WHAT IS ELECTROPLATING AND POLARIZATION

Electroplating:

Electroplating uses the principle of electrolysis to apply a thin coat of one metal to another metal. Some practical applications include the tinplating of steel, silver-plating of nickel alloys and chromium-plating of steel. If two copper electrodes connected to a battery are placed in a beaker containing copper sulphate as the electrolyte it is found that the cathode (i.e. the electrode connected to the negative terminal of the battery) gains copper whilst the anode loses copper.

The Simple Cell:

The purpose of an electric cell is to convert chemical energy into electrical energy. A simple cell comprises two dissimilar conductors (electrodes) in an electrolyte. Such a cell is shown in the figure below, comprising copper and zinc electrodes. An electric current is found to flow between the electrodes.

Other possible electrode pairs exist, including zinc-lead and zinc-iron. The electrode potential (i.e. the p.d. measured between the electrodes) varies for each pair of metals. By knowing the e.m.f. of each metal with respect to some standard electrode the e.m.f. of any pair of metals may be determined. The standard used is the hydrogen electrode. The electrochemical series is a way of listing elements in order of electrical potential, and the below given table shows a number of elements in such a series.

TABLE : Part of the electrochemical series:

Potassium.
sodium.
aluminium.
zinc.
iron.
lead.
hydrogen.
copper.
silver.
carbon.
In a simple cell two faults exist — those due to polarization and local action.

Polarization:

If the simple cell shown in the above Figure is left connected for some time, the current I decreases fairly rapidly. This is because of the formation of a film of hydrogen bubbles on the copper anode. This effect is known as the polarization of the cell. The hydrogen prevents full contact between the copper electrode and the electrolyte and this increases the internal resistance of the cell. The effect can be overcome by using a chemical depolarizing agent or depolarizer, such as potassium dichromate which removes the hydrogen bubbles as they form. This allows the cell to deliver a steady current.

Local action:

When commercial zinc is placed in dilute sulphuric acid, hydrogen gas is liberated from it and the zinc dissolves. The reason for this is that impurities, such as traces of iron, are present in the zinc which set up small primary cells with the zinc. These small cells are short-circuited by the electrolyte, with the result that localized currents flow causing corrosion. This action is known as local action of the cell. This may be prevented by rubbing a small amount of mercury on the zinc surface, which forms a protective layer on the surface of the electrode.
When two metals are used in a simple cell the electrochemical series may be used to predict the behaviour of the cell:
(i) The metal that is higher in the series acts as the negative electrode, and vice-versa. For example, the zinc electrode in the cell shown in the above figure is negative and the copper electrode is positive.

(ii) The greater the separation in the series between the two metals the greater is the e.m.f. produced by the cell.

The electrochemical series is representative of the order of reactivity of the metals and their compounds:

(i) The higher metals in the series react more readily with oxygen and vice-versa.

(ii) When two metal electrodes are used in a simple cell the one that is higher in the series tends to dissolve in the electrolyte.

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DEFINE MAGNETIC FLUX AND MAGNETIC FLUX DENSITY

Magnetic flux:

Magnetic flux is the amount of magnetic field (or the number of lines of force) produced by a magnetic source. The symbol for magnetic flux is  Φ (Greek letter ‘phi’). The unit of magnetic flux is the weber, Wb.

Magnetic Flux density:

Magnetic flux density is the amount of flux passing through a defined(specific) area that is perpendicular to the direction of the flux:
Magnetic flux density = magnetic flux / area.
The symbol for magnetic flux density is B. The unit of magnetic flux density is the tesla, T, where 1 T = 1 Wb/m^2. 
Hence,
B = Φ/A tesla , where A( m^2) is the area.

