### Welcome to Talha's Physics Academy

To Help Teachers and Students.

Talha's Physics Academy is an exploration environment for concepts in physics which employs free Physics Books and other linking strategies to facilitate smooth navigationThe entire environment is interconnected with thousands of links, reminiscent of a neural network.

New content for Talha's Physics Academy will be posted as it is developed,It is my intent to keep this material continuously available except for brief maintenance times.

All the Branches of Physics are covered.

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### Water-Proof Hanky

A great excuse to threaten to pour water over your audience – but with a surprise twist thanks to physics. Ingredients

• large glass

• ashtray or similar

• water

• handkerchief Instructions

1. Push the centre of the handkerchief into the glass, so that the edges are hanging over the outside of the rim of the glass.

2. Pour water into the glass, through the loose handkerchief. Make sure that your audience can see the water easily passing through the handkerchief into the glass. Keep pouring the water until the glass is roughly half full.

3. Pull the corners of the handkerchief so that the material is taut over the top of the glass. Hold the glass and handkerchief so that the material stays tightly stretched over the opening. For younger audiences you may like to say some 'magic words' that make the hanky water proof.

4. Place the ashtray on the top of the glass and tip it all upside down, being careful to keep the handkerchief pulled tight.

5. Choose a likely suspect from your audience to threaten with a drenching!

Hold the upside-down glass and ashtray above their head, making sure that the glass is vertical and the handkerchief is tight. Remove the ashtray and voila! – nothing happens! The water stays inside the glass. How does it work?

How does it work?
This demonstration is based on surface tension. When the handkerchief is loose, the water can pour through the gaps in the fabric. However, when the handkerchief is pulled tight, the water molecules can form a single surface or membrane across the handkerchief. The surface tension of this membrane is sufficient to overcome gravity. Tips for Success Don't try to substitute a paper tissue for the handkerchief as it won't work! If the glass isn't held vertically, some water may dribble out where the membrane attaches to the edge of the glass. Serving Suggestions This trick will work in almost any environment and with any age group. Did You Know? Galileo was among the earliest to demonstrate the existence of surface tension on water by showing that an iron needle can be floated lengthways on water, but not on it’s point.

### How in common collector BJT configuration voltage gain is less than unity ?

The most striking characteristic of this configuration is that the input signal source must carry the full emitter current of the transistor, as indicated by the heavy arrows in the first illustration. As we know, the emitter current is greater than any other current in the transistor, being the sum of base and collector currents. In the last two amplifier configurations, the signal source was connected to the base lead of the transistor, thus handling the least current possible.

Because the input current exceeds all other currents in the circuit, including the output current, the current gain of this amplifier is actually less than 1 (notice how Rload is connected to the collector, thus carrying slightly less current than the signal source). In other words, it attenuates current rather than amplifying it. With common-emitter and common-collector amplifier configurations, the transistor parameter most closely associated with gain was Î². In the common-base circuit, we follow another basic transistor parameter: the ratio between collector current and emitter current, which is a fraction always less than 1. This fractional value for any transistor is called the alpha ratio, or Î± ratio.

### What is the difference between AC and DC Generator?

A generator produces electricity in a form that looks like a linear sine wave: first it is positive and then negative.

To create AC electricity, the central shaft carrying windings - which is called the rotor - has slip rings connected to the ends of the winding. In a single-phase generator (more correctly called an "alternator") the outer slip ring is attached to one end of the rotor's winding and the inner slip ring is attached to the other end of the rotor's winding. (In a three-phase alternator there are three separate windings and three sets of slip rings. Each slip ring is connected to the ends of one pair of the windings in such a manner that no windings are shorted-out.) The slip rings are touched by fixed brushes to take off the AC current.

To generate DC electricity, the central shaft carries a part called a "commutator" which has many separate segments. Each segment in sequence around the commutator is connected to the opposite ends of the rotor's winding . As the rotor spins round, two fixed brushes diametrically opposite one another connect to those segments one by one. Thus, as the rotor spins, one brush always picks up the positive wire from the winding and the opposite brush picks up the negative wire from the winding. So, as the shaft rotates, the two brushes always remain positive or negative.

