistanceis measured inchs. The resistance of a conductor may be determined by passing a steady current (1) through it and atthe same time recording the corresponding voltage across it

stig 7A rman The SI Units of Electromotive Force and Potential Difference ‘State the SI units of electromotive force and potential difference Eleetromotive Force (e.m.f) of @ source is the energy converted from non-electrical to electrial form when one coulomb of positive charge passes through the source.

Sumit: Volt (V)

F-W/Q, where B= emf, W = work done by source, Q + amount of postive 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)

points, Q= amount of positive change IMPORTANT: There can be e:m.f, without a closed cireuit, BUT there cannot be a potential difference without closed cireuit Electromotive Force of a Cell and Potential Difference Measure electromotive force of a cell and potential difference across a conductor Potentiometer is a device used to compare the emf (electromotive force) of two cells, 0 measure the intemal resistance of a eell, 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 @ cireuit, 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 curtent

flows through it, Electromotive force is also known as voltage, and it is measured in volts, Flectromotive force is not truly a force: rather, it isa measurement of energy per unit charge Measuring potential difference Potential 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 volimeter in parallel to measure the potential difference across 2 component in

acireuit, A circuit diagram showing a volimeter in parallel with a lamp When two components are connected in parallel, you eannot follow the circuit through both components fiom one side to the other without lifting your finger or going back over the path you have already taken

Resistance to Electric Current

The Concept of Electric Current in a Conductor

Explain the concept of electric current ina conductor Resistance of a wire: The resistance of a wire depends on the length (L) and cross sectional area of the conductor. R is directly proportional to L.

Also R=UA Combine eqn (1) and (11) R=UA

R=KUA

K =Constant called Resistivity (J)

R=IUA

J=RAL

Resistivity (J) 1s the resistance of a Imetre length of a pioce of a conductor whose eros — sectional area is equal to 1 meter square (m2),

Example 1

The resistance of copper wire is found to be 100 Calculate the resistance of a copper wire of the same length but whose radius is twice that ofthe first wire,

Solution

Let, rand J be the radius, length and resistivity of the wire whose resistance is 100. Thus

R=JUA

But

R=JUA. @

Let 2r, Land J be the radius length and Resistivity of the wire of Resistance (R)

R= ILMOr? (ii)

Divide Eqn Gi) /()

R100 = GLAM) £ (4)

Resistance. RE of the conductor 2.50

Factors which Determine the Resistance of a Conductor

Describe factors which determine the resistance of a conductor There are three external factors that influence the resistance in a conductor. Thickness (cross sectional area of the wire), length, and temperature all have some effect on the amount of resistance created m a conductor. The fourth factor is the conductivity of the material we are using, Some metals are just more electrically conductive than others, This however, is considered

an internal factor rather than an external one. 1, Cross Sectional Area:The cross-sectional area of a conductor (thickness) is similar to the cross seetion of a hallway. If the hall is very wide, it will allow a high current through it While a narrow hall would be difficult to get through due to it's restriction to 2 high rate of flow.

The animation at the left demonstrates the comparison between a wire with a small cross sectional area (A) and a larger one (A), Notice that the eleettons seem to be moving at the same speed in each one but there are many more electrons in the larger wire. Tis results in a larger ‘current which leads us to say thatthe resistance is less in a wire with a larger eross sectional area 2 Length of the Conductor: he length of a conductor is similar to the length of a hallway.

A shorter hallway would allow people to move through at a higher rate than a longer one.

• Temperature:The temperature of a conductor has a less obvious effect on the resistance

of the conductor, It would be as hard to apply the hallway analogy as itis hard to say whether a hot hallway would make us move faster or slower than a cold hallway. To truly understand the effect you must picture what happens in a conductor as it is heated. Remember, heat on the atomic or molecular scale isa direct representation of the vibration of the atoms or molecules Higher temperature means more vibrations, Imagine a hallway full of people, Half of the people

(the electrons) are trying to move in the same direction you are and the other half (the protons) are evenly spaced but stationary in the hallway. This would represent a cold wire, Since the wire is cold the protons ate not vibrating much so the electrons ean run between them faisly rapidly As the conductor (hallway) heats up, the protons start vibrating and moving slightly out of position, As their motion becomes more erratic they are more likely to get in the way and disrupt

the flow of the electrons. As a result, the higher the temperature, the higher the resistance. A prime example of this is when you tum on a light bulb, The first instant, the wire (filament) is cold and has a low resistance but as the wire heats up and gives off light it mnereases in resistance. As a result we can say that Ohm's law holds true unless temperature changes At extremely low temperatures, some materials have no measurable resistance, This is called

superconductivity. The materials are known as superconductors. Gradually, we are creating materials that become superconductors at higher temperatures and the race is on to find or ereate materials that superconduet at room temperature. We are painfully far away from the finish line.

