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Current of Electricity


Current electricity is defined as the rate of flow of electrons through a conductor. Moving electrons produces the electricity. Unlike static electricity, current electricity must flow through a conductor. Current of electricity is compared with the current of a river. Because, a moves from one point from another with a speed of water. Here current is flowing instead of water in a current of electricity. Electricity current is always going right back where it came from, kind of in a circle. It is a measure of the quantity or amount of energy transferred during a particular time period. This energy is termed as flow of electrons. The flow of current through a conductor causes heat.

Electric Current

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The electric current through a particular conductor or machine must not be too large or too small for it to work correctly. For example, too little current may fail to light a lamp or turn an electric motor, but too much current could burn out the filament of the lamp or turn the motor too quickly. So it is important to be able to measure the electric current in a circuit or through a conductor. To get an idea of the strength or size of an electric current we can compare the flow of electric charge with the flow of water in a river. We would say that the current was strong if a large quantity of water was flowing quickly down a river. The strength or size of this current could be measured in litres of water flowing past a point in the river in a certain time. We would in fact be measuring the rate of flow of the water. Similarly the strength or size of an electric current is a measure of the rate of flow of electric charge past a point in an electric circuit or through a conductor.

Current of Electricity Formula

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To use the relation between the size of an electric current and the rate of flow of charge we need symbols and units for these quantities. The quantity of electric charge Q is measured in coulombs (C). The size or strength of an electric current I is measured in amperes (A). In everyday usage we usually say 'amps' but the only correct written abbreviation for ampere is A. 

The relation between current and charge is the following.
Current = $\frac{Charge}{Time}$

I = $\frac{Q}{t}$

Which gives as:
A current of 1 ampere is a flow of charge at the rate of 1 coulomb per second.

Potential Difference

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When we state the height of a hill it is given as metres above a certain level, usually sea level or ground level, because what is important is the difference in height between the top and the bottom. In the same way, the potential difference is an electric circuit between two points is the most important factor. A difference in water levels causes a current of water to flow downhill, while no difference of levels in a pond results in no flow. 

In exactly the same way a potential difference in an electric circuit causes charge to flow downhill and the current depends upon the size of the potential difference. The word potential difference is often abbreviated to p.d, but its symbol in formulas is V. The unit of potential difference is the volt and its symbol is also V. We often talk about voltage when we really mean potential difference in volts.

Resistance and Resistivity

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The resistance of a wire depends on several factors. A long wire has a larger resistance than a short wire. A fat wire has a lower resistance than a thin wire. Resistance is proportional to length and inversely proportional to cross sectional area and it depends on a property of the material called resistivity. So,
Resistance = Resistivity$\times$ $\frac{length}{area}$

R = $\frac{\rho l}{A}$
Resistivity, ρ is a property of the material (whereas resistance is a property of a component). Resistivity is a measure of how the material opposes the current through it. Metals have a low resistivity; insulators have a high resistivity. Semiconductors, as their name implies, are somewhere in the middle.

Sources of Electromotive Force

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An external energy source is required by most electrical circuits to move charge through the circuit. The circuit therefore must include a device that maintains a potential difference between two points in the circuit, just as a circulating fluid requires an analogous device (a pump) that maintains a pressure difference between two points. Any device that performs this task in an electrical circuit is called a source (or a seat) of electromotive force. It is sometimes useful to consider a source of emf as a mechanism that creates a hill of potential and moves charge uphill, from which the charge flows