Notebook

How do an induction cooker work?

Bibi Mohanan June 19, 2016

An induction cooktop is simply an electromagnet you can cook with. Inside the glass cooktop, there's an electronically controlled coil of metal. When you turn on the power, you make a current flow through the coil and it produces a magnetic field all around it and (most importantly) directly above it. it generates a constantly changing magnetic field. It does not generate heat directly. You can put your hand on top of it and you won't feel a thing. (Warning: Don't ever put your hand on a cooktop that has recently been used for cooking because it may have become dangerously hot from the cooking pan that's been standing on top of it.)

Image result for how induction cooker works

Image result for how induction cooker works

When you stand a suitable cooking pan on top of an induction cooktop that's powered up, the magnetic field produced by the cooktop penetrates the metal of the pan. So we have a fluctuating magnetic field moving around inside a piece of metal (the base and sides of the pan)—and that makes an electric current flow through the pan too (that's all that induction means). Now this is not quite the same as the electric current that flows through a wire, carrying electrical energy in a straight line from (say) a battery to a flashlight bulb. It's a kind of whirling, swirling electric current with lots of energy but nowhere to go; we call it an eddy current. As it swirls around inside the metal's crystalline structure, it dissipates its energy. So the metal pan gets hot and heats up whatever food is inside it, first by conduction (it passes its heat energy directly to the food) but also by convection (liquid food rises and falls in the pan carrying heat with it).

How do an induction cooker work?

Reviewed by Bibi Mohanan on June 19, 2016 Rating: 5

Power measurement in a transmission line

Bibi Mohanan June 19, 2016

In a transmission line, power is measured by means of a voltmeter between the conductors,and an ammeter in series with one of the conductors (Fig.). Then the power, P (in watts) is equal to the product of the voltage E (in volts) and the current I(in amperes). This technique can be used in any transmission line, be it for 60-Hz utility service, or in a radio transmitting station. But is this indication of power the same as the power actually dissipated by the load at the end of the line? Not necessarily.

Recall, from the discussion of impedance, that any transmission line has a characteristic impedance. This value, Zo, depends on the size of the line conductors, the spacing between the conductors, and the type of dielectric material that separates the conductors. For a coaxial cable, Zo can be anywhere from about 50 to 150 Ω. For a parallel-wire line, it can range from about 75 Ω to 600 Ω.If the load is a pure resistance R containing no reactance, and if R _ Zo, then thepower indicated by the voltmeter/ammeter scheme will be the same as the true power

dissipated by the load. The voltmeter and ammeter must be placed at the load end of the transmission line.

If the load is a pure resistance R, and R < Zo or R > Zo, then the voltmeter and ammeter will not give an indication of the true power. Also, if there is any reactance in the load, the voltmeter/ammeter method will not be accurate.

The physics of this is rather sophisticated, and a thorough treatment of it is beyond

the scope of this course. But you should remember that it is always desirable to have the load impedance be a pure resistance, a complex value of R _ j0, where R _ Zo. Small discrepancies, in the form of a slightly larger or smaller resistance, or a small reactance, can sometimes be tolerated. But in very-high-frequency (VHF), ultra-high-frequency (UHF) and microwave radio transmitting systems, even a small impedance mismatch between the load and the line can cause excessive power losses in the line.

An impedance mismatch can usually be corrected by means of matching transformersand/or reactances that cancel out any load reactance.

Power measurement in a transmission line

Reviewed by Bibi Mohanan on June 19, 2016 Rating: 5

Primary and secondary cells

Bibi Mohanan June 19, 2016

Primary and secondary cells

Some electrical cells, once their potential (chemical) energy has all been changed to electricity and used up, must be thrown away. They are no good anymore. These arecalled primary cells.

Other kinds of cells, like the lead-and-acid unit depicted above, can get their chemical

energy back again. Such a cell is a secondary cell.

Primary cells include the ones you usually put in a flashlight, in a transistor radio,

and in various other consumer devices. They use dry electrolyte pastes along with

metal electrodes. They go by names such as dry cell, zinc-carbon cell, alkaline cell,

and others. Go into a department store and find the panel of batteries, and you’ll see

various sizes and types of primary cells, such as AAA batteries, D batteries, camera batteries, and watch batteries. You should know by now that these things are cells, not truebatteries. This is a good example of a misnomer that has gotten so widespread that

store clerks might look at you funny if you ask for a couple of cells. You’ll also see real

batteries, such as the little 9-V transistor batteries and the large 6-V lantern batteries.

