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WIKIBOOKS
DISPONIBILI
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ARTICLES IN THE BOOK

  1. AAAA battery
  2. AAA battery
  3. AA battery
  4. A battery
  5. Absorbent glass mat
  6. Alessandro Volta
  7. Alkaline battery
  8. Alkaline fuel cell
  9. Aluminium battery
  10. Ampere
  11. Atomic battery
  12. Backup battery
  13. Baghdad Battery
  14. Batteries
  15. Battery charger
  16. B battery
  17. Bernard S. Baker
  18. Beta-alumina solid electrolyte
  19. Betavoltaics
  20. Bio-nano generator
  21. Blue energy
  22. Bunsen cell
  23. Car battery
  24. C battery
  25. Clark cell
  26. Concentration cell
  27. Coulomb
  28. 2CR5
  29. Daniell cell
  30. Direct borohydride fuel cell
  31. Direct-ethanol fuel cell
  32. Direct methanol fuel cell
  33. Dry cell
  34. Dry pile
  35. Duracell
  36. Duracell Bunny
  37. Earth battery
  38. Electric charge
  39. Electric current
  40. Electricity
  41. Electrochemical cell
  42. Electrochemical potential
  43. Electro-galvanic fuel cell
  44. Electrolysis
  45. Electrolyte
  46. Electrolytic cell
  47. Electromagnetism
  48. Electromotive force
  49. Energizer Bunny
  50. Energy
  51. Energy density
  52. Energy storage
  53. Flashlight
  54. Float charging
  55. Flow Battery
  56. Formic acid fuel cell
  57. Fuel cell
  58. Fuel cell bus trial
  59. Galvanic cell
  60. Gel battery
  61. Grove cell
  62. Half cell
  63. History of the battery
  64. Hybrid vehicle
  65. Lead-acid battery
  66. Leclanché cell
  67. Lemon battery
  68. List of battery sizes
  69. List of battery types
  70. List of fuel cell vehicles
  71. Lithium battery
  72. Lithium ion batteries
  73. Lithium iron phosphate battery
  74. Lithium polymer cell
  75. LR44 battery
  76. Luigi Galvani
  77. Manganese dioxide
  78. Memory effect
  79. Mercury battery
  80. Metal hydride fuel cell
  81. Methane reformer
  82. Methanol reformer
  83. Michael Faraday
  84. Microbial fuel cell
  85. Molten carbonate fuel cell
  86. Molten salt battery
  87. Nickel-cadmium battery
  88. Nickel-iron battery
  89. Nickel metal hydride
  90. Nickel-zinc battery
  91. Open-circuit voltage
  92. Optoelectric nuclear battery
  93. Organic radical battery
  94. Oxyride battery
  95. Panasonic EV Energy Co
  96. Peukert's law
  97. Phosphoric acid fuel cell
  98. Photoelectrochemical cell
  99. Polymer-based battery
  100. Power density
  101. Power management
  102. Power outage
  103. PP3 battery
  104. Primary cell
  105. Prius
  106. Proton exchange membrane
  107. Proton exchange membrane fuel cell
  108. Protonic ceramic fuel cell
  109. Radioisotope piezoelectric generator
  110. Ragone chart
  111. RCR-V3
  112. Rechargeable alkaline battery
  113. Reverse charging
  114. Reversible fuel cell
  115. Searchlight
  116. Secondary cell
  117. Short circuit
  118. Silver-oxide battery
  119. Smart Battery Data
  120. Smart battery system
  121. Sodium-sulfur battery
  122. Solid oxide fuel cell
  123. Super iron battery
  124. Thermionic converter
  125. Trickle charging
  126. Vanadium redox battery
  127. Volt
  128. Voltage
  129. Voltaic pile
  130. Watch battery
  131. Water-activated battery
  132. Weston cell
  133. Wet cell
  134. Zinc-air battery
  135. Zinc-bromine flow battery
  136. Zinc-carbon battery

 

 
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BATTERIES
This article is from:
http://en.wikipedia.org/wiki/Voltage

All text is available under the terms of the GNU Free Documentation License: http://en.wikipedia.org/wiki/Wikipedia:Text_of_the_GNU_Free_Documentation_License 

Voltage

From Wikipedia, the free encyclopedia

 
International safety symbol "Caution, risk of electric shock" (ISO 3864), colloquially known as high voltage symbol.
International safety symbol "Caution, risk of electric shock" (ISO 3864), colloquially known as high voltage symbol.

