Sunday, January 3, 2010

13-Current Electricty

Current Electricity

Electric Current – Electric current is the constant current which, if maintained in two straight parallel conductors of infinite length and negligible circular cross-section, when placed 1 metre apart in a vacuum would produce a force of 2 × 10-7 newton per metre of length between these conductors.

Coulomb – One coulomb is the charge that in one second crosses a section of a circuit in which there is a current of one ampere.
The conventional current direction is that in which the positive charges will drift, that is, in the same direction of the electric field, E.

Potential difference - The potential difference between 2 points, V, is equal to the amount of electrical energy, W, which is converted to other forms of energy per unit charge, Q, that passes from the point at higher potential to the point at lower potential. i.e. V=W/Q
Or The potential difference between 2 points V is the power dissipated , P, or rate of conversion of electrical energy to i.e. V = P/I other forms of energy, per unit current, I, between the points.

Volt – 1 volt is the potential difference V, between two points in a circuit when the amount of electrical energy, J, which is converted to
other forms of energy is 1 joule per coulomb , C, of electric charge that passes from one point to the other.
i.e. 1 V = 1 JC-1

Electromotive force (emf) - The electromotive force , ε, of a source of electrical energy is the energy converted into electrical energy, W, per unit charge , Q, supplied.

Resistance - The resistance R, of a conductor is defined as the ratio of the potential difference, V, across it to the current, I, flowing through it.

Ohm’s Law – Ohm’s Law states that the current through a metallic conductor is directly proportional to the potential difference across it provided that the physical conditions (such as temperature, stress, etc) are constant.

The resistance R (= V / I ) of a metallic conductor does not change with potential difference, provided the temperature and other physical conditions are constant.
Ohm – 1 ohm is the resistance of a conductor in which the current is 1 ampere, A, when a potential difference of 1 volt, V, is applied across it. ie 1 Ω = 1 V / 1 A
Resistivity – is numerically equal to the resistance of a sample of unit length and unit cross-sectional area at a particular constant temperature.


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