Electricity
Electricity Units and Quantities
The key quantities of electrical engineering are shown in Table 1.
For example, the voltage of the battery can be 12 V, i.e. 12 volts, or for example 9 V, and
in a standard socket in the wall in Finland the voltage is 230 V (rms value,
alternating current, do not mind about this). Electrical current occurs in
everyday life, for example when looking at fuse sizes. Normal fuse sizes in
standard house electrical systems are, for example, 10 A and 16 A fuse sizes.
The resistance rarely appears directly in the device manufacturer's
information. However, we may need resistance with calculations. The unit of
resistance is the ohm and the symbol for the unit is the Greek letter capital
Omega "W". The unit of
power is the watt and power can be found in many places. For example, in a
microwave oven, the choices for power can be found to be, for example, 170 W
(i.e. 170 watts), 340 W, 510 W, 680 W and 850 W.
With the electrical engineering the prefixes mega, kilo, milli and micro are widely
used. Mega is a million, kilo is a thousand, milli is a thousandth and micro is
a millionth. In the SI system, changing these prefixes to another always means
just moving the decimal separator. Table 2 shows the use of prefixes with
quantities in electrical engineering.
Kuva Pixabay.
For example, the voltage of the battery can be 12 V, i.e. 12 volts, or for example 9 V, and
in a standard socket in the wall in Finland the voltage is 230 V (rms value,
alternating current, do not mind about this). Electrical current occurs in
everyday life, for example when looking at fuse sizes. Normal fuse sizes in
standard house electrical systems are, for example, 10 A and 16 A fuse sizes.
The resistance rarely appears directly in the device manufacturer's
information. However, we may need resistance with calculations. The unit of
resistance is the ohm and the symbol for the unit is the Greek letter capital
Omega "W". The unit of
power is the watt and power can be found in many places. For example, in a
microwave oven, the choices for power can be found to be, for example, 170 W
(i.e. 170 watts), 340 W, 510 W, 680 W and 850 W.
With the electrical engineering the prefixes mega, kilo, milli and micro are widely
used. Mega is a million, kilo is a thousand, milli is a thousandth and micro is
a millionth. In the SI system, changing these prefixes to another always means
just moving the decimal separator. Table 2 shows the use of prefixes with
quantities in electrical engineering.
Kuva Pixabay.
Formulas
The laws of physics are written in formulas that use symbols of the quantity. Below are the
two key formulas of electricity, Ohm's law which is the formula (1) and the
power formula which is the formula (2).
U = RI (1)
P = UI (2)
Using Ohm's law, the power formula can also be written in the forms P = U2/R and P = RI2.
Ohm's law states that voltage is equal to resistance times current. The power formula
says that power is equal to voltage times current, or power is equal to voltage
to the power of two (to exponent two) divided by resistance, or power is equal
to resistance times current to the power of two (to exponent two). Power
(exponent) tells how many times the thing has been multiplied by itself, e.g. I2
= I × I and U2 = U × U. Yes, the word ”power” can here mean
the quantity of power with units of watts, or the mathematical operation.
Example 1.
Here are examples about exercises in electrical engineering.