Numericals:

Problem 1. 
A magnetic pole face has a rectangular section having dimensions 200 mm by 100 mm. If the total flux emerging from the pole is 150 µWb, calculate the flux density:
Solution.
Flux  Φ = 150 µWb = 150 x 10^ -6 Wb
Cross sectional area A = 200 x 100 = 20000 mm^2
                                                         = 20000 x 10^ -6 m^2
Flux density B =  Φ/A
                        =  150 x 10^- 6/20000 x 10^- 6
                        =  0.0075 T or 7.5 mT (Ans)
Problem 2.
The maximum working flux density of a lifting electromagnet is 1.8 T and the effective area of a pole face is circular in cross-section. If the total magnetic flux produced is 353 mWb, determine the radius of the pole face:
Solution.
Flux density B = 1.8 T; flux  Φ = 353 mWb = 353 x 10^-3 Wb
Since B =  Φ/A
or
cross-sectional area, A =  Φ/B
                                     = 353 x 10^-3/1.8
                                     = 0.1961 m^2
The pole face is circular, hence 
area =  π r^2, where r is the radius.
Hence  π r^2 = 0.1961
from which r^2 = 0.1961/ π and radius r = (0.1961)^1/2 = 0.250 m
i.e. the radius of the pole face is 250 mm.(Ans)

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DEFINE CONDUCTORS,INSULATORS AND SEMI-CONDUCTORS

Some materials allow electric current to flow more freely than others. These materials are called conductors. Other materials are resistant to the flow of electric current. These materials are called insulators. Conductors and insulators are both important in the field of electronics. 

Conductors:

Conductors allow electric current to flow easily because of the make up of their atoms. In a conductor, the outer electrons of the atom are loosely bound and can freely move through the material when an electric charge is applied. 

About Conductive Materials:

In general, the best electrical conductors are metals. Metals tend to have electrons in the outer layer of their atoms that are freely shared. The most conductive of all the elements is silver. Unfortunately, silver is too rare and expensive to use in most electrical equipment. Today, the most commonly used electrical conductor is copper. Copper is used in electrical wiring and electrical circuits throughout the world.

Relation Between Conductance and Resistance:

Another way to think of conductance is as the opposite of resistance. The resistance of a material is a measurement of how well a material opposes the flow of electric current. Sometimes conductance is represented by the letter "G" where G is the inverse of resistance, R. 

G = 1/R

Using Ohm's law we know that resistance is equal to voltage divided by current or R = V/I, therefore,

G = I/V

About Superconductors:

A superconductor is a material that is a perfect conductor. It has an electrical resistance of zero. All of the superconductors that have been discovered by scientists to date require a very cold temperature on the order of minus 234 degrees C in order to become superconductors.

Insulators:

The opposite of a conductor is an insulator. An insulator opposes the flow of electricity. Insulators are important to keep us safe from electricity. The wire that carries electricity to your computer or television is covered with a rubber-like insulator that protects you from getting electrocuted. Good insulators include glass, the air, and paper. 

Semiconductors:

Some materials behave in between a conductor and an insulator. These materials are called semiconductors. Semiconductors are important in electronics such as computers and mobile phones because their conductivity can be controlled allowing for current to flow in just one direction or only under certain circumstances. The most commonly used semiconductor in electronics today is silicon. 

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DEFINE OHMS LAW

One of the most important and basic laws of electrical circuits is Ohm's law which states that the current passing through a conductor is proportional to the voltage over the resistance. 

Equation Of Ohms Law:

Ohm's law may sound a bit confusing when written in words, but it can be described by the simple formula:

I=V/R

where I = current in amps, V = voltage in volts, and R = resistance in ohms. 

This same formula can be also be written in order to calculate for the voltage or the resistance: 

I=V/R  OR  V=IR  OR R=V/I

Ohms Law Triangle:

If you ever need help in remembering the different equations for Ohm's law and solving for each variable (V, I, R) you can use the triangle below. 

As you can see from the triangle and the equations above, voltage equals I times R, current (I) equals V over R, and resistance equals V over I. 

Ohms Law Circuit Diagram:

Here is a diagram showing I, V, and R in a circuit. Any one of these can be calculated using Ohm's law if you know the values of the other two.

Explanation By Example:

Ohm's law describes the way current flows through a resistance when a different electric potential (voltage) is applied at each end of the resistance. One way to think of this is as water flowing through a pipe. The voltage is the water pressure, the current is the amount of water flowing through the pipe, and the resistance is the size of the pipe. More water will flow through the pipe (current) the more pressure is applied (voltage) and the bigger the pipe is (lower the resistance). 