Voltage is the difference of electrical potential between two points of an electrical or electronic circuit, expressed in volts [1]. It measures the potential energy of an electric field to cause an electric current in an electrical conductor. Depending on the difference of electrical potential it is called extra low voltage, low voltage, high voltage or extra high voltage.

Explanation

Between two points in an electric field, such as exists in an electrical circuit, the difference in their electrical potentials is known as the electrical potential difference. This difference is proportional to the electrostatic force that tends to push electrons or other charge-carriers from one point to the other. Potential difference, electrical potential, and electromotive force are measured in volts, leading to the commonly used term voltage. Voltage is usually represented in equations by the symbols V, U, or E. (E is often preferred in academic writing, because it avoids the confusion between V and the SI symbol for the volt, which is also V.)

Electrical potential difference can be thought of as the ability to move electrical charge through a resistance. At a time in physics when the word force was used loosely, the potential difference was named the electromotive force or EMF—a term which is still used in certain contexts.

Voltage is a property of an electric field, not individual electrons. An electron moving across a voltage difference experiences a net change in energy, often measured in electron-volts. This effect is analogous to a mass falling through a given height difference in a gravitational field.

When using the term 'potential difference' or voltage, one must be clear about the two points between which the voltage is specified or measured. There are two ways in which the term is used. This can lead to some confusion.

Voltage with respect to a common point

One way in which the term voltage is used is when specifying the voltage of a point in a circuit. When this is done, it is understood that the voltage is usually being specified or measured with respect to a stable and unchanging point in the circuit that is known as ground or common. This voltage is really a voltage difference, one of the two points being the reference point, which is ground. A voltage can be positive or negative. "High" or "low" voltage may refer to the magnitude (the absolute value relative to the reference point). Thus, a large negative voltage may be referred to as a high voltage. Other authors may refer to a voltage that is more negative as being "lower."

Voltage between two stated points

Another usage of the term "voltage" is in specifying how many volts are dropped across an electrical device (such as a resistor). In this case, the "voltage," or, more accurately, the "voltage drop across the device," is really the first voltage taken, relative to ground, on one terminal of the device minus a second voltage taken, relative to ground, on the other terminal of the device. In practice, the voltage drop across a device can be measured directly and safely using a voltmeter that is isolated from ground, provided that the maximum voltage capability of the voltmeter is not exceeded.

Addition of voltages

Voltage is additive in the following sense: the voltage between A and C is the sum of the voltage between A and B and the voltage between B and C. The various voltages in a circuit can be computed using Kirchhoff's circuit laws.

Two points in an electric circuit that are connected by an "ideal conductor," that is, a conductor without resistance and not within a changing magnetic field, have a potential difference of zero. However, other pairs of points may also have a potential difference of zero. If two such points are connected with a conductor, no current will flow through the connection.

Hydraulic analogy

Main article: Hydraulic analogy

If one imagines water circulating in a network of pipes, driven by pumps in the absence of gravity, as an analogy of an electrical circuit, then the potential difference corresponds to the fluid pressure difference between two points. If there is a pressure difference between two points, then water flowing from the first point to the second will be able to do work, such as driving a turbine.

