Get to know the 4 basic electrical quantities

There’s no way around it! You’re surrounded by electricity in EVERY aspect of your life.The alarm clock that woke you up, the water you drank, the shower you took, the food you ate, your work/school/college computer, your phone for that awesome social media post…

Everything! We’re ultra-dependent on electricity. Without it, we’d be back in the Stone Age, and chaos would unfold—because, let’s be honest, I don’t know how to make fire using stones, wood, magnifying glasses, or whatever they use in movies... Oh! And don’t forget that even lighters and matches are made in factories that also need electricity to be produced.

So then…

If it’s so vital to everyone, do you know the basic electrical quantities like voltage, current, resistance, and power?

Well, if it’s that important, it’s only natural that we should know about it. So, here’s a summary below explaining what electrical quantities are, along with concepts, definitions, and other key information.

What is electric voltage?

Measured in Volts [V], electric voltage is the potential difference (PD) between two points (initial and final) in the presence of an electric field. Each point has an electric potential associated with the amount of work “W” measured in joules required to move a charge “q” measured in coulombs. Therefore, voltage is the difference between these two points.

It's easier to understand with water. Similarly, imagine a water tank on top of a house. The water tank is our "energy source." It’s connected to a faucet near the ground.

In the case of the water tank, the hydrostatic pressure associated with the height of the tank is like our electrical potential. There will be a flow of water from the point of higher pressure to the point of lower pressure. This is the mechanics of electricity.

Electric current (which is a flow of charges) flows from the point of higher electric potential to the point of lower potential due to this "electromotive force" (electric voltage between the + and - points). Note that for the current to flow, we need a medium (wire or conductor, analogous to the plumbing), and this medium limits the amount of water (flow rate) or current in the electrical analogy. This is the operating mechanism of the first OHM'S LAW, which relates the electrical quantities: Electric Voltage, Electric Current, and a constant called Electrical Resistance.

Direct voltage and alternating voltage

We have two well-defined types of voltage in our daily life, which are:

Direct Current (DC) Voltage

Constant voltage that does not change polarity over time. Examples include: 1.5V or 9V batteries, 12V, 24V, or 48V car batteries, smartphone charger outputs (5V), among others. It is common to refer to direct current voltage as VCC (Voltage Common Collector), VDC (Voltage Direct Current), or simply as DC (Direct Current). Both cases refer to systems with continuous voltage and current. Also, it is important to note that we are surrounded, in large part, by voltages with values between one or two tens.

Alternating Voltage

The voltage found in the standard outlets of our homes, on power poles, in motors, or among countless other applications. It is the voltage that sustains the entire Energy Supply System in the National Interconnected System. Alternating voltage has its polarity alternated over time at a defined frequency. In the outlet, the nominal frequency is 60Hz, meaning it alternates 60 times per second between positive and negative.

As a rule, our energy generation and entire distribution are in alternating voltage, as it is safer for transport and less costly. Since most electro-electronic devices require direct voltage, it is natural to always have a "source" that converts AC (Alternating Current) into DC (Direct Current). This is the famous AC/DC, referring to this conversion, and also to that famous band.

O que é corrente elétrica?

Qual a definição de corrente elétrica?

A corrente elétrica “i” é definida como um fluxo ordenado de cargas em um  meio condutor. Imagine um fio condutor cortado perpendicularmente por um plano hipotético. Corrente é a quantidade de cargas que passam em um intervalo de tempo neste plano em um mesmo sentido.

A corrente elétrica é medida em Ampére [A], conforme o Sistema Internacional de Unidades “SI”. Como se trata de um movimento de carga em um espaço pequeno de tempo, tem-se que:  1 Ampere = 1 Coulomb/1 segundo ; A=C/s.

É natural que o leitor pergunte o porquê da seta “i” apontar pro sentindo real de movimento das cargas. Respondo-lhes, foi convencionado a muito tempo atrás, globalmente, que a corrente sai de um potencial/pólo positivo, para um negativo. Chamamos de sentido convencional da corrente.

Estas cargas são elétrons livres que se movem, em função da diferença do potencial elétrico entre as extremidades do condutor. Lembre-se, a diferença de potencial elétrico é estabelecida a partir de um campo elétrico que forma níveis diferentes de energia potencial, resultando na tensão necessária para gerar o movimento das cargas elétricas.

What is electrical resistance?

Definition of electrical resistance

We already know what voltage and electric current are. Now, our third star is missing! Electrical resistance.

In our previous example, the amount of water flowing directly depends on the diameter of the pipe. Larger pipes, with a bigger "gauge" or section, allow a greater volume of water to pass through. Remember the wires and conductors in your house.

The wires that connect an electric shower or higher power loads, like a motor or a super washer, have a larger section and are "thicker" than the wire in a socket that charges a smartphone, for example. The shower needs much more current to maintain a hot shower. And a lot more… about 30 times the current of a smartphone. With this analogy, we qualitatively define what electrical resistance is.

Electrical resistance is defined as the opposition to the flow of electric current. It varies according to the conductivity or resistivity of the material, width, length, and temperature. Electrical resistance is measured in "Ohms," symbolized by the Greek letter omega [?].

Thus, 1 = 1V1A, meaning that a voltage of 1 Volt is needed to flow 1 Ampere of current, with a conductor of 1Ω (read as: one ohm). So, to have more current, we need to apply more voltage. We finally arrive at the fundamental concept of electricity: Ohm’s First Law.

Ohm's Law

Ohm's Law states that a conductor at constant temperature has a constant electrical resistance associated with the ratio between voltage and current. In other words, the voltage difference or electric tension, divided by the electric current, is equal to the resistance of the resistor, R, which represents the electrical resistance of the conductor.

From this, we can size, calculate, design, and model various electrical circuits and systems, providing us with the foundation to understand something that's hard to visualize, but extremely present and necessary: electricity.

Remember the electric shower? Ever heard of the circuit breaker tripping in the middle of a shower, giving the person a cold shock? Why does that happen? Ohm's Law explains it! What current should I choose to size a circuit breaker? We know that in a household, we have a voltage of 127 or 220VAC here in Brazil, and the power ratings indicated on the shower's packaging are around ±6000W… But wait! Power? What is electrical power?

O que é potência elétrica?

Electrical power is the amount of electrical energy used to perform work. A refrigerator converts the electrical power supplied by the outlet into temperature to keep food cool. A toaster converts electrical power and consumes it, transforming it into heat to make a nice toast.

Note that power is a physical quantity that can be transformed, meaning, for example, converting electrical power into mechanical power. And that’s exactly what we want! We want electrical energy to work for us! See the difference in the brightness of light bulbs with higher power:

Power is defined in Ohm's first law as:

• P = V * I (Power equals voltage multiplied by current);

• The unit of electrical power is the Watt [W].
  

It is natural, therefore, since we are talking about power, to evaluate what electrical consumption is, as they are directly related!

The difference between power and consumption

he energy consumption represents the amount of electrical energy used, that is, consumed over a certain period, meaning we have "time" associated with power. Consumption can vary depending on the power of the devices and the number of minutes, hours, or days they remain on.

To calculate electrical energy consumption, we just need to know the power of the device, as well as the time the device is running. The formula we use to calculate electrical energy consumption is the following:

P – power (kW)

?t – time interval of use (h)

This formula shows that the electrical energy consumption, which is measured in kWh (Kilo-Watt-hour), can be calculated by the product of the power (in kW = 1000W, because k = 10³), which is usually provided on the device, and the time interval that the device operates (in hours).

If we still want to find the value in reais of the consumption, just multiply the kWh by the rate applied by the local utility company.

Basic Electrical Quantities and Formulas

The main electrical quantities, in many applications, can be manipulated according to Ohm's First Law. Below, we present the most commonly used ones and a table with the possible manipulations:

Electric Voltage Formula:

• V = R * I (Voltage is equal to resistance multiplied by current);

• V = P / I (Voltage is equal to power divided by current);

Electric Power Formula:

• P = V * I (Power is equal to voltage multiplied by current);

• P = R * I² (Power is equal to resistance multiplied by the square of the current);

Electric Resistance Formula:

• R = P / I² (Resistance is equal to power divided by the square of the current);

• R = V / I (Resistance is equal to voltage divided by current);

Electric Current Formula:

• I = P / V (Current is equal to power divided by voltage);

• I = V / R (Current is equal to voltage divided by resistance).

Electrical Quantities and Units of Measurement

As we have defined each of the electrical quantities, below are the units of measurement adopted by the International System of Units (SI):

Basic quantities in the International System are those that cannot be reduced or expressed by other quantities. Derived quantities can be expressed through combinations of basic quantities, such as voltage, which is measured in Volts—and a Volt can be expressed as kg·m²/(s³·A).

• Electric current: Ampere (A) – Basic unit

• Electric voltage: Volts (V)

• Electric resistance: Ohm (Ω)

• Electric power: Watt (W)

• 
  

APPLICATION OF OHM’S LAW

We return, finally, to the shower, which could also be an air conditioner, refrigerator, electric oven, etc.

How to calculate the circuit breaker current?

Check the power and voltage of the load. In this case, the nominal power is 5200W, but to ensure safety, we use the upper limit of the class, 5700W. If in doubt, use the highest indicated power.

Apply Ohm’s Law:

I=PV=5700W220V≈25.9AI = \frac{P}{V} = \frac{5700W}{220V} \approx 25.9AI=VP​=220V5700W​≈25.9A

Choose the next commercially available current rating above 25.9A.In this case, go with a 32A circuit breaker.

How to calculate electric consumption?

In the case of the shower, find out how long it stays on:

• Duration of one shower: 10 minutes/day;

• Considering a month = 30 days, we have 30 showers of 10 minutes each;

• 30×10=300 minutes of shower time;

• 300÷60=5 hours of shower time;
  

The monthly consumption will be:

• 5700W×5 hours=28,500 watt-hours5700W \times 5 \text{ hours} = 28,500 \text{ watt-hours}5700W×5 hours=28,500 watt-hours

  
  

Since we want the result in kWh, we divide by 1000 (because 1 kW = 1000 W):

Monthly consumption=28,5001000=28.5 kWh\text{Monthly consumption} = \frac{28,500}{1000} = 28.5 \text{ kWh}Monthly consumption=100028,500​=28.5 kWh

Did you like the article? Then download it as a PDF to consult it whenever you want: Conheça as 4 grandezas elétricas básicas.