NOTE: It is generally applied only to direct current (DC) circuits, not alternating current (AC) circuits. In AC circuits, because the current is constantly changing, other factors such as capacitance and inductance must be taken into account.

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DEFINE RESISTORS,CAPACITORS & INDUCTORS

The three basic elements used in electronic circuits are the resistor, capacitor, and inductor. They each play an important role in how an electronic circuit behaves. They also have their own standard symbols and units of measurement. 

Resistors:

A resistor represents a given amount of resistance in a circuit. Resistance is a measure of how the flow of electric current is opposed or "resisted." It is defined by Ohm's law which says the resistance equals the voltage divided by the current. 

Resistance = voltage/current 
or 
R = V/I


Resistance is measured in Ohms. The Ohm is often represented by the omega symbol: Ω. 

The symbol for resistance is a zigzag line as shown below. The letter "R" is used in equations. 

Capacitors:

A capacitor represents the amount of capacitance in a circuit. The capacitance is the ability of a component to store an electrical charge. You can think of it as the "capacity" to store a charge. The capacitance is defined by the equation.

C = q/V 
where q is the charge in coulombs and V is the voltage.


In a DC circuit, a capacitor becomes an open circuit blocking any DC current from passing the capacitor. Only AC current will pass through a capacitor. 

Capacitance is measured in Farads. 

The symbol for capacitance is two parallel lines. Sometimes one of the lines is curved as shown below. The letter "C" is used in equations. 

Inductors:

An inductor represents the amount of inductance in a circuit. The inductance is the ability of a component to generate electromotive force due to a change in the flow of current. A simple inductor is made by looping a wire into a coil. Inductors are used in electronic circuits to reduce or oppose the change in electric current. 

In a DC circuit, an inductor looks like a wire. It has no affect when the current is constant. Inductance only has an effect when the current is changing as in an AC circuit. 

Inductance is measured in Henry. 

The symbol for inductance is a series of coils as shown below. The letter "L" is used in equations. 

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DEFINE CIRCUIT LOADS

Circuit Loads:

A load generally refers to a component or a piece of equipment connected to the output of an electric
circuit. In its fundamental form, the load is represented by any one or a combination of the following:

1.Resistor(R)

2.Inductor(L)

3.Capacitor(C)

A load can either be of resistive, inductive or capacitive nature or a blend of them. For example,a light bulb is a purely resistive load where as a transformer is both inductive and resistive. A circuit load can also be referred to as a sink since it dissipates energy whereas the voltage or current supply can be termed as a source.
Table shows the basic circuit elements along with their symbols and schematics used in an electric circuit.The R,L and C are all passive components i.e. they do not generate their own emf whereas the DC voltage and current sources are active elements.

Common circuit elements and their representation in circuit.

Standard quantities and their units and symbols that are commonly found in electric circuit.

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DEFINE VOLTAGE

Voltage or potential difference between two points in an electric circuit is 1V if 1J (Joule) of energy is expended in transferring 1C of charge between those points.

It is generally represented by the symbol same letter, however, it rarely causes any confusion.V and measured in volts (V). Note that the the unit of voltage are both denoted by the same letter, however, it rarely causes any confusion.

The symbol the symbol V also signifies a constant voltage (DC) whereas a time varying (AC) voltage is represented by the symbol v or v(t).

Voltage is always measured across a circuit element as demonstrated in Figure 2.2

Figure 2.2:A voltmeter is connected in parallel with the circuit element,R to measure the voltage across it.

A voltage source provides the energy or emf (electromotive force) required for current flow.However,current can only exist if there is a potential difference and a physical path to flow.A potential difference of 0 V between two points implies 0 A of current flowing through them.The current I Figure 2.3 is 0 A since the potential difference across R(2) is 0 V. In this case, a physical path but there is no potential difference. This is equivalent.

Figure 2.3:The potential difference across R(2) is 0V,hence the current I is 0A where V(s) and I(s) are the voltage and current sources respectively.

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