The electric magnitudes & their measure methods

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In the 2nd part of our "Circuit Analysis" we will start defining some of the main circuit concepts, the "electric magnitudes", which are voltage, current, resistance, conductarce, charge and power, their units and their measure methods. Do you want to come with us, yes? Then...

Here we go!

Definitons:

Voltage: the voltage between two ends of a path is the electrical force that would drive an electric current between those points. Specifically, voltage is equal to energy per unit charge. Voltage can be measured by a voltmeter. The unit of measurement is the volt (V). Other names (electrostatic case): potential difference, battery.

Current: the current is the rate of flow of electric charge, typically in a conductor such as wire. Current can be measured by an ammeter. The International System of Units (SI) unit of electric current is the ampere (A).

Resistance: the electrical resistance of an electrical element measures its opposition to the passage of an electric current. The SI unit of electrical resistance is the ohm (Ω).

Conductance: it is the inverse quantity of resistance and measures how easily electricity flows along a certain path. It is measured in siemens (S).

Charge: electric charge is a physical property of matter which causes it to experience a force when near other electrically charged matter. Electric charge comes in two types. These are called positive and negative. The SI unit of electric charge is the coulomb (C).

Power: electric power is the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt (W).


How can I calculate it...?:

Ok, as we know what are the main electric magnitudes our next step is: how can we calculate it?

The components of an electronic circuit can be connected in many different ways. The two simplest of these are called series and parallel. These methods get differents values in some magnitudes, then  we normally have to difference between them in the next analysis chapters.

Calculating resistances:

Resistors in serie: we only have to add his values, one by one, obtaning at the end the global/final value of all resistances.

 

For example:

a

Resistors in parallel: this is a little bit more difficult than in series. The operation we have to perform is:

 

For example:

 

Normally, the value of the resistance is printed in the resistor, using differents rings with different colors which means one or other value, as you can see in the next table/image:

Also it is an easy method to know the value of a resistor, we normally use a multimeter conected in parallel with the resistance. With this tool we can easy know what is the value of our resistor, because the multimeter doesn´t have any more additional element to show in his display the resistance value if we are conected it properly to the resistor, right in parallel.

Calculating voltages:

Calculate the total voltage of a circuit is as easy as adding the voltages (power supply/bateries) that the circuit contains.

It´s important to know that we can only measure the voltage if the bateries are located in serie, and we can not meause it if they are located in parallel.

The total voltage of a circuit is:               Vt = V1 + V2 + ... Vn

For example:

If we have a circuit with 3 bateries of 4,5 V, 9 V and 15 V located in serie, the total voltage is:

Vt = 4,5 + 9 + 15 = 28,5 Volts (V)

Normally, to calculate the total voltage of a circuit we use a voltmeter, which is an instrument used for measuring the electrical potential difference between two points in an electric circuit. 

A voltmeter is placed in parallel with a circuit element to measure the voltage drop across it.

However, we can also use the multimeter to measure voltage if we don´t have a voltmeter, working like this:

Calculating currents:

Now, to calculate currents we have a powerful tool, the Ohm´s Law:

Voltage (V) = Current (I) x Resistance (R)

V = I x R

Remember it, is very important! 

For example:

Imagine you have this circuit: 

To calculate the current I:

Voltage = 9 V

Resistance = 3 + 3 + 3 = 9 Ω 

Current = ?

9 V = I x 9 Ω => I = 9V/9Ω = 1 A

Then the current we are looking for is:

1 A ; 1 ampere

We can also calculate the current with instruments in order to do it with hand. To calculate the current we use an ammeter, which is a measuring instrument used to measure the electric current in a circuit. The ammeter is connected in series with the circuit carrying the current to be measured.

However, we can also use the multimeter to measure current if we don´t have an ammeter.

Ending with this chapter I bring you an useful table to remember the concepts we have view/learn in this post:

Now we have the skills to start with the real analysis of an electronic circuit. We are going to start on it in the next chapter/post. 

Please don´t forget us and... See you in the next Post!