Showing posts from January, 2014

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 y…

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 β. In the common-base circuit, we follow another b…

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…

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 CurrentDirect CurrentAmount of energy that can be carriedSafe 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 electronsRotating magnet along the wire.Steady magnetism along the wire.FrequencyThe frequency of alternating current is 50Hz or 60Hz depending upon the country.The frequency of direct current is zero.Dire…

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 potenti…

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.

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.

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.

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.
 See the complete explaination: Download the document

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?

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 el…


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/vol…

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 me…

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 Transforme…

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…

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 Curren…

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.

View Online

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. Cond…


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 - r…

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 …

Quantum Theory of Solids-2nd ed-Charles_Kittel


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.