Secondary cells can also be found increasingly in consumer stores. Nickel-cadmium

(Ni-Cd or NICAD) cells are probably the most common. They’re available in

some of the same sizes as nonrechargeable dry cells. The most common sizes are AA, C,and D. These cost several times as much as ordinary dry cells, and a charging unit also costs a few dollars. But if you take care of them, these rechargeable cells can be usedhundreds of times and will pay for themselves several times over if you use a lot of “batteries” in your everyday life.

The battery in your car is made from secondary cells connected in series. These cells

recharge from the alternator or from an outside charging unit. This battery has cells likethe one in Fig.

. It is extremely dangerous to short-circuit the terminals of such a battery,

because the acid (sulfuric acid) can “boil” out and burn your skin and eyes.

An important note is worth making here: Never short-circuit any cell or battery, because

it might burst or explode.

Lead acid battery

Primary and secondary cells

Reviewed by Bibi Mohanan on June 19, 2016 Rating: 5

Cells and batteries

Bibi Mohanan June 19, 2016

ONE OF THE MOST COMMON AND MOST VERSATILE SOURCES OF DC IS THE CELL.

The term cell means self-contained compartment, and it can refer to any of various different things in (and out of) science. In electricity and electronics, a cell is a unit source of dc energy. There are dozens of different types of electrical cells.

When two or more cells are connected in series, the result is known as a battery.

Kinetic and potential energy .

Energy can exist in either of two main forms. Kinetic energy is the kind you probably think of right away when you imagine energy. A person running, a car moving down a freeway, a speeding aircraft, a chamber of superheated gas—all these things are visible manifestations of kinetic energy, or energy in action. The dissipation of electrical power, over time, is a form of kinetic energy too.

Potential energy is not as vividly apparent. When you raise a block of concrete into

the air, you are creating potential energy. You remember the units called foot pounds, the best way to measure such energy, from school physics classes. If you raise a one-pound weight a foot, it gains one foot pound of potential energy. If you raise it 100 feet, it gains 100 foot pounds. If you raise a 100-pound weight 100 feet, it will gain 100 × 100, or 10,000, foot pounds of potential energy. This energy becomes spectacularly evident if you happen to drop a 100-pound weight from a tenth-story window.

Electrochemical energy

In electricity, one important form of potential energy exists in the atoms and molecules of some chemicals under special conditions. Early in the history of electrical science, laboratory physicists found that when metals came into contact with certain chemical solutions, voltages appeared between the pieces of metal. These were the first electrochemical cells.

A piece of lead and a piece of lead dioxide immersed in an acid solution (Fig.)will show a persistent voltage. This can be detected by connecting a galvanometer between the pieces of metal. A resistor of about 1,000 ohms should always be used in series with the galvanometer in experiments of this kind; connecting the galvanometer directly will cause too much current to flow, possibly damaging the galvanometer and causing the acid to “boil.”

The chemicals and the metal have an inherent ability to produce a constant exchange of charge carriers. If the galvanometer and resistor are left hooked up between the two pieces of metal for a long time, the current will gradually decrease, and the electrodes will become coated. The acid will change, also. The chemical energy, a form of potential energy in the acid, will run out. All of the potential energy in the acid will have been turned into kinetic electrical energy as current in the wire and galvanometer. In turn, this current will have heated the resistor (another form of kinetic energy), and escaped into the air and into space.

Cells and batteries

Reviewed by Bibi Mohanan on June 19, 2016 Rating: 5

Bar-graph meters

Bibi Mohanan June 19, 2016

A cheap, simple kind of meter can be made using a string of light-emitting diodes (LEDs) or a liquid-crystal display (LCD) along with a digital scale, to indicate approximate levels of current, voltage or power. This type of meter has no moving parts to break, just like a digital meter. But it also offers the relative-reading feeling you get with an analog meter.

Figure below is an example of a bar-graph meter that is used to show the power output, in kilowatts, for a radio transmitter. It indicates 0.8 kW or 800 watts, approximately.

