THE electrodynamics is the branch of physics that studies the electrical charges in motion. The main concepts studied in this area are electrical current (i), electrical resistance (R) and electrical power (P).
THE electric current is the ordered movement of charges and is determined by the amount of charge (ΔQ) that passes in a given time (Δt). Its unit of measure is the ampere (A).
THE electrical resistance is found through the 1st and 2nd Ohm's Law, which relate resistance to voltage (U) and current (i), as well as resistance to the type of material the conductor is made of. Its unit of measure is the ohm (Ω).
THE electric power it is the efficiency of the device to transform energy, in this case electrical energy. Its unit of measure is the watt (w).
Read too: Ohm's Laws — fundamental laws for the study of electricity
Summary
- Electrodynamics studies charge in motion.
- The three main concepts of electrodynamics are: electric current, electric resistance and electric power.
- Electric current (i) is the amount of charge that passes through a conductor in a given time.
- Electrical resistance is the difficulty of passing current in a conductor.
- The electrical resistance obeys the 1st and 2nd Ohm's Law, formulated by Georg Simon Ohm.
- The 1st law of ohm relates voltage (U) to electrical current (i).
- If the resistance of a conductor is constant, we call this resistor an ohmic.
- The 2nd law of ohm relates electrical resistance to the type and shape of the material the conductor is made of.
- Electric power is the efficiency of transforming energy and can be found through the voltage and current of a device.
What is electrodynamics?
It is a sub-area of Physics that is within andlectricity. THE concern in this area is to study the movement of electrical charges. Therefore, the study of electrodynamics consists of understanding and applying electrical current, electrical resistance and electrical power.
Main concepts of electrodynamics
Electrodynamics is concerned with understanding the effects of moving charges. Thus, its main concepts are: electrical current, electrical resistance and electrical power
Electric current
THE electric current is the orderly movement of electrical charges within a conductor due to a potential difference (ddp). The current intensity (i) is calculated by the amount of charges (ΔQ) that pass on the conductor in a given time (Δt):
i: electric current (C/s or A)
Q: electric charge (C)
t: time(s)
→ Video class: Electrodynamics in Enem — electric current
electrical resistance
THE rresistance andelectricalis the difficulty in passing electrical current. It obeys the 1st and 2nd law of Ohm (laws formulated by Georg Simon Ohm about the functioning of the electrical resistance).
→ 1st law of Ohm
THE1st law of ohm determines that the electric current (i) is proportional to the voltage (U) to which the conductor is subjected. And if this relationship is constant, that is, if the electrical resistance (R) is constant, we call these resistors ohmics.
i: electric current (A)
R: electrical resistance (Ω)
U: voltage (V)
→ 2nd law of ohm
THEOhm's second lawdetermines that electrical resistance is a characteristic of the body and depends on the shape (length and area) and the material the body is made of, the resistivity (ρ). Ohm's 2nd law relates these two features.
L: length of conductor (L)
R: electrical resistance (Ω)
A: conductor area (m2)
ρ: resistivity (Ω. M2)
→ Videoclass: Electrodynamics in Enem — electrical resistance and Ohm's laws
Electric power
Power is the equipment's efficiency in transforming energy, that is, how fast the device is able to transform one energy (ΔE) into another. It is measured in watts (W).
In the case of electrical power, we have the efficiency to transform electrical energy into other energies, such as thermal, luminous and sonorous.
P: electrical power (A.V or W)
i: electric current (A)
U: voltage (V)
To find electrical power in resistors, we can modify this first electrical power equation together with the electrical resistance equation. Isolating the voltage (U), in the electrical resistance equation, we have:
Substituting the U in the electrical power equation, we have:
And we can still find another equation isolating the current (i) in the electrical resistance equation and substituting it in the electrical power equation:
Read too: Electrical circuits — connections that allow the circulation of electrical current
Electrodynamics in Enem
Electrodynamics can be found in everyday life easily in any electrical device we use. So this is one of the most demanded subjects, in Physics, at Enem.
With this in mind, issues involving circuits, such as the electric shower and light bulbs, which involve energy transformation, among others, are issues of electrodynamic analysis. Let's look at an example below.
(Enem 2016) An LED (light emitting diode) lamp, which runs on 12V and 0.45A direct current, produces the same amount of light as an incandescent lamp with a power of 60W.
What is the value of the power consumption reduction when replacing the incandescent lamp with the LED one?
Resolution
Using the power equation and placing the information in the statement, we have:
As the exercise asks for power reduction, we have that the power of the incandescent lamp was 60 W, and that of the LED, 5.4 W. Subtracting one by the other, we have a reduction of 54.6 W.
Solved Exercises on Electrodynamics
1. (Enem 2017) The capacity of a battery with accumulators, such as that used in an automobile's electrical system, is specified in ampere hours (Ah). A 12V, 100Ah battery provides 12J for each coulomb of charge that flows through it.
If a generator, with negligible internal resistance, which provides an average electrical power equal to 600 W, connected to the described battery terminals, how long would it take to recharge it completely?
a) 0.5 h
b) 2 h
c) 12 h
d) 50 h
e) 100 h
Resolution
Alternative B.
To know the time, we need to figure out the amount of total energy when the charge is complete, that is, when the amount of charge Q equals 100A.h. As the load is normally seen in coulomb, let's transform the unit of measure. As in an hour, we have 3600 seconds, we can multiply the 100 A.h by 3600 seconds, leaving us with 360000 C.
If 1 C provides 12 J of energy, for rule of three, 360000 C provide 432000 J:
Using the power equation and isolating time (t):
Turning seconds into hours, we have 7200 seconds = 2 hours.
2. (Enem 2016) An electrician must install a shower that is rated 220V - 4400W to 6800W. For the installation of showers, it is recommended a proper network, with wires of adequate diameter and a circuit breaker sized to power and electrical current, provided with a close tolerance margin of 10%. Circuit breakers are safety devices used to protect electrical installations from short circuits and electrical overloads and must be disarmed whenever there is a passage of electrical current greater than that allowed in the device.
To make a safe installation of this shower, the value of the maximum current of the circuit breaker must be:
a) 20 A
b) 25 A
c) 30 A
d) 35 A
e) 40 A
Resolution
Alternative D.
To find the maximum current that can flow through the circuit breaker, we need to use the maximum power value (6800W) in the electrical power equation:
But the statement states that the circuit breaker predicts 10% more current, so to calculate this difference:
Adding the two together, we have an approximate value of 33 A.
by Gabriela de Oliveira
Physics teacher