This hydraulic analogy is a useful method of teaching a range of electrical concepts. In a hydraulic system, the work done to move water is equal to the pressure multiplied by the volume of water moved. Similarly, in an electrical circuit, the work done to move electrons or other charge-carriers is equal to 'electrical pressure' (an old term for voltage) multiplied by the quantity of electrical charge moved. Voltage is a convenient way of quantifying the ability to do work. In relation to electric current, the larger the gradient (voltage or hydraulic) the greater the current (assuming resistance is constant).

Mathematical definition

The electrical potential difference is defined as the amount of work needed to move a unit electric charge from the second point to the first, or equivalently, the amount of work that a unit charge flowing from the first point to the second can perform. The potential difference between two points a and b is the line integral of the electric field E:

V_a - V_b = \int _a ^b \mathbf{E}\cdot d\mathbf{l}

Useful formulas

DC circuits

V = \sqrt{PR}
V = \frac{P}{I}
V = IR \!\

Where V=voltage, I=current, R=resistance, P=power

AC circuits

V = \frac{P}{I\cos\phi}
V = \frac{\sqrt{PZ}}{\sqrt{\cos\phi}} \!\
V = \frac{IR}{\cos\phi}

Where V=voltage, I=current, R=resistance, P=true power, Z=impedance, φ=phasor angle between I and V

AC conversions

V_{avg} = .637\,V_{pk} = \frac{2}{\pi} V_{pk} = \frac{\omega}{\pi}\int_0^{\pi/\omega} V_{pk} \sin(\omega t - k x) {\rm{d}}x \!\
V_{rms} = .707\,V_{pk} = \frac{1}{\sqrt{2}} V_{pk} = V_{pk} \sqrt{\langle \sin^2(\omega t - k x) \rangle} \!\
V_{pk} = 0.5\,V_{ppk} \!\
V_{avg} = .319\,V_{ppk}\!\
V_{rms} = .354\,V_{ppk} = \frac{1}{2 \sqrt{2}} V_{ppk}\!\
V_{avg} = 0.900\,V_{rms} = \frac{2 \sqrt{2}}{\pi} V_{rms}\!\

Where Vpk=peak voltage, Vppk=peak-to-peak voltage, Vavg=average voltage over a half-cycle, Vrms=effective (root mean square) voltage, and we assumed a sinusoidal wave of the form Vpksin(ωtkx), with a period T = 2π / ω, and where the angle brackets (in the root-mean-square equation) denote a time average over an entire period.

Total voltage

Voltage sources and drops in series:

V_T = V_1 + V_2 + V_3 + ... + V_n \!\

Voltage sources and drops in parallel:

V_T = V_1 = V_2 = V_3 = ... = V_n \!\

Where n \!\ is the nth voltage source or drop

Voltage drops

Across a resistor (Resistor R):

V_R = IR_R  \!\

Across a capacitor (Capacitor C):

V_C = IX_C  \!\

Across an inductor (Inductor L):

V_L = IX_L  \!\

Where V=voltage, I=current, R=resistance, X=reactance.

Measuring instruments

A multimeter set to measure voltage.
A multimeter set to measure voltage.

Instruments for measuring potential differences include the voltmeter, the potentiometer (measurement device), and the oscilloscope. The voltmeter works by measuring the current through a fixed resistor, which, according to Ohm's Law, is proportional to the potential difference across it. The potentiometer works by balancing the unknown voltage against a known voltage in a bridge circuit. The cathode-ray oscilloscope works by amplifying the potential difference and using it to deflect an electron beam from a straight path, so that the deflection of the beam is proportional to the potential difference.

Safety

Electrical safety is discussed in the articles on High voltage and Electric shock.

See also

 
  • Alternating current (AC)
  • Direct current (DC)
  • Mains electricity (an article about domestic power supply voltages)
  • Ohm's Law
  • Voltage drop

References

  1. ^ "voltage", A Dictionary of Physics. Ed. John Daintith. Oxford University Press, 2000. Oxford Reference Online. Oxford University Press.

External link

  • Elementary explanation of voltage at NDT Resource Center
Retrieved from "http://en.wikipedia.org/wiki/Voltage"