The chief disadvantage of the bar-graph meter is that it isn’t very accurate. For this reason it is not generally used in laboratory testing. The LED or LCD devices sometimes also flicker when the level is “between” two values given by the bars. This can be annoying to some people.

Bar-graph meters

Reviewed by Bibi Mohanan on June 19, 2016 Rating: 5

Pen recorders

Bibi Mohanan June 19, 2016

A meter movement can be equipped with a marking device, usually a pen, to keep a graphic record of the level of some quantity with respect to time. Such a device is called a pen recorder. The paper, with a calibrated scale, is taped to a rotating drum. The drum, driven by a clock motor, turns at a slow rate, such as one revolution per hour or one revolution in 24 hours. A simplified drawing of a pen recorder is shown in Fig.

A device of this kind, along with a wattmeter, might be employed to get a reading

of the power consumed by your household at various times during the day. In this way you might tell when you use the most power, and at what particular times you might be using too much.

Pen recorders

Reviewed by Bibi Mohanan on June 19, 2016 Rating: 5

Light meters or illumination meter./Lux meter

Bibi Mohanan June 19, 2016

Light intensity is measured by means of a light meter or illumination meter. You might think that it’s easy to make this kind of meter by connecting a milliammeter to a solar (photovoltaic) cell. And this is, in fact, a good way to construct an inexpensive light meter (Fig.).

More sophisticated devices might use dc amplifiers to enhance sensitivity and to allow for several different ranges of readings. One problem with this design is that solar cells are not sensitive to light at exactly the same wavelengths as human eyes. This can be overcome by placing a colored filter in front of the solar cell, so that the solar cell becomes sensitive to the same wavelengths, in the same proportions, as human eyes. Another problem is calibrating the meter. This must usually be done at the factory, in units such as lumens or candela. Its not important that you know the precise definitions of these units in electricity and electronics.

Light meters or illumination meter./Lux meter

Reviewed by Bibi Mohanan on June 19, 2016 Rating: 5

VU and decibel meters

Bibi Mohanan June 19, 2016

In high-fidelity equipment, especially the more sophisticated amplifiers (“amps”), loudness meters are sometimes used. These are calibrated in decibels, a unit that you will sometimes encounter in reference to electronic signal levels. A decibel is an increase or decrease in sound or signal level that you can just barely detect, if you are expecting the change.

Audio loudness is given in volume units (VU), and the meter that indicates it is called a VU meter. Usually, such meters have a zero marker with a red line to the right and a black line to the left, and they are calibrated in decibels (dB) above and below this zero marker (ref fig).

The meter might also be calibrated in watts rms, an expression for audio power.

As music is played through the system, or as a voice comes over it, the VU meter

needle will kick up. The amplifier volume should be kept down so that the meter doesn’t go past the zero mark and into the red range. If the meter does kick up into the red scale, it means that distortion is probably taking place within the amplifier circuit.

Sound level in general can be measured by means of a sound-level meter, calibrated in decibels (dB) and connected to the output of a precision amplifier with a microphone of known, standardized sensitivity (Fig. 3-13). You have perhaps heard that a vacuum cleaner will produce 80 dB of sound, and a large truck going by might subject your ears to 90 dB. These figures are determined by a sound-level meter. A VU meter is a special form of sound-level meter.

VU and decibel meters

Reviewed by Bibi Mohanan on June 19, 2016 Rating: 5

Frequency counters

Bibi Mohanan June 19, 2016

The measurement of energy used by your home is an application to which digital metering is well suited. It’s easier to read the drum type, digital kilowatt-hour meter than to read the pointer type meter. When measuring frequencies of signals, digital metering is not only more convenient, but far more accurate.

The frequency counter measures by actually counting pulses, in a manner similar

to the way the utility meter counts the number of turns of a motor. But the frequency counter works electronically, without any moving parts. It can keep track of thousands, millions or even billions of pulses per second, and it shows the rate on a digital display that is as easy to read as a digital watch. It measures frequency directly by tallying up the number of pulses in an oscillating wave, even when the number of pulses per second is huge.

The accuracy of the frequency counter is a function of the lock-in time. Lock-in is usually done in 0.1 second, 1 second or 10 seconds. Increasing the lock-in time by a factor of 10 will cause the accuracy to be good by one additional digit. Modern frequency counters are good to six, seven or eight digits; sophisticated lab devices will show frequency to nine or ten digits.