### Difference between AC and DC

Electricity flows in two ways; either in alternating current (AC) or in direct current (DC). Electricity or 'current' is nothing more than moving electrons along a conductor, like a wire, that have been harnessed for energy. Therefore, the difference between AC and DC has to do with the direction in which the electrons flow. In DC, the electrons flow steadily in a single direction, or "forward." In AC, electrons keep switching directions, sometimes going "forwards" and then going "backwards."

## Comparison chart

Alternating Current
Direct Current
Amount of energy that can be carried
Safe to transfer over longer city distances and can provide more power.
Voltage of DC cannot travel very far until it begins to lose energy.
Cause of the direction of flow of electrons
Rotating magnet along the wire.
Frequency
The frequency of alternating current is 50Hz or 60Hz depending upon the country.
The frequency of direct current is zero.
Direction
It reverses its direction while flowing in a circuit.
It flows in one direction in the circuit.
Current
It is the current of magnitude varying with time
It is the current of constant magnitude.
Flow of Electrons
Electrons keep switching directions - forward and backward.
Electrons move steadily in one direction or 'forward'.
Obtained from
A.C Generator and mains.
Cell or Battery.
Passive Parameters
Impedance.
Resistance only
Power Factor
Lies between 0 & 1.
it is always 1.
Types
Sinusoidal, Trapezoidal, Triangular, Square.
Pure and pulsating.

### What is the difference between emf and voltage?

Potential difference is simply a voltage difference between two points in a closed electrical circuit with a voltage source circuit (or in free space). So, the interesting fact is the potential difference can be a source of emf if it is used to move charges. The term ‘potential difference’ is a general term and found in all the energy fields such as electric, magnetic and gravitational fields. But emf is only pertaining to electrical circuits. Although, both ‘electrical potential difference’ and emf are measured in Volts (V), there are many differences between them.

Potential Difference
Potential is a function of the location, and potential difference between point A and point B is calculated by subtracting the potential of A from potential of B. In an electric field, it is the amount work to be done to move a unit charge (+1 Coulomb) from B to A. Electric potential difference is measured in V (Volts). In an electrical circuit, current flows from the higher potential to lower potential.

EMF (Electromotive Force)
EMF is the electrical potential difference provided by an energy source like battery. Varying magnetic fields also can generate an EMF according to the Faraday’s law. Although EMF is also a voltage and measured in Volts (V), it is all about the generation of a potential difference.

So the important differences between Voltage and EMF is:

1. The term ‘potential difference’ is used in all energy fields (electric, magnetic, gravitational), and ‘EMF’ is only used in electric circuits.

2. EMF is the electrical potential difference generated by a source like battery or generator.

3. We can measure potential difference between any two points, but EMF exists only between the two ends of a source.

4. Sum of ‘potential drops’ around a circuit is equal to total EMF according to Kirchhoff’s second law.

### Commutator is the only difference b/w AC &DC motors or some thing else?

Ans. Following the the three main differences between AC and DC motor.

1.
The most basic difference is the power source. A.C. motors are powered from alternating current (A.C.)

D.C. motors are powered from direct current (D.C.), such as batteries, D.C. power supplies or an AC-to-DC power converter.

2.
A.C. induction motors do not use brushes; they are very rugged and have long life expectancies.

D.C wound field motors are constructed with brushes and a commutator, which add to the maintenance, limit the speed and usually reduce the life expectancy of brushed D.C. motors.

3.
The final basic difference is speed control. The speed of an A.C. motor is controlled by varying the frequency, which is commonly done with an adjustable frequency drive control.

The speed of a D.C. motor is controlled by varying the armature winding’s current.

### Applications of constraints in classical physics

In classical mechanics, a constraint is a relation between coordinates and momenta (and possibly higher derivatives of the coordinates). In other words, a constraint is a restriction on the freedom of movement of a system of particles.
Types of constraint

1.First class constraints and second class constraints
2.Primary constraints, secondary constraints, tertiary constraints, quaternary constraints.
3.Holonomic constraints, also called integrable constraints, (depending on time and the coordinates but not on the momenta) and Non-holonomic constraints
4.Pfaffian constraints
5.Scleronomous constraints (not depending on time) and rheonomous constraints (depending on time).
6.Ideal constraints: those for which the work done by the constraint forces under a virtual displacement vanishes.

