Introduction to the electrical electricity and phenomena

2º international System and change of units.

Each PU, for example the Kilo when we spoke of weight, has a series of multiples and sub-multiple to refer to weights majors or minors to us, for example, if we are in a laboratory, we cannot say that this tablet or on contains 0.005 of paracetamol. But normal and the comfortable thing is to speak of 5 milligrams of paracetamol, because the previous thing, among other things, can take to errors. We will see a table where is the change of a unit, between multiples (but great) and sub-multiple (but small) In our case, each step is going to be thousand times greater (or minor).

There are other multiples majors, as the Tera (case of the Terabyte) and smaller others as femto.

3. Exponential numbers

In the technology very great numbers are handled and very small and, for this reason, we have seen before the table of multiples and sub-multiple, but when we must work with operations, but the practitioner is to use the exponential numbers.

**3.1** What is. It is an annotation of two numbers. The one down is called base and the one of above, exponent and have 5 form ³, where 5 are the base and 3 the exponent

**3.2** What represents. The answer would be a compressed number, so that a number without as much number can be shelp, for example, the previous case, 5 ³, as 5 are high to 3, to obtain the number, we would have to multiply 5 so many times as it says the exponent, that is 5*5*5 = 125.

- We have some particular cases.
- A lifted number to 1 is the same number, for example 101 = 10, 51 = 5, etc.

Any lifted base to 0 is worth 1, for example 250 = 1

We will see because this ahead but

**3,3 Cuidadin** with the signs **negatin**. Then that, taken care of with the negatives that according to where they are a thing is worth different from another one. For example, -34 is not the same a (- 3) ^{4}. In the first case, we have:

-34 = – (3 • 3 • 3 • 3) = -81

and in the second (- 3) ^{4} = -3 • -3 • -3 • -3 = 81.

In this case, the result has been different. What would happen if we changed the exponent to an odd number.

We will see now: -35 = – (3 • 3 • 3 • 3• 3) = -243

(- 3) 5 = -3 • -3 • -3 • -3 • -3 = -243

**In this case they have agreed**! This is due to that uneven by odd number he is even, but even * uneven is uneven. We remembered?

- 3,4 Rules To calculate Exponents
- If we have a parenthesis, first it is to conduct the operations that we have inside and soon the exponent. For example, (4 + 6)
^{2}= (10)^{2}= 10 • 10 = 100. We cannot elevate and soon to add. If we do this we have 42 + 62 = 16 + 36 = 52. You see? - 2 numbers in exponential form with the same base can take the same base and work with the exponents. For example, in case they multiply, we have (23) (24), that would be the same to write (2 • 2 • 2) * (2 • 2 • 2 • 2), that is to say, that we have 7 numbers 2 that are multiplying, therefore, we got to obtain the exponential number in 27.
**When two****exponential****numbers with the same base multiply, simply extreme the exponents.**

Resistivity

**Resistivity** is called to the specific opposition that has each material to be against to the passage of an electrical current. Ρ imagines by the Greek letter and it is moderate in [Ω·mm ² /m].

Each material, independent of its dimensions, has a unique resistivity, but it can raise or lower when modifying the room temperature.

A cable with some concrete dimensions, is going to have a total resistance that comes by the expression:

where:

l = Length of the cable

A = Cross-sectional Area

ρ = resistivity

Example

To calculate the resistance of a rectangular bar of side 2 * 3 cm and length 4 km if 0.0020 resistivity is Ω*mm2/m

Solution

First it is to calculate the surface of the bar that comes dice by base X height, therefore:

S = 2 * 3 cm ² = 6 cm ² = 600 mm ²

Now, we passed 4km to meters, that give 4000 meters us

Problems

- 1º Pasa the following units to the international system:
- These three values of currents. 21 mA, 68 KA, 0,05 MA
- For the time, these three 45 values ms, 2200000 µs, 0.005 Ks
- In tension, these three: 98 kV, 228 MV and 12566 mV
- Also it is possible to be asked that pass to a multiple or specific submultiple. For example, to pass 0.05 KΩ to pΩ
- To pass 56522236 mV to KV

To pass 25000 mA to KA

- 2º Exercises with expoenciales numbers
- It demonstrates with traditional divisions that 105: 102 = 10
^{(5 - 2)}= 10^{3} - To calculate the value of 105
^{*}102 3º - To calculate 105 * 102 * 107: 104
- To obtain the turn out to divide 105 between 10
^{-7} - To calculate in exponential form (23 • 22)
- Previously we have shelp that any lifted number to 1 is the base and any number lifted to zero is worth 1. It invents an exercise, for example 10, so that the result of the operation is 10.

Beam just like in section 6, but in this case, the result of the operation is 1, and thus to demonstrate 2º postulated (the one that says us that any number lifted to zero is 1)

- 3º Exercises on electricity, energy and heat
- A circuit is fed with a tension of 14 volts and absorbs an intensity of 2 amperes. To calculate the power that is being provided
- A lamp of 400 mW is connected during two hours to a taking of 220 volts. To calculate the energy that produces in that time in Julies and calories
- An electrical heating engineer is connected to the network of 220 V. If the electrical resistance is of 10 ohms What power consumes?
- An electrical resistance of a heating engineer has a value of 0.094 kΩ and works to a tension of 220 volts. If we considered that we have a room with a perfect isolation (we do not have losses of heat by any site) a) How long it has to be connected the heating engineer to warm up that room, that needs 14 K calories? and b) To make the same calculations in the same circumstances if the feeding tension is of 110 volts.

To calculate the electrical charge, if through a circuit passes an intensity of 23 mA in a time of 4 s.

- 3º Exercises resistivity
- To calculate the resistance of a cable of silver of 3 km and section 16 mm
^{2} - To calculate the resistance of 4400 cm of silver cable, if it has a diameter of 20 centimeters.

1º

To mount the circuit of the book with the program crocodile and to answer the questions. To raise DRIVE a pdf with the circuit (screenshot) and the answers.

The relay

The relay is an electromechanical device composed of a winding that, to the fed being, moves a system of mechanical elements that bring about the commutation of the electrical terminals. Very they are used in the automobile to handle high currents (current of lights, to activate motor starts, etc when activating the controls that we have in steering wheel, key contact, etc, that they handle small currents. For example, when activating the ignition key, takes place a commutation in a small commutator of the bowler, that the winding feeds on a relay and this it causes that the terminals of the same are closed to feed a starter engine that needs great amperage.

We will see an explanatory video

3º Hacer just like in exercise 2, using a relay, with double contacts.

Work on electrical consumption in the house As a showing a experience on the cost of electricity in a house, I leave a study you on if he is interesting to contract night rate