Electricity

Let Us Learn About Electricity.

Electricity is one of the most important sources of energy. Lights, fans, motors, radios and television are some common appliances which work on electricity. In general usage, the word "electricity" is adequate to refer to a number of physical effects. In scientific usage, however, the term is vague, and these related, but distinct, concepts are better identified by more precise terms.

It can also define as, Electricity is a form of energy produced by the movement of electrons. Electricity is electrical power or an electric current. This form of energy can be sent through wires in a flow of tiny particles. It is used to produce light and heat and to run motors.

Now Let Us learn about Electric Charges

The word'electric' is derived from the Greek word 'elektron' meaning amber. The existences of charges were known when charged particles were produced by rubbing (due to friction) of suitable materials. These facts are demonstrated by simple experiments.

a) uspend a glass rod rubbed with silk. Bring another glass rod rubbed with silk nearer to the suspended rod. It is observed that, the suspended rod will swing away, showing repulsion.

b)Similarly, suspend a plastic rod rubbed with fur and bring another plastic rod rubbed with fur closer to it, we observe that the rods repel.

c)Again, if we suspend a glass rod rubbed with silk and bring a plastic rod rubbed with fur nearer to the glass rod, we observe that the rods attract.

From the above, we find that glass rod rubbed with silk acquire 'something' different from that of plastic rubbed with fur. That 'something' is called 'charge'.

To know about the electric charges we must know the structure of atom. It was discovered in 1911, by Rutherford performed by Hans Geiger and Earnest Marsden, under the direction of Rutherford that an atom is made up from the protons and neutrons in the nucleus, with a very small contribution from the or biting electrons

An atom consists of nucleus centrallyplaced with protons consisting positive charge. They seem to be surrounded by a kind of invisible force field.

This is called a a electrostatic field

The neutral particles are neutrons.

The electrons are the negative charged that revolves around the nucleus. This negatively charged electrostatic field is exactly the same strength as the electrostatic field in protons.

The negative charge of an electron is the same as the positive electrical charge of the much larger in size proton. This way the atom stays electrically neutral. The value of one charge is 1.6×10⁻¹⁹ coulombs (+ for protons and – for electrons).

In Physics we usually call the charge as electric charge, electrostatic charge, electrical charge or simply charge. We denote it by q. +q means protons and –q means electrons.

In a neutral atom the number of protons and number of electrons are same hence it does not carry any charge. But when the number or protons and number or electrons vary then atom acquires charge. If there are fewer electrons than protons, the atom has a positive charge. The amount of charge carried by an atom is always a multiple of the elementary charges, the elementary charge is e (e = ± .6×10⁻¹⁹ coulombs). When an atom losses electrons than it acquires positive charge and when it gains electrons it acquires negative charge. Hence, the net charge on an atom is q = ne, where n is number or electrons lost or gained.

THERMAL EXPANSION

Let Us Learn About Thermal Expansion

Thermal expansion is the tendency of matter to change in volume in response to a change in temperature. When a substance is heated, its particles begin moving and become active thus maintaining a greater average separation. Materials which contract with increasing temperature are rare; this effect is limited in size, and only occurs within limited temperature ranges. The degree of expansion divided by the change in temperature is called the material's coefficient of thermal expansion and generally varies with temperature.

It is our common experience that most substances expand on heating and contract on cooling. A change in the temperature of a body causes change in its dimensions. The increase in the dimensions of a body due to the increase in its temperature is called thermal expansion. The expansion in length is called linear expansion. The expansion in area is called area expansion. The expansion in volume is called volume expansion

Scale Of Temperature

Important scales of temperature


Let us learn about scale of temperature

A reference scale with respect to which the temperatures can be measured is known as 'scale of temperature'. Various scales of temperatures are in use. Important scales of temperature are:

• Celsius scale

• Fahrenheit scale

• Kelvin Scale

Lower and upper fixed point of temperature

To devise a scale of temperature, fixed reference points (temperature) are required, with respect to which all other temperatures are measured. For both Celsius and Fahrenheit Scales of temperatures, the fixed points are as follows:

Lower fixed point:

Melting point of pure ice at normal atmospheric pressure is regarded as the lower fixed point.