A very Good explanation by :
Dr. M Ramegowda
Dept. of Physics
Govt. College (Autonomous), Mandya

### What is Tesla Coil ?

The Tesla coil is one of Nikola Tesla's most famous inventions. It is essentially a high-frequency air-core transformer. It takes the output from a 120vAC to several kilovolt transformer & driver circuit and steps it up to an extremely high voltage. Voltages can get to be well above 1,000,000 volts and are discharged in the form of electrical arcs. Tesla himself got arcs up to 100,000,000 volts, but I don't think that has been duplicated by anybody else. Tesla coils are unique in the fact that they create extremely powerful electrical fields. Large coils have been known to wirelessly light up florescent lights up to 50 feet away, and because of the fact that it is an electric field that goes directly into the light and doesn't use the electrodes, even burned-out florescent lights will glow.

### How the capacitor is work in electrostatic induction?

Definition:
A capacitor is most simply defined as two conductors separated by a dielectric. It is easier to grasp the significance of this definition by looking at a commonly used model for a capacitor that is shown here. A capacitor is also called a condenser.

A dielectric is a material that is a good insulator (incapable of passing electrical current), but is capable of passing electrical fields of force.

Some examples dielectric materials:
vacuum  ,air  ,aluminum oxide  ,various ceramics  ,Barium titanate   ,glass  ,water  ,mica  ,oil etc .

Charged Capacitor

A capacitor is said to be charged when there are more electrons on one conductor plate than on the other.
The plate with the larger number of electrons has the negative polarity. The opposite plate then has the positive polarity.
When a capacitor is charged, energy is stored in the dielectric material in the form of an electrostatic field.

Electrostatic Induction
When an electron is added to one plate of a capacitor, one electron is driven away from the opposite plate.
Or you can say that when an electron is pulled away from one plate of a capacitor, another electron is drawn to the opposite plate.

No matter how you look at it, this is the principle of electrostatic induction at work in a capacitor.

When this electrostatic effect increases the imbalance of electrons between the two plates:
The electrostatic field grows stronger.
The amount of energy stored in the dielectric increases.
The capacitor is said to be charging.
When this electrostatic effect decreases the imbalance of electrons between the two plates:

The electrostatic field grows weaker.
The amount of energy stored in the dielectic decreases.
The capacitor is said to be discharging.

### MEASUREMENT OF DENSITY

Introduction

In order to classify and identify materials of a wide variety, scientists use numbers called physical constants (e.g. density, melting point, boiling point, index of refraction) which are characteristic of the material in question. These constants do not vary with the amount or shape of the material, and are therefore useful in positively identifying unknown materials.  Standard reference works have been complied containing lists of data for a wide variety of substances. The chemist makes use of this in determining the identity of an unknown substance, by measuring the appropriate physical constants in the laboratory, consulting the scientific literature, and then comparing the measured physical constants with the values for known materials. This experiment illustrates several approaches to the measurement of the density of liquids and solids.
Density is a measure of the “compactness” of matter within a substance and is defined by the equation:

Density =  mass/volume                                                           eq 1.

The standard metric units in use for mass and volume respectively are grams and milliters or cubic centimeters.  Thus, density has the unit grams/milliter (g/ml) or grams/cubic centimenters (g/cc).  The literature values are usually given in this unit.  Density may be calculated from a separate mass and volume measurement, or, in the case of liquids, may be determined directly by the use of an instrument called hydrometer.
Volume measurements for liquids or gases are made using a graduated containers, for example, a graduated cylinder.  For solids, the volume can be obtained either from the measurement of the dimensions of the solid or by displacement.  The first method can be applied to solids with regular geometric shapes for which the mathematical formulas can be used to calculate the volume of the solid from the dimensions of the solid.  Alternatively, the volume of any solid object, irregular or regularly shaped, can be measured by displacement.  The solid is submerged in a liquid in which it is not soluble, and the volume of liquid displaced measured.
The hydrometer measures density directly.  An object that is less dense than a liquid will float in that liquid density to a depth such that the mass of the object submerged equals the mass of the of the liquid displaced (Archimedes' Principle).  Since mass equals density X volume (see equation 1), an object floated in liquids of different densities will displace different volumes of liquid.  A hydrometer is a tube of constant mass that has been calibrated to measure density by floating the hydrometer in liquids of known densities and recording on a scale the fraction of the hydrometer submerged.  Any hydrometer can be used over a limited range of densities because the hydrometer must float in the liquid being studied and the hydrometer level must be sufficiently submerged to obtain an on scale reading.  Hydrometers may be calibrated in g/ml or some other unit of density.
In the following experiment, the identities of three colorless liquids will be determined by measuring the densities of the liquids by two methods and then comparing the density of the liquid to literature (reference) values for the three liquids.  The identity of an unknown metal will be established in a similar manner.