The Relationship between Potential Difference across the Conductor and Current Determine the relationship between potential difference acrass the conductor and current The relationship between voliage, V and current ,I in a metal conductor was discovered by George Ohm and formulated in a law called as Ohm's law ‘Ohm's law state: The potential across a metal conductor is directly proportional to the current ‘owing through the conductor, provided that its temperature remains constant

(Or Val , iF T remains constant ‘Types of Resistors Hdeniify pes of resistors Resistors can be classified on various types based on Various factors Some of the classification of resistors are Based on the conductive properties ofa resistor resistors ean be classified as: 1 Linear Resistor A Tinear resistor is the type of resistor whose resistance remains constant with increase in the potential difference or voltage applied to it. Or the Resistance or

Current passed through the resistor does not changes as the applied voltage ( PD) changes. The V-l characteristics of such resistor is a straight line as shown on the figure below or in other words these types of resistors fallowsOhm’s Law very strieily

• Non Linear Resistor: Non-Linear Resistor are those types of resistors in which the

Current passedithroughit is not exactly diteetly proportional to the Potential Difference applied to it, These types of resistors have non-liner V-Icharacteristiesand does not strictly follows ohm’s Law Based on the resistance value of the resistor the resistors can be classified into following groups:

• Fixed Value Resistor: Fixed value resistors are those types of resistors whose value is

fixed already durmg manufacturing and cannot be changed during its usage Fixed resistor

• Variable Resistor or Potentiometer: Variable Resistors or Potentiometers are those

types of resistors whose value can be changed during its usage

Variable Resistors

‘The Equivalent Resistance of more than two Resistors in Series and Parallel Determine the equivalent resistance of more than two resistors in series and parallel Resistor in series:To connect two or more resistors in series is to joins the resistors one to another in succession. Consider a total of N Resistors connected in series.

R R, a

ta 6a ka Let a current flow through cach of the Resistor RI, R2, R3 RN let their total resistance be represented by only one resistor R called the Equivalent Resistance

Nn = IRN

The voltage V across the set of Resistor is the sum of the voltages a crass each resistor Thus Ri a] i v Let:The current, 12, 13 In flow in the resistor RI, R2., R3 RN Respectively and equivalent Resistances R [enn IN

VIR = VIRI + V/R2 +VIR3 YNIRN

VIV (UR) = VIV (IRI + UR2~ 1/R3 URN)

Hence the reciprocal of the equivalent Resistance in parallel is equal to the sum of the reciprocals of the individual Resistors,

Example 2

Two resistor of 30 and SQ are connected in series and then in parallel. Find the equivalent resistance when they are in series and paralle

Solution

Equivalent resistance in series, R

R=RI+R2

Equivalent Resistonee in parallel, R

UR=URI+UR2 URN

The Mode of Action of a Wheatstone Bridge Explain the mode of action of a Wheatstone bridge 1a Wheat stone bridge is an electrical circuit – In wheat-stone bridge four resistance Ry, Rz, Ruand Fl ' pz i ee f. de

Simple Electric Circuit

Remarks– While the current in a resistor has a fixed relationship with the voltage across if, the Switeh

Lamp A

== Battery

Lamp B

Lamp ¢

Effects of an Electric Current

‘The Mechanism of Heating by Electric Current Explain the mechanism of heating by electric current Electricity is the form of energy, and it can be demonstrated as follows: When you close key K an electric current flows thought the wire resistor and the water heats up, showing that the electrical energy 1s being converted to heat. Suppose the increase in temperature of the water is Q. Then, the heat gained by the water is given by

H=McQ

Factors which determines the Quality of Heat Generated in a Conductor due

toa Current Describe factors which determine the quality of heat generated in a conductor due to a current His directly proportional to Q.

• Heat absorbed by the water is proportional to the increase in its temperature
• The arrangement used in experiment ean be also used to investigate how the heat (or

temperature) varies with current and resistanee of the wire

• His directly proportional to Q
• Heat is direetly proportional to the temperature.
• His directly proportional to I
• Also Heat is directly proportional tothe time (t).
• His directly proportional to
• Also Heat is directly proportional to the Resistance, R
• His directly proportional to R.

When we combine the equations, wefind that His proportional to FRt Hence

+ He KER

+ HER) It

• V=IRH=Vit
• Hen

By Ohm's law

• 1=0VIR)

+ H=(vIR)(V)

+ H=VUR

S1-Unit of energy is Joule (I)

Joule is the work done when a change of one coulomb flows through a conductor with a potential diference (pd) of 1 volta cross it in one second

Example3

An cleetric Kettle has a wire of Resistance $0. Ikg of water is to be heated fiom room temperature (300k) up to its boiling point (373k). Using the kettle, if we ignore the thermal capacity of the kettle, what current must flow in the resistance wie if the water isto be heated in 10 minutes?