Upper fixed point

Boiling point of pure water at normal atmospheric pressure is regarded as the lower fixed point.

Celsius Scale of Temperature Celsius scale of temperature was devised by a Swedish astronomer Anders Celsius (1701 1744). In this scale, the lower fixed point (the temperature of melting ice at normal atmospheric pressure) is taken as zero degree Celsius, written as 0⁰C. The upper fixed point (the temperature of pure boiling water at normal pressure of 76 cm of mercury) is considered to be hundred degree Celsius, written as 100^OC. The interval between 0⁰C and 100⁰C is divided into hundred equal parts. Each part represents 1⁰C. This is a convenient scale of temperature, which is widely used.

Fahrenheit Scale of Temperature

This scale of temperature was devised by Gabriel Fahrenheit (1687-1736). The lower and upper fixed points in this scale are considered as 32⁰F and 212⁰F respectively. The interval of 180⁰ F is divided into 180 equal parts. Each part is known as 1⁰F. This is widely used by doctors.

TEMPERATURE AND HEAT

MEASUREMENT OF TEMPERATURE


Before learning how to measure temperature, let us learn what is temperature

Temperature is a relative measure, or indication of hotness or coldness. A hot utensil is said to have a high temperature, and ice cube to have a low temperature.


















A measure of temperature is obtained using a thermometer. ( A thermometer is a device that measures temperature using a variety of different principles)

1. About Digital Thermometers

2. Appliance Thermometers

3. Bimetallic-Coil Thermometers

4. Calibrating the Thermometers

5. Cooking Temperatures

6. Disposable Thermometers

7. Food Safety Temperature Guide

8. Instant-Read Bimetallic Coil Thermometers

9. Oven Cord Thermometers

10. Oven Safe Bimetallic Coil Thermometers

11. Oven Thermometers

12 Placing the Thermometer

13. Refrigerator and Freezer Thermometers

14. Thermistor Thermometers

15. Thermocouple Thermometers

16. Thermometer Fork

Many physical properties of materials change sufficiently with temperature to be used as the basis for constructing thermometers. The commonly used property is variation of the volume of a liquid with temperature. For example, a common thermometer (the liquid-in-glass type) with
which you are familiar. Mercury and alcohol are the liquids used in most liquid-in-glass
thermometers. Thermometers are calibrated so that a numerical value may be assigned to a given temperature.

For the definition of any standard scale, two fixed reference points are needed. Since all substances change dimensions with temperature, an absolute reference for expansion is not available. However, the necessary fixed points may be correlated to physical phenomena that always occur at the same temperature. The ice point and the steam point of water are two convenient fixed points and are known as the freezing and boiling points. These two points are the temperatures at which pure water freezes and boils under standard pressure. The two familiar temperature scales are the Fahrenheit temperature scale and the Celsius temperature scale. The ice and steam point have values 32 °F and 212 °F respectively, on the Fahrenheit scale and 0 °C and 100 °C on the Celsius scale. On the Fahrenheit scale, there are 180 equal intervals between two reference points, and on the celsius scale, there are 100.


'scale of temperature' A reference scale with respect to which the temperatures can be measured is known as 'scale of temperature'. Various scales of temperatures are in use. Important scales of temperature are:

• Celsius scale

• Fahrenheit scale

• Kelvin Scale

Lower and upper fixed point of temperature

To devise a scale of temperature, fixed reference points (temperature) are required, with respect to which all other temperatures are measured. For both Celsius and Fahrenheit Scales of temperatures, the fixed points are as follows:

Lower fixed point:

Melting point of pure ice at normal atmospheric pressure is regarded as the lower fixed point

DETECTION OF AMPLITUDE MODULATED WAVE

Let Us learn about intermediate frequency

First of all let us understand meaning of intermediate frequency (IF)

The transmitted message gets attenuated in propagating through the channel. The receiving antenna is therefore to be followed by an amplifier and a detector. In addition, to facilitate further processing, the carrier frequency is usually changed to a lower frequency by what is called an intermediate frequency (IF) stage preceding the detection. The detected signal may not be strong enough to be made use of and hence is required to be amplified.