Procedure

1)  Weigh a clean, dry 50ml graduated cylinder.  Add approximately 30ml of liquid to your weighed 50ml graduated cylinder without bothering to measuring out the liquid accurately. Now carefully read and record whatever amount of liquid there is in the cylinder.  Weigh the cylinder and liquid, and then calculate the density of the liquid. Repeat this procedure to find the density of each liquid

2)  Determine the density of each of the above, using a hydrometer and an ungraduated cylinder.  Read the density from where the liquid crosses the hydrometer's scale.

3)  Weigh and record the mass of an unknown metal cylinder.  Also record the identity of the unknown metal cylinder (A, B, C, or D).  Calculate the volume of the metal cylinder by measuring (in cm) the height (h) and diameter (d) of the metal cylinder and then applying the formula: Volume (cc) = = h x 0.785d2.  Also, measure the volume of the metal cylinder by displacement of water in a 50ml graduated cylinder.  Calculate the density of the metal cylinder for each method of measuring volume and identify the metal by comparing the value obtained with the literature values for various metals.

4)  Using any appropriate procedure learned above, find the density of one of the following more objects:  a coin, a piece of chalk, a small cork.,

Formulas for volumes of regular shaped objects
Area of circle ¼ p d2, where d = diameter, and p = 3.14159
Volume of a cylinder = area of base x height
Volume of a sphere 1/6 p d3

Data and Calulations

Name___________________ Date_________________Lab section________

Liquid A                     Liquid B                      Liquid C

Wt.of cyl + liquid                   ________g                  _________g                _________g

Wt. of liquid                           ________g                  _________g                _________g

Volume of liquid                     ________ml                _________ml              _________ml

Density                                    _________g/ml           _________g/ml           _________g/ml

b) Density as determined with hydrometer:

_________g/ml           _________g/ml           _________g/ml

Literature value:

_________g/ml           _________g/ml           __________g/ml

c)  Data for metal cylinder

unknown No._________________
unknown color________________
weight _____________________g
height ____________________cm
diameter __________________cm
volume(a)_________________cc  (by calculation)
volume(b)_________________ml (by displacement, 1 ml = 1 cc)
density(a)_________________(b) _____________g/cc(g/ml)
identity of metal_______________
literature value of density___________
literature source__________________

Density for special materials

1)  Identity and description of material:

Mass of material                      ________________

Volume of material                 ________________

Density of material                 ________________

2)  Identity and description of material:

Mass of material                      ________________

Volume of material                 ________________

Density of material                 ________________

3)  Identity and description of material:

Mass of material                      ________________

Volume of material                 ________________

Density of material                 ________________

### Thermistors- Working and Advantages (College Level)

Thermistors are variable resistance type of transducers. Let us see what they are and their working.

What are Thermistors?
Thermistors are one of the most commonly used devices for the measurement of temperature. The thermistors are resistors whose resistance changes with the temperature. While for most of the metals the resistance increases with temperature, the thermistors respond negatively to the temperature and their resistance decreases with the increase in temperature. Since the resistance of thermistors is dependent on the temperature, they can be connected in the electrical circuit to measure the temperature of the body.

Materials used for Thermistors and their Forms

The thermistors are made up of ceramic like semiconducting materials. They are mostly composed of oxides of manganese, nickel and cobalt having the resistivities if about 100 to 450,000 ohm-cm. Since the resistivity of the thermistors is very high the resistance of the circuit in which they are connected for measurement of temperature can be measured easily. This resistance is calibrated against, the input quantity, which is the temperature, and its value can be obtained easily.

