Lesson summary on technology grade 8. Household heaters and lamps

The principle of operation of an incandescent lamp

The first and most famous application of thermal action is the glow of a lamp, heating the conductor to white heat.

Incandescent lamps, which are widespread today, are designed quite simply (Fig. 1).

Fig. 1. Incandescent lamp

The main part of the lamp is the filament, which is made of tungsten (or an alloy that includes tungsten). Tungsten is used because it is a very refractory material, the melting point of tungsten is more than 30,000C. Tungsten filament, when heated, glows brightly and creates intense light radiation.

In addition to the filament, the lamp has lead contacts.

A tungsten filament, if heated in air, will burn out quickly enough. This is because it oxidizes and breaks down when heated. Therefore, in an incandescent lamp, a tungsten filament is placed inside a glass bulb, from which air is removed. The ends of the tungsten filament are connected to the contacts. Two contacts connect to two important points of the lamp - one contact connects to the spiral, which is screwed into the socket, the second contact connects to one of the contacts at the bottom of the base. This ensures the flow of electric current.

When an electric current flows, the filament of an incandescent lamp can heat up to, which provides a sufficiently bright glow.

There are various lamps: some burn brightly, give a lot of light, others dim enough. It depends on which coil is used. If the coil is thinner, the lamp will burn brighter. If the spiral is thicker, respectively, its resistance is different, this lamp will burn dimmer.

Lesson summary for grade 8 "Electric heating appliances"

a) Electric stoves, hotplates and ovens.

3rd student. Electric cookers are now gaining more and more popularity. Research by specialists has established that electric stoves have a number of indisputable advantages over gas ones. The main one is clean air. Indeed, in the atmosphere of an electrified kitchen, there are no products of incomplete combustion of gas. Surveys by hygienists show that in fully electrified apartments, residents, and especially children, are less ill than in apartments with gas stoves.

Electric stoves are explosion and fire hazardous, easy to use and, like all electrical appliances, are easily automated.

A traditional electric stove is generally similar to a gas one. In the upper part of the body there is a panel with 3-4 burners. There is a cooking cabinet under the hob and an auxiliary cabinet underneath for storing kitchen utensils. The power switches for the hotplates and the broiler are located between the hob and the broiler.

The main element of the electric stove is the burner.

In stationary floor plates, cast iron burners are used. They owe their name to the cast-iron body. Their power, depending on the diameter, can be from 1000 to 2000 W. Such burners can be easily removed from the body and replaced with another, which is very convenient for repairs.

In addition, heating elements can be installed on electric stoves.

When cooking, it is necessary to change the amount of heat given off by the burner.

Modern models of electric stoves have a seven-stage power control. An electric stove is a rather complex and expensive electrical appliance. And it must work for at least 15 years.This is only possible if it is handled carefully and correctly. The electric stove is the most powerful device in the apartment and more than half of the electricity bill falls on it. A few tips for the rational use of the electric stove can save the family a certain amount.

The widespread belief that cooking on electric stoves takes much longer than on gas stoves is wrong. Rather, it happens if the electric stove is used without knowing some of the features of its operation. And first of all, the dishes are “to blame” for this.

The fact is that heat is transferred from the surface of the burners to the bottom of the cookware due to the tight contact. To achieve the highest contact density, the working surface of the burner is subjected to special turning. The bottom of the dishes should be the same level. It is very easy to check the deflection of the pan bottom. To do this, just take a ruler and attach it to the bottom, as shown in the table "Using different utensils for cooking". Even if all pans have a flat bottom, remember that crumbs adhering to the bottom of the pans create an air gap between the burner and the bottom, with all the unpleasant consequences.

The diameter of the cookware also plays an important role, since the economy of cooking and its quality depends on it. If the diameter of the pan is less than the diameter of the burner, then the diameter of the pan is too large, then the heating of the day will be uneven and the food may burn.

How to care for an electric stove? Well, firstly, clean it every day, because if the stove is not washed for several days, then food residues will burn to its surface and form a dense crust that is difficult to wash off. It is already difficult to clean it with conventional means, and using metal objects and sandpaper for this, you can damage the enamel.

The bodies of the burners, as we have already said, are made of cast iron. But cast iron is quite fragile and requires careful handling. First of all, it must be borne in mind that when heated, thermal stresses arise in the body of the burner, which are the greater, the higher the temperature.

They also lead to cracking of the burners. To prevent this from happening, the following guidelines must be followed.

Do not turn on the hotplate at full power without cookware!

Do not place a pot of cold water on a hot hotplate!

It is strictly forbidden to use the electric stove as a heating device!

Do not clean the enamel surfaces with emery paper, as the enamel is damaged and it will become more difficult to clean it next time.

Other heating devices

In addition to incandescent lamps, there are other heating devices. These are various heaters, carpets, electric stoves. On heating, that is, on the Joule-Lenz law, devices such as boilers, irons, hair dryers, etc. are based.

All of these devices use the same principle: heating the conductor when an electric current flows. All heating devices use a heating element, which is presented either in the form of a tape or a sufficiently powerful wire (Fig. 2).

Fig. 2. Various heating elements

Electrical appliances use heating elements of various shapes and configurations. In irons, this can be one heating element, and in an electric stove, another. All heating devices have the goal of creating a comfortable and large amount of heat that can be used.

Constant heating temperature of electrical appliances

Why does the heating temperature remain constant in all heaters?

Heating of a conductor is related to electrical resistance. And the electrical resistance of the conductor depends on the temperature. The lower the conductor temperature, the lower the conductor resistance.If a conductor heats up, its resistance increases. The change in electrical resistance during heating leads to the fact that the same power is maintained all the time, which is released during heating in the conductor.

Heating elements are of particular importance today. In the near future, we can expect the appearance of heated sidewalks, heated streets, and not just the use of heating devices in the premises.

Types of electric heaters

There is no generally accepted classification of electric heaters. It certainly exists and is described in sufficient detail in GOSTs and other regulatory documents, but few adhere to it. Everything is explained by the multitude of units, equipment and devices in all areas of daily life that fall under this classification as a heating device.

Some sources suggest classifying electric heating devices according to the design of heating elements, for example:

open type;
closed type;
hermetically sealed.

However, such a classification does not give an idea of the ultimate purpose of the device itself.

A classification by the end product of heating is also proposed, for example:

water heaters;
air heaters.

However, the question arises to which class should be attributed, for example, microwave ovens or electric stoves.

Most likely, for the correct classification of electric heating devices, it is necessary to introduce their division by scope, as:

for water heating: in heating systems;
in water supply systems;
for cooking food;
for washing;

for air heating:

in heating equipment;

in drying equipment;

for cooking by;

heating the surface with utensils for cooking;

heating directly dishes with food.

However, this classification cannot cover the entire range of electric heating devices and requires a detailed decoding of each subclass.

electric heating appliances for cooking: general use, for example, electric stoves (photo 1), electric grills (photo 2), etc.
making drinks, for example, an electric kettle (photo 3) or a coffee maker (photo 4);
for baking, stewing food or frying, for example, an electric oven or electric steamer (photo 5);
to maintain the temperature of cooked food, for example, to warm baby food bottles (photo 6) or food warmers (photo 7);
for heating water, for example, boilers (photo or instantaneous water heaters (photo 9);
for washing (automatic washing machines (photo 10) and ironing (irons (photo 11), ironing boards (photo 12) things);
for space heating in the form of fan heaters (photo 13), electric radiators (photo 14) or "warm floor" systems (photo 15);
electrically heated clothing, such as blankets (photo 16) or electric heating pads (photo 17) to warm the body;
tools with electrical heating elements, such as soldering irons (photo 18) or wood burners (photo 19).

All electrical appliances, including electric heating ones, are classified according to protection classes. At the moment, only five classes are being considered for protecting a person from electric shock - zero "0", zero first "01", first "I", second "II" and third "III".

The zero class includes electric heating devices, in which the insulation of the wire connecting to the stationary household electrical network acts as a protection. An example of an electric heating device with a zero protection class is an electric iron.

Electric frying pans or electric coffee makers have protection class "I", which is characterized by the presence of an additional protective connection with the conductive parts of the devices.If the main insulation is damaged, the conductive parts of the equipment are not energized and cannot harm human health or pets.

Devices of the II class of protection do not have an additional protective conductor, but at the same time, double insulation is used in their design and the case is made of non-conductive materials. A striking example of such devices is an electric kettle with a plastic body.

There are no electric heaters with protection class III yet, as this protection class is applicable to devices with low voltage power supply.

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