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Electric Current

 

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The Basics - SOURCE

Ohm's Law

For many conductors of electricity, the electric current which will flow through them is directly proportional to the voltage applied to them. When a microscopic view of Ohm's law is taken, it is found to depend upon the fact that the drift velocity of charges through the material is proportional to the electric field in the conductor. The ratio of voltage to current is called the resistance, and if the ratio is constant over a wide range of voltages, the material is said to be an "ohmic" material. If the material can be characterized by such a resistance, then the current can be predicted from the relationship:


Data can be entered into any of the boxes below. Specifying any two of the quantities determines the third. After you have entered values for two, click on the text representing to third in the active illustration above to calculate its value.


Amperes = volts / ohms

AC version of Ohm's law

Index

DC Circuits

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Voltage Law

The voltage changes around any closed loop must sum to zero. No matter what path you take through an electric circuit, if you return to your starting point you must measure the same voltage, constraining the net change around the loop to be zero. Since voltage is electric potential energy per unit charge, the voltage law can be seen to be a consequence of conservation of energy.

The voltage law has great practical utility in the analysis of electric circuits. It is used in conjunction with the current law in many circuit analysis tasks.


Current law

Resistor combinations

Analogy in water circuit

Index

DC Circuits

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SOURCE


Current Law

The electric current in amperes which flows into any junction in an electric circuit is equal to the current which flows out. This can be seen to be just a statement of conservation of charge. Since you do not lose any charge during the flow process around the circuit, the total current in any cross-section of the circuit is the same. Along with the voltage law, this law is a powerful tool for the analysis of electric circuits.


Voltage law

Resistor combinations

Analogy in water circuit

Index

DC Circuits

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Electric Current

Electric current is the rate of charge flow past a given point in an electric circuit, measured in coulombs/second which is named amperes. In most DC electric circuits, it can be assumed that the resistance to current flow is a constant so that the current in the circuit is related to voltage and resistance by Ohm's law.


Microscopic view of electric current

Measurement with ammeter

Conventional electric current direction

Index

Electric Circuits

 

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Electric Charge

The unit of electric charge is the coulomb. Ordinary matter is made up of atoms which have positively charged nuclei and negatively charged electrons surrounding them. Charge is quantized as a multiple of the electron or proton charge:

The influence of charges is characterized in terms of the forces between them (Coulomb's law) and the electric field and voltage produced by them. One coulomb of charge is the charge which would flow through a 120 watt lightbulb (120 volts AC) in one second. Two charges of one coulomb each separated by a meter would repel each other with a force of about a million tons!

 

The rate of flow of electric charge is called electric current and is measured in amperes.

In introducing one of the fundamental properties of matter, it is perhaps appropriate to point out that we use simplified sketches and constructs to introduce concepts, and there is inevitably much more to the story. No significance should be attached to the circles representing the proton and electron, in the sense of implying a relative size, or even that they are hard sphere objects, although that's a useful first construct. The most important opening idea, electrically, is that they have a property called "charge" which is the same size, but opposite in polarity for the proton and electron. The proton has 1836 times the mass of the electron, but exactly the same size charge, only positive rather than negative. Even the terms "positive" and "negative" are arbitrary, but well-entrenched historical labels. The essential implication of that is that the proton and electron will strongly attract each other, the historical archtype of the cliche "opposites attract". Two protons or two electrons would strongly repel each other. Once you have established those basic ideas about electricity, "like charges repel and unlike charges attract", then you have the foundation for electricity and can build from there.

From the precise electrical neutrality of bulk matter as well as from detailed microscopic experiments, we know that the proton and electron have the same magnitude of charge. All charges observed in nature are multiples of these fundamental charges. Although the standard model of the proton depicts it as being made up of fractionally charged particles called quarks, those fractional charges are not observed in isolation -- always in combinations which produce +/- the electron charge.

An isolated single charge can be called an "electric monopole". Equal positive and negative charges placed close to each other constitute an electric dipole. Two oppositely directed dipoles close to each other are called an electric quadrupole. You can continue this process to any number of poles, but dipoles and quadrupoles are mentioned here because they find significant application in physical phenomena.

One of the fundamental symmetries of nature is the conservation of electric charge. No known physical process produces a net change in electric charge.

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Coulomb's Law

Electromagnetic force

 

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Conventional Electric Current

Although it is electrons which are the mobile charge carriers which are responsible for electric current in conductors such as wires, it has long been the convention to take the direction of electric current as if it were the positive charges which are moving. Some texts reverse this convention and take electric current direction as the direction the electrons move, an obviously more physically realistic direction, but the vast majority of references use the conventional current direction and that convention will be followed in most of this material. In common applications such as determining the direction of force on a current carrying wire, treating current as positive charge motion or negative charge motion gives identical results. Besides the advantage of agreeing in direction with most texts, the conventional current direction is the direction from high voltage to low voltage, high energy to low energy, and thus has some appeal in its parallel to the flow of water from high pressure to low (see water analogy).

Index

Electric Circuits

 

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