Thermistors are available in various shapes like disc, rod, washer, bead etc. They are of small size and they all can be fitted easily to the body whose temperature has to be measured and also can be connected to the circuit easily. Most of the thermistors are quite cheap.

Thermistor Shapes

Principle of Working of Thermistors
As mentioned earlier the resistance of the thermistors decreases with the increase its temperature. The resistance of thermistor is given by:

R = Ro ek

K = Î²(1/T – 1/To)

Where R is the resistance of the thermistor at any temperature T in oK (degree Kelvin)

Ro is the resistance of the thermistors at particular reference temperature Toin oK

e is the base of the Naperian logarithms

Î² is a constant whose value ranges from 3400 to 3900 depending on the material used for the thermistors and its composition.

The thermistor acts as the temperature sensor and it is placed on the body whose temperature is to be measured. It is also connected in the electric circuit. When the temperature of the body changes, the resistance of the thermistor also changes, which is indicated by the circuit directly as the temperature since resistance is calibrated against the temperature. The thermistor can also be used for some control which is dependent on the temperature.

Here are some of the advantages of the thermistors

1) When the resistors are connected in the electrical circuit, heat is dissipated in the circuit due to flow of current. This heat tends to increase the temperature of the resistor due to which their resistance changes. For the thermistor the definite value of the resistance is reached at the given ambient conditions due to which the effect of this heat is reduced.

2) In certain cases even the ambient conditions keep on changing, this is compensated by the negative temperature characteristics of the thermistor. This is quite convenient against the materials that have positive resistance characteristics for the temperature.

3) The thermistors are used not only for the measurement of temperature, but also for the measurement of pressure, liquid level, power etc.

4) They are also used as the controls, overload protectors, giving warnings etc.

5) The size of the thermistors is very small and they are very low in cost. However, since their size is small they have to be operated at lower current levels.

### In ohmic law the voltage is directly proportional to current while in case of electric transformer the situation is quite reverse voltage is inversely proportion to current.clarification is required.

Actually, according to Ohm,s Law I= V/R, clearly Current is directly proportional to the Voltage, But according to P=VI or I=P/V, it shows that current is inversely proportional to the Voltage.

It depends on how you increase the voltage if you increase it by keeping the power of the source constant or not,if the power of the source is constant then the current would decrease when voltage increasing ....if you don't care about the power and just simply replace the battery with a new one's with higher power rating this can increase the current.

In Transformer, when voltage increases then current decrease because power remains constant...both side power is P=VI

By Ohm's Law, Current (I) is directly proportional to the Voltage (V) if Resistance (R) and Temperature remain same.
I = V/R.....or...R=V/I.....or......V=IR.

According to P=VI...or...I=P/V....or ...V=P/I,..... It says that Current inversely proportional to the voltage if power remain same.As we know that in Transformer, If power remain same, and voltage increase, then current decreases in Step Up Transformer. also Voltage decreases when current increases as in Step Down Transformer.

### Surface Tension-Unit-Examples

Surface tension is a phenomenon in which the surface of a liquid, where the liquid is in contact with gas, acts like a thin elastic sheet. This term is typically used only when the liquid surface is in contact with gas (such as the air). If the surface is between two liquids (such as water and oil), it is called "interface tension."
Causes of Surface Tension

Various intermolecular forces, such as Van der Waals forces, draw the liquid particles together. Along the surface, the particles are pulled toward the rest of the liquid, as shown in the picture to the right.

Surface tension (denoted with the Greek variablegamma) is defined as the ratio of the surface force Fto the length d along which the force acts:
gamma = F / d

Units of Surface Tension

Surface tension is measured in SI units of N/m (newton per meter), although the more common unit is the cgs unit dyn/cm (dyne per centimeter).

In order to consider the thermodynamics of the situation, it is sometimes useful to consider it in terms of work per unit area. The SI unit in that case is the J/m2 (joules per meter squared). The cgs unit is erg/cm2.

These forces bind the surface particles together. Though this binding is weak - it's pretty easy to break the surface of a liquid after all - it does manifest in many ways.

Examples of Surface Tension

Drops of water. When using a water dropper, the water does not flow in a continuous stream, but rather in a series of drops. The shape of the drops is caused by the surface tension of the water. The only reason the drop of water isn't completely spherical is because of the force of gravity pulling down on it. In the absence of gravity, the drop would minimize the surface area in order to minimize tension, which would result in a perfectly spherical shape.