A block diagram of a typical receiver is shown here (a)

Detection is the process of recovering the modulating signal from the modulated carrier wave. We just saw that the modulated carrier wave contains the frequencies ωc and ωc ± ωm. In order to obtain the original message signal m(t ) of angular frequency ωm

a simple method is shown in the form of a block diagram(b)The modulated signal of the form given in (a) is passed through a rectifier to produce the output shown in (b). This envelope of signal (b) is the message signal. In order to retrieve m(t ), the signal is passed through an envelope detector.

Let Us learn the reason for using intermediate frequency.

Intermediate frequencies are used for three general reasons.

First reason at very high frequencies, signal processing circuitry performs poorly. Active devices such as transistors cannot deliver much amplification (gain) without becoming unstable. Ordinary circuits using capacitors and inductors must be replaced with cumbersome high frequency techniques such as striplines and waveguides. So a high frequency signal is converted to a lower IF for processing.

A second reason to use an IF, in receivers that can be tuned to different stations, is to convert the various different frequencies of the stations to a common frequency for processing. It is difficult to build amplifiers, filters, and detectors that can be tuned to different frequencies, but easy to build tunable oscillators. Superheterodyne receivers tune in different stations simply by adjusting the frequency of the local oscillator on the input stage, and all processing after that is done at the same frequency, the IF. Without using an IF, all the complicated filters and detectors in a radio or television would have to be tuned in unison each time the station was changed, as was necessary in the early tuned radio frequency receivers.

Third reason for using an intermediate frequency is to improve frequency selectivity. In communications circuits a very common task is to separate out or extract signals or components of a signal that are close together in frequency. This is called filtering. Some examples are, picking up a radio station among several that are close in frequency, or extracting the chrominance subcarrier from a TV signal. With all known filtering techniques the filter's bandwidth increases proportionately with the frequency. So a narrower bandwidth and more selectivity can be achieved by converting the signal to a lower IF and performing the filtering at that frequency.

Our next topic we shall learn about Frequency Modulation.

Waves

Let Us learn about Sky waves

First of all let us learn what are waves

The wave is a physical phenomenon that is found in a variety of contexts. You undoubtedly know about ocean waves and have probably played with a stretched slinky toy, producing undulations which move rapidly along the slinky. Other examples of waves are sound, vibrations in solids, and light.

let us learn meaning of Ground wave, Sky waves and Space wave.

Ground wave is also called surface wave. a surface wave is a mechanical wave that propagates along the interface between differing media, usually two fluids with different densities. A surface wave can also be an electromagnetic wave guided by a refractive index gradient. In radio transmission, a ground wave is a surface wave that propagates close to the surface of the Earth.

sky wave, often called the ionospheric wave, is radiated in an upward direction and returned to Earth at some distant location because of refraction from the ionosphere. This form of propagation is relatively unaffected by the Earth's surface and can propagate signals over great distances. Usually the high frequency (hf) band is used for sky wave propagation. The following in-depth study of the ionosphere and its effect on sky waves will help you to better understand the nature of sky wave propagation.

Space waves A radio wave that follows two distinct paths from the transmitting antenna to the receiving antenna—one through the air directly to the receiving antenna, the other reflected from the ground to the receiving antenna. The primary path of the space wave is directly from the transmitting antenna to the receiving antenna. So, the receiving antenna must be located within the radio horizon of the transmitting antenna. Although space waves suffer little ground attenuation, they nevertheless are susceptible to fading. This is because space waves actually follow two paths of different lengths the receiving site and, therefore, may arrive in or out of phase. If these two component waves are received in phase, the result is a reinforced or stronger signal. Alternately, if they are received out of phase, they tend to cancel one another, which results in a weak or fading signal.

Television broadcast, microwave links and satellite communication are some examples of communication systems that use space wave mode of propagation.