Data Given

• Specific Heat capacity, C = 4200 1/K gk
• Change in Temperature = (373-300) K=73K
• Mass of water, M= Ke
• Resistanee of wire, R= SO
• Tame ofthe current to flow, t= 1Umin = 6005

Solution

The the:mal energy Gained by water H=Mco

Electrical energy delivered by a current flowing is H=FR

3X10?

Electrical Power

P = (Electrical energy)

P= (IRI

Calculate (a) the current taken and (b) the resistance of the plate of an electric iron rated 240,

• Since

Pav I=)

240v

• Since

P=VvY/R

The Power Rating of Electrical Appliances

Interpret the power rating of electrical appliances The commercial Unit of Electrical energy is the Kilowatt ~ Hour, abbreviated as KWh 1 Kwh is the energy supplied in one Hour by an appliance working atthe rate of 1000 wats

36x 10

Examples

What is the cost of using an electric Iron rated 240V, 2000W for 10 Hours if the Electrical

The cost of using the appliance

Electric Installation

‘The Meaning of the Letter E (Earthing) L (Live) and N (Neutral) in Electrical Wiring Explain the meaning of the letter E (Earthing) L (Live) and N (Neutral) in Electrical Wiring Domestic lectricitys the form of electricity which is wired in the house LIVE WIRE (Ly Is connected to one ofthe lower holes using a pin Brown / Red in colour NEUTRAL WIRE (N): Is connected 1 one ofthe lower holes using a pin is blue in colour

EARTH (E): The upper hole is connected ta the Earth wire wit stripes of green and yellow lines or just ren st is tg od wt pase cose —~ © a ‘The Function ofa Fuse and a Circuit Breaker insalated with rubber or plastic which melts when current exceeds its normal value. Fuse can current passes thought i ve Circuit Breakers: These are sensitive switches that turn off the current when there is a surge of current following a fault they can be reset simply by flicking the switch to the ‘on’ position,

Wiring on a Board Perform wiring on a board The plug; Is the device that is connected to the cable that supplies electricity to the appliance on ‘one side and is pushed into a socket connected to the source of the mains electricity supply on the other,

Elecrical Faults in Domestic Appliances

Check and rectify electrical faults in domestic appliances Most faults you will encounter are from a fairly simple cause. Sometimes these are easy to track down sometimes not. The key to success isto use a logical and systematic approach when trying to pinpoint the cause A good stating point is to get familiar with your consumer unit whether it is a fuse type or a cireuit breaker type, Hentify what circuits you have and what they da:You'll probably have several lighting circuits,

probably one on each floor and several socket circuits (ring circuits) one on each floor. Additionally you may have several circuits for individual appliancestike cookers, electric showers, alarms, out-door power, ete In the event of a fault you may find a fuse blows or a circuit breaker trips on one of these cireuits. Clearly the problem is limited to that one circuit, You can try to reset the breaker or change the fuse. This may well solve the problem if it has been caused by @ temporary

overload Ifthe fuse blows again or the breaker trips you still have a fault and need to investigate that circuit Either, the eitcuit is drawing too much current{whieh could be the result of a faulty appliance or you've got too many appliances for the rating of that cireuit),Or, there is a short circuit which means somewhere you have a live wire touching something it shoulda't, which will be du to one of several possible causes.

Appliances not working?

• Try them in another socket

2 Check the fuse in the plug (see using a continuity checker),

• Ifother appliances works on the socket and you've checked the fuse then the appliance is

broken! Replace it or try and get it repaired by a specialist. Cells Simple cellis the cell consists of copper and zinc cathode with dilute sulphuric acid as Flectralyt. The Mode of Action of a Dry Cell (Leclanche) Describe the mode of action of a dry cell (Leclanche) Action of simple cells, ry oo8 aay At cathode: The zine plate dissolves in the sulphuric acid solution and liberates electrons into the extemal cireuit, The metal diseus 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 zine plates dipped in dilute sulphunic acid. Volta called these devices, arranged in serics. The * Crown of cups” zm – 2c – Zn 2+ The 2n7* ions go into solution, b. At anode: Positively charged hydrogen lons (I+) are attracted towards the negatively charged copper plate2H~+ 2e –— Hy The chemical reaction in the cell creates a potential

different between plates, causing clectrons to flow when the two plates are joined with a wire The electrons flow is maintained by the chemical change that occurs when the zine dissolves in the sulphuric acid. Since simple cell, which is able to adrive an electrie eurrent through 2 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

When 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 of ofthe open connector on your second battery

a oe — sav ow w Connecting in series (double voltage, same eapacity [ah Connecting in parallel When 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, ete. Use a jumper wire between the positives of both batteries and another jumper wite between the negatives of both batteries,

Connect your positive and negative wires to the same battery to run to your application pas ov wv w a Connecting in parallel (double voltage, same capacity [ah]

The Cell Defects

Iennify the cell defects asi Simple Have two main defects which cause the current to diminish quickly when the cell is being used, Two defeets of simple cell are: a Polarisation

b. Local Action

Polarisation Polarisation 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 ftom the copper plate to become neutral Also hydrogen ions accumulating at the copper plate repels other Hydrogen ions ( This defect is called Baek e mf and opposes or weakens the main e.m.fof 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

Local action is the process by which a cell is used up when no extemal current is Aowing Commercial zine normally contains atoms of Iron, lead, carbon, etc called impurities, When commercially zine 15 used in a simple cell, bubbles of hydrogen are seen escaping from the zine plate (evidence of local Action) The Impurities on the surface of the Zine act as a second plate of a cell. As a result, Zine dissolves in the acid even when the cell is not in use (This

process wastes the zine) How to minimize local action The problem of local action can be overcome by using amalgamated zinc plate (zine coated with mercury). Pure zine in mercury form zine amalgam. This is done by rubbing some mercury on the zine plate.

The leclanche cell: Is the cell uses an aqueous solution of ammonium chloride (sal ammoniac) as the electrolyte, amalgarmated ine rod as cathode. The carbon rod as anode fixed in a porous pot, containing a powdered mixture of earbon and manganese (iv) oxide.

The carbon makes the mixture more conducting, and the manganese (IV) oxide (manganese dioxide MnO;) aets as a depolarizer.

The Dry cell

Is a modified leclanche cell in which the main electrolyte is a paste of stareh and ammonium chloride. The aetion of the cell is similar to that of the wet leclanche cell ‘The paste is prevented from drying by scaling 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: Hence it is useful for a greater variety of applications However, Local action eannot be eliminated completely in these calls, so thatthe cells have a storage (or shelf) life ranging from 2 few months to up to several years if stored in a cool place The leclanche cell is called a primary cell and this type of cell current is produced from a non recoverable or irreversible chemical reaction.

Example: When all the zine has been dissolved in the simple cell it can never be recovered to its

‘original form by passing a current thought the cell im the opposite direction. Secondary cell This 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 eurrent in an external eircuit Also called storage cells or accumulator some eommon accumulators are

1 Lead Acid accumulators 2 Nickel — Cadmium accumulators

• Alkaline and chloride accumulators

The main advantage of this type of cell is that it bas @ very low intemal resistance and can therefore give a large current with very Title drop in the terminal potential difference The Mode of Action of Lead-acid accumulator Describe the mode of action of lead-aci accumulator The lead — acid accumulator cell consists of two plates of lead immersed in sulphuric acid.The acid is ina plastic container Two or more cells may be connected to forms battery

The positive terminalis lead (iv) oxide and the negative terminalis 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 she charging and discharging phenomenon of an accumulator Charging is done by connecting across its plates as source of direct current. Ifthe current of 2A,

may be allowed to flow across the terminal ofthe 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 LF the cell is now fully charged the p.d across its terminals on open circuit is 2v, andthe cell is ready for use I ean light says a2v electric lamp connected across its terminals, The eurrenis will flow in the reverse direction to the charging current

Discharging Accumulator

• Isthe 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 eel falls,
• Recharging will restore the plate of lead (iv) oxide and lead, and the relative density of

acid will rise to 1.2

• The accumulator must not allowed to discharge below the stated values of pd and

Relative density, or it will not be possible to recover it on recharging

• The capacity of an accumulator i the amount of current in amperes that the cell ean send

thought a circuit is measured m ampere — Hour (Ab)

• The charging rate an Accumulator is a current in amperes, numerically equal to one —

tenth (1/10) ofthe capacity required in Recharging

• Lead-abid accumulators have high e.m.f(2v per cell) and allow intemal Resistance
• Accumulators are best cared for by a regular check of the level of the sulphuric avi.
• Any oss due to evaporation must be replenished with distilled water only
• Noacid should be added unless there has been same spillage from the cell
• Accumulators must be recharged regularly using the charging current recommended by

the manufuctures,

• They should not left in discharged condition for a long time.
• When not in use they should be recharged at least onee every month
• Anaccumulator should never be short-cireuited,
• 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

• Blectrie cells are very usefial when no mains supply of electricity is available or when

connecting to a main supply of electricity would be inconvenient.

• Portable radios, torches, calculators and watches are example of devices that use primary

calls

• Its possible to buy rechargeable batteries for these devices. These are secondary cells to

start the engine and to run all the electrical circuits

• This cell is recharged by the alternator when the car isin use