Insects walking on water. Several insects are able to walk on water, such as the water strider. Their legs are formed to distribute their weight, causing the surface of the liquid to become depressed, minimizing the potential energy to create a balance of forces so that the strider can move across the surface of the water without breaking through the surface. This is similar in concept to wearing snow shoes to walk across deep snowdrifts without your feet sinking.

Needle (or paper clip) floating on water. Even though the density of these objects are greater than water, the surface tension along the depression is enough to counteract the force of gravity pulling down on the metal object. Click on the picture to the right, then click "Next," to view a force diagram of this situation or try out the Floating Needle trick for yourself.

### Transistor Basics

Transistors can be regarded as a type of switch, as can many electronic components. They are used in a variety of circuits and you will find that it is rare that a circuit built in a school Technology Department does not contain at least one transistor. They are central to electronics and there are two main types; NPN and PNP. Most circuits tend to use NPN. There are hundreds of transistors which work at different voltages but all of them fall into these two categories.

Transistors are manufactured in different shapes but they have three leads (legs).
The BASE - which is the lead responsible for activating the transistor.
The COLLECTOR - which is the positive lead.
The EMITTER - which is the negative lead.
The diagram below shows the symbol of an NPN transistor. They are not always set out as shown in the diagrams to the left and right, although the ‘tab’ on the type shown to the left is usually next to
 the ‘emitter’. How they are related to each other ??? Since the Transistor is a Current device, any signal Voltage must first be Converted to a Current.  Voltage to Current Converter First, you must convert the input voltage to a current by using a Voltage to Current Convertor--a resistor. Since the Transistor is a Current in/Current out device, any Current Output is Converted to a Voltage Drop by the Current flowing thru a Load Resistor. Current to Voltage Converter: Next, you convert the output current into a voltage by using a Current to Voltage Converter in the collector circuit--you guessed it--a resistor.

### Nodal analysis

The aim of nodal analysis is to determine the voltage at each node relative to the reference node (or ground). Once you have done this you can easily work out anything else you need.The following document shows the step by step Nodal Analysis of Circuit.

### What is difference b/w conductors and insulators?

The behavior of an object that has been charged is dependent upon whether the object is made of a conductive or a nonconductive material.

Conductors are materials that permit electrons to flow freely from atom to atom and molecule to molecule. An object made of a conducting material will permit charge to be transferred across the entire surface of the object. If charge is transferred to the object at a given location, that charge is quickly distributed across the entire surface of the object. The distribution of charge is the result of electron movement. Since conductors allow for electrons to be transported from particle to particle, a charged object will always distribute its charge until the overall repulsive forces between excess electrons is minimized. If a charged conductor is touched to another object, the conductor can even transfer its charge to that object. The transfer of charge between objects occurs more readily if the second object is made of a conducting material. Conductors allow for charge transfer through the free movement of electrons.

In contrast to conductors, insulators are materials that impede the free flow of electrons from atom to atom and molecule to molecule. If charge is transferred to an insulator at a given location, the excess charge will remain at the initial location of charging. The particles of the insulator do not permit the free flow of electrons; subsequently charge is seldom distributed evenly across the surface of an insulator.

While insulators are not useful for transferring charge, they do serve a critical role in electrostatic experiments and demonstrations. Conductive objects are often mounted upon insulating objects. This arrangement of a conductor on top of an insulator prevents charge from being transferred from the conductive object to its surroundings. This arrangement also allows for a student (or teacher) to manipulate a conducting object without touching it. The insulator serves as a handle for moving the conductor around on top of a lab table. If charging experiments are performed with aluminum pop cans, then the cans should be mounted on top of Styrofoam cups. The cups serve as insulators, preventing the pop cans from discharging their charge. The cups also serve as handles when it becomes necessary to move the cans around on the table.

Examples of conductors include metals, aqueous solutions of salts (i.e., ionic compounds dissolved in water), graphite, water and the human body. Examples of insulators include plastics, Styrofoam, paper, rubber, glass and dry air. The division of materials into the categories of conductors and insulators is a somewhat artificial division.

