Electric current
From Free net encyclopedia
Electric current is the flow of electric charge. Natural examples include lightning and the solar wind, the source of the polar aurora. The most familiar artificial form of electric current is the flow of conduction electrons in metal wires, such as the overhead power lines that deliver electrical energy across long distances and the smaller wires within electrical and electronic equipment. In electronics, other forms of electric current include the flow of electrons through resistors or through the vacuum in a vacuum tube, the flow of ions inside a battery, and the flow of holes within a semiconductor.
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Definition
The magnitude of an electric current is defined as the time derivative of electric charge:
- <math>I = {dQ \over dt}</math>
Formally this is written as
- <math>i(t) = {dq(t) \over dt}</math> or inversely as <math>q(t_0) = \int_{-\infty}^{t_0} i(t)\, dt</math>
(the amount of charge Q flowing per unit of time t) is I, from the German word Intensität, which means 'intensity'.
Conventional current
Conventional current was defined early in the history of electrical science as a flow of positive charge. In solid metals, like wires, the positive charges are immobile, and only the negatively charged electrons flow in the direction opposite conventional current, but this is not the case in most non-metallic conductors. In other materials, charged particles flow in both directions at the same time. Electric currents in electrolytes are flows of electrically charged atoms (ions), which exist in both positive and negative varieties. For example, an electrochemical cell may be constructed with salt water (a solution of sodium chloride) on one side of a membrane and pure water on the other. The membrane lets the positive sodium ions pass, but not the negative chlorine ions, so a net current results. Electric currents in plasma are flows of electrons as well as positive and negative ions. In ice and in certain solid electrolytes, flowing protons constitute the electric current. To simplify this situation, the original definition of conventional current still stands.
There are also instances where the electrons are the charge that is physically moving, but where it makes more sense to think of the current as the movement of positive "holes" (the spots that should have an electron to make the conductor neutral). This is the case in a p-type semiconductor.
Units
The SI unit of electric current is the ampere (A), which is equal to a flow of one coulomb of charge per second.
The drift speed of electric charges
The mobile charged particles within a conductor move constantly in random directions. In order for a net flow of charge to exist, the particles must also move together with an average drift rate. Electrons are the charge carriers in metals and they follow an erratic path, bouncing from atom to atom, but generally drifting in the direction of the electric field. The speed at which they drift can be calculated from the equation:
- <math>I=nAvQ \!\ </math>
where
- I is the current
- n is number of charged particles per unit volume
- A is the cross-sectional area of the conductor
- v is the drift velocity, and
- Q is the charge on each particle.
Electric currents in solid matter are typically very slow flows. For example, in a copper wire of cross-section 0.5 mm², carrying a current of 5 A, the drift velocity of the electrons is of the order of a millimetre per second. To take a different example, in the near-vacuum inside a cathode ray tube, the electrons travel in near-straight lines ("ballistically") at about a tenth of the speed of light.
However, we know that electric current signals are waves which propagate at very high speed. As with any wave, the speed of the waves in a medium have little relation to the speed of that medium as it moves. For example, in AC power lines, the waves of current propagate rapidly from a source to a distant load, while the charges themselves only move back and forth over a tiny distance. The velocity of flowing charges can be quite low. Yet, any changes in electric current can travel at the speed of light, though it might be slower in certain media. The percentage of speed in a medium compared to the speed of light in vacuum is called velocity factor, and is proportional to refractive index.
Current density
Current density is the current per unit (cross-sectional) area.
Mathematically, current is defined as the net flux through an area. Thus:
- <math>
I = j \cdot A </math>
where, in the MKS or SI system of measurement,
- I is the current, measured in amperes
- j is the "current density" measured in amperes per square metre
- A is the area through which the current is flowing, measured in square metres
The current density is defined as:
- <math>
j=\int_i n_i \cdot x_i \cdot \mathbf{u_i} </math>
where
- n is the particle density (number of particles per unit volume)
- x is the mass, charge, or any other characteristic whose flow one would like to measure.
- u is the average velocity of the particles in each volume
Current density is an important consideration in the design of electrical and electronic systems. Most electrical conductors have a finite, positive resistance, making them dissipate power in the form of heat. The current density must be kept sufficiently low to prevent the conductor from melting or burning up, or the insulating material failing. In superconductors, excessive current density may generate a strong enough magnetic field to cause spontaneous loss of the superconductive property.
Electromagnetism
Every electric current produces a magnetic field. The magnetic field can be visualized as a pattern of circular field lines surrounding the wire.
Electric current can be directly measured with a galvanometer, but this method involves breaking the circuit, which is sometimes inconvenient. Current can also be measured without breaking the circuit by detecting the magnetic field it creates. Devices used for this include Hall effect sensors, current clamps, current transformers, and Rogowski coils.
Ohm's law
Ohm's law predicts the current in an (ideal) resistor (or other ohmic device) to be applied voltage divided by electrical resistance:
- <math>
I = \frac{V}{R} </math>
where
- I is the current, measured in amperes
- V is the potential difference measured in volts
- R is the resistance measured in ohms
Reference direction
When studying electrical circuits, it is possible that the actual direction of current flow in a specific circuit element is not known at the start. Consequently, we arbitrarily assign each current variable a reference direction. After current values are solved for, some of them might display negative values. Hence, for the negative current variables, the actual current flows in the direction opposite to the reference direction which was originally selected.
Electrical safety
The most obvious hazard is electric shock, where a current through part of the body can cause effects from a slight tingle to cardiac arrest or severe burns. It is the current that passes that determines the effect, and this depends on the nature of the contact, the condition of the body part, the current path through the body and the voltage of the source. The effect also varies considerably from individual to individual. (For approximate figures see Shock Effects under electric shock.)
Because of this and because in practical situations the current that may pass cannot be predicted any supply of over 50 volts should be considered a possible source of dangerous electric shock. In particular, note that 110 volts can certainly be lethal.
Electric arcs, which can occur with supplies of any voltage (for example, a typical arc welding machine has a voltage between the electrodes of just a few tens of volts), are very hot and emit ultra-violet (UV) and infra-red radiation (IR). Proximity to an electric arc can therefore cause severe thermal burns, and UV is damaging to unprotected eyes and skin.
Accidental electric heating can also be dangerous. An overloaded power cable is a frequent cause of fire. A battery as small as an AA cell placed in a pocket with metal coins can lead to a short circuit heating the battery and the coins which may inflict burns. NiCad, NiMh cells, and Lithium batteries are particularly risky because they can deliver a very high current due to their low internal resistance.
See also
- Alternating current
- Direct current
- Electrical conduction for more information on the physical mechanism of current flow in materials
- SI electromagnetism units
External links
- Interactive Current Conversion Table - convert selected unit to all other units of current
- Which direction does electricity really flow?
- All about circuits - a useful site introducing electricity and electronics, as well as some mathematics involved with circuit calculations.ar:تيار كهربائي
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