When it is connected a continuous conductive path, like a wire with a battery terminal, we obtain an electric circuit. The device which receives charging from a battery is not always a wire, it can be a light bulb, a device sound or anything else. When this circuit is formed, the charge can pass through it from one terminal to another stack. In this load flow it is called “Direct-Current”.
More properly, the electrical current of a driver is defined as the net amount of load that pass through it per unit of time, in any of its points. Thus, the mean current I is defined as:
Where ∆Q is the amount of load which passes through a section of the conductor at a given point during the interval ∆t. Electrical current is measured in Coulombs per second, unit that receives the special name of ampere (abbreviated amp or A) in honor of the French physicist Andre Ampere.
When a conductive wire is connected to the terminals of a battery, in fact are electrons negatively charged which flows through the wire. When it is connected first the wire, the potential difference between the battery terminals creates an electric field inside the wire parallel thereto. Thus, free electrons from one end of the wire are attracted to the positive terminal, while at the other end, electrons leave the negative terminal of the stack and enter the driver. Thus, there is a continuous flow of electrons through the wire, which begins as soon as it is connected to both terminals.
When we talk about the current which passes through a circuit, it is understood that is the direction it would take a positive charge. To this, it is usually called conventional current. When it is wanted to talk about the direction of flow of electrons, specifically it will mention that it is the electron current. In liquids and gases can be moved both loads, such as positive or negative ions.
To produce an electrical current in a circuit is needed a potential difference. It was Georg Ohm who established experimentally that a metallic conductor current is proportional to the potential difference V applied to its ends.
The amount of current passing through a conductor not only depends on the voltage, but also the resistance that the driver provides to the electron flow, and the higher the resistance, the lower the voltage V current for determined, being the relationship:
Materials or devices that don’t comply with Ohm’s law are called non-ohmic.
All electrical items, from light bulbs to heaters or stereo amplifiers, offer resistance to current flow. Usually, the connecting cables have a very low resistance. In many circuits, especially in electronic devices, resistors are used to control the amount of current. Resistors have resistances less than one ohm since even millions of ohms. The two main types of resistors are:
- Potentiometers: which consisting on a fine wire coil.
- Composition: which normally are manufactured with semiconductor coal.
Experimentally it has been found that the resistance of a metal wire is directly proportional to its length L and inversely proportional to its cross section area A. That is:
Wherein, φ, the constant of proportionality is called resistivity and it depends on the material used. This equation makes sense, since we expect the resistance of a thick wire is less than the small one, since one thick has more area through which electrons can pass.
Typical values of φ, whose units are Ω·m, depend on the purity, the heat treatment temperature and other factors. In general, the resistance of metals increases with increasing temperature, at higher temperatures, atoms move faster and therefore more expected to interfere with the flow of electrons. If the temperature change is not too large, the resistivity of metals increases almost linearly with temperature. That is to say:
Where ϕ0 is the resistivity at a given reference temperature and α it is the resistivity thermal coefficient.
At low temperatures, the resistivity of certain metals and their compounds or alloys becomes zero, as indicated by the more precise measurement techniques. When a material is under these conditions is called “superconductor”. The phenomenon of superconductivity was first observed by H.K.Onnes in 1911 when he cooled mercury at -4,2K. Onnes found that, at that temperature, the mercury resistance suddenly down to zero. In general, superconductors acquire this status only when they fall beyond a certain transition temperature, which is usually a few degrees above absolute zero. It has been observed that in the absence of a potential difference, current can flow for years by a superconducting material whose form is a ring, without an appreciable decrease. Measurements show that the resistivity of superconductors is less than 4 Ω·1025 Ω·m, which is more than 1016 times lower than for copper and, in practice, it is considered as zero.
An important use of superconductivity is to circulate the current through an electromagnet. In large magnets a huge amount of energy is needed just to maintain current, and that energy is wasted as heat. The use of superconductors at temperatures would allow to motors and generators be much smaller. The power transmission over long distances through superconductors lines also requires much smaller and less expensive transmission.
Electric power is useful because it can be transformed easily into other forms of energy. The devices convert electrical energy into heat or light because, generally, the current is large and many shocks occur between the moving electrons and atoms driver. In each collision, part of the kinetic energy of the electron is transferred to the atom against which collides. As a result, the kinetic energy of atoms increases and therefore increases the temperature of the element. The resulting thermal energy, which is internal energy, can be transmitted in the form of heat by conduction or convection air in a heating or food in an oven for bread toaster radiation or may radiate as light.
To calculate the power which is the rate of energy conversion, that is:
The charge passing per second, Q / I, is simply the electric current I. Therefore, we get:
where: n = number of electrons per unit volume; of = electron charge; s = section of the conductor; v = velocity of the electrons.
where: σ= conductivity.
Ohm’s law for a thread:
Factors from which depends the resistance:
Where: l= length of conductor; S= section.
Resistivity variation with temperature:
Work of an electric current:
Power of an electric current:
Potential difference between its terminals:
Generalized Ohm Law:
Potential difference between two points of a circuit:
Group of Resistors:
Association of n identical generators:
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