### WHAT IS THE SYMBOL OF POINT CONTACT DIODE?

Ans. There is no Specific Symbol for Point Contact Diode.The specific symbols are given below .

### What is the difference between alternating current and direct current.(College Level)

A battery cell gives "d.c." or "direct current" which means that a steady voltage is available to drive your radio or whatever. Less well understood is "a.c." which stands for "alternating current". However, it's important to understand the difference because it could cost you money! You wouldn't dream of connecting your 12 volt radio directly to a mains power plug because you know that it gives at least 230 volts. But do you know that a 12 volt a.c. transformer can do almost as much damage? The reason is that electronic equipment needs not only LOW voltage but low D.C. voltage. Let's take a quick look at the method of making electricity.
sine wave voltage

In a power station, electricity can be made most easily and efficiently by using a motor to spin magnetic wire coils. The resultant voltage is always "alternating" by virtue of the motor's rotation. Fig. indicates how the voltage goes first positive then negative - rather like turning a battery cell continually backwards and forwards in its clip.

There are several ways but the simplest is to use a transformer to reduce the voltage to, say 12 volts a.c.  This lower voltage can be fed through a "rectifier" which combines the negative and positive alternating cycles so that only positive cycles emerge. This "rectified" voltage is suitable for running things like filament bulbs and electric trains but it is still no good for electronic circuits. So a "12 volt dc transformer" is no good for electronic devices. What you need is "regulated d.c." which truly simulates the steady voltage that you get from a battery.

The first step is to connect a large value capacitor to the output of the rectifier. A capacitor acts as a voltage reservoir and has the effect of smoothing the "ripples". This is still not the same as a battery produces but it's often good enough for charging batteries in mobile phones, personal stereo equipment and similar full wave rectified voltage with reservoir capacitor

The final step is to pass this "rippling d.c." through a regulator unit. This effectively chops off the ripple to leave almost pure "regulated d.c." at a steady voltage.

So, to provide a suitable voltage for electronic circuits you need a power supply which gives a "regulated d.c." output. If your power supply doesn't use those words then it may not be suitable for use with electronic circuits. In this case, saving cost might lead to expensive smoke!

### What is the significance of AC load line?

The ac load line is a graph that represents all possible combinations of  and  for a given amplifier. Under normal circumstances, the ac and dc load lines for a given amplifier are not identical. A typical ac and dc load line combination is shown in Figure 11-1a. Note that the two lines intersect at the circuit Q-point. The endpoints of the ac load line are defined as shown in Figure 11-1b. As shown, the ac saturation and cutoff points can be defined using circuit Q-point values.

It's Significance

•The ac load line of a given amplifier will not follow the plot of the dc load line.
•This is due to the dc load of an amplifier is different from the ac load.
•The ac load line is used to tell you the maximum possible output voltage swing for a given common-emitter amplifier.
•In other words, the ac load line will tell you the maximum possible peak-to-peak output voltage (Vpp ) from a given amplifier.
•This maximum Vpp is referred to as the compliance of the amplifier.
(AC Saturation Current  Ic(sat)  ,  AC Cutoff Voltage  VCE(off) )

### Do bends in a wire effect its resistance?

Ans.It shouldn't, unless the bend is specifically a coil, where the physics is different. There are 4 factors that can affect R:

1. Length of the conductor
2. Area of cross section of the conductor
3. Electrical Resistivity of Substances
4. Effect of Temperature

But if any bend produces stress in the wire (compression on the inside, tension on the outside) and this stress affects resistivity and hence resistance.

Secondly, because metals are more easily stretched than compressed, in general the wire will get SLIGHTLY longer and thinner when its bent (or straightened)
and if its kinked the effect will be even greater.

### Why is the internal resistance of a cell not constant?

Ans. This can also answer to the question that "Why  internal resistance of battery increases with usage "?

The internal resistance depends on the concentration and mobility of the ions present in the liquid or paste, and on the surface-resistance of the internal 'plates'.

Over time, due to chemical reactions, the ionic concentrations reduces. And you get deposition of high resistance materials (e.g. insoluble sulphates) on the plate surfaces. Both of these increase internal resistance.

There are many types of cell so the exact chemical changes depends on the cell-chemistry.

For more details, you could try asking under Chemistry, as battery-chemistry is a fairly specialist area.