Thursday, February 4, 2021

Electric Devices: UNIZOR.COM - Physics4Teens - Electromagnetism

Notes to a video lecture on

Electric Devices

In this chapter we will discuss different usages of electricity by grouping them according to certain criteria.

Two major groups to differentiate are electric and electronic devices.
To the category of electric devices we relate devices that use electricity for two primary purposes - to produce mechanical work, like rotation, and to produce heat, like in electric stove, including producing light by means of heating, like in incandescent lamps.

Regardless of such a simple definition of this category, the number of devices in this group is enormous, and these devices are the first ones invented to make our lives easier. Examples of these electric devices are a subject of this lecture.

To the category of electronic devices we relate all other devices that use electricity and whose primary purpose is not just to heat or to do mechanical work. They will be discussed in the next lecture.

Mechanical Work

The easiest and most common example of the usage of electricity to produce mechanical work is an electric motor. This is a device that converts electricity into rotation.

For alternating current numerous one-phase and three-phase motors are used all around us.
The primary motion they produce is rotation, which in some cases is converted into other forms of motion.

They pump water, rotate fans, work in refrigerators, rotate wheels of electric trains, lift elevator cabins, drill for oil and gas, operate machinery at manufacturing plants, move construction cranes.

Direct current in most cases comes from batteries and is used in direct current electric motors, like the one that starts the car engine, rotates a hard disk in a computer, rotates the battery powered electric drill etc.

Electric wall clock is another example of using electricity to move the wheels of a clock. Some electric clocks work off alternating current, some off direct one.

Washing machine has an electric pump to deal with water pumped in and out and another motor to rotate the drum.

Just as an illustration, let's calculate the technical characteristics of a motor that should supply water to a building, where I live in.

The water is pumped to the roof tank, then it flows down to all apartments.

We have 12 floors, each about 3 meters high, 200 apartments, each apartment needs about 100 liters of water during 3 hours in the morning.
So, the pump should pump 200·100=20,000 liters of water to the height 12·3=36 meters during 3 hours time.

This allows us to calculate work W performed during this time and power P of the pump needed to perform this work.

Each liter of water has a mass of 1 kg and, therefore, the weight of 9.8 N.
W = 9.8·20,000·36 =
= 7,056,000 J

Since the time to do this work is T = 3 hours and each hour has 3,600 seconds, the power of the motor is
≅ 653 J/sec

Usually, we need some excess of power to prevent shortage during some extra work requirements and to account for losses of power in the motor itself due to friction and heating, so a motor of about 1000 watt (1 kilowatt) should suffice, if we allow it to work without interruption.

In practice, the motor should start and stop periodically, depending on the level of water in the tank, so it has to pump faster than water is consumed and we need a more powerful motor, say 1.5 KW.
With voltage to such a motor at the level of 220V the current flowing through this motor is
I = 1500W/220V ≅ 6.8A

In addition, considering that things break and we need an uninterrupted water supply, we need the same pump with the motor of the same power to be ready to automatically pick up the load in case the main pump breaks.
That makes a design a bit more complicated with two pumps working in parallel, alternating their work and, in case one breaks, another working alone. This requires some electronic switching mechanism.


Electric heater and incandescent lamp represent this group of electric devices. They warm and light up our homes.

We use electric stove to prepare our food.

Electric hair drying fan is an example of a combined mechanical (to push the air) and heating (to heat up the air) electric device.

Drying machine uses electricity to rotate a drum, produce heat and blow it into the drum using a fan.

For illustration, let's do some calculations related to incandescent lamps.
Consider a lamp with marked power consumption of P=100W and voltage U=120V.
Here we are talking about alternating current and, therefore, all characteristics are effective.

The effective electric current running through it is
I = 100W/120V ≅ 0.8333A
The resistance of the spiral in this lamp is
R = 120V/0.8333A = 144Ω

Obviously, we can check that
P = U²/R = I²·R
With given voltage in the circuit, the power consumed by an incandescent lamp will be more when the resistance is less. That's why a lamp consuming 100W has a thicker spiral (with less resistance) than a lamp consuming 40W for the same voltage.

Another interesting example of using electricity to produce heat is welding. This is a process when an electric arc between two electrodes is formed and used to melt metal.

There are many different types of welding machines. An important characteristic is the electric current going through the arc formed between electrodes. Usually it's in hundreds of amperes, like 500A-1000A with voltage in the range 30V-60V.
That makes the power consumption of a welding machine to be somewhere from 15KW to 60KW, which is a lot, comparing to a power of about 1.5KW needed for a water pump described above.

These characteristics fluctuate as the welding process goes, they depend on the length of an electric arc and materials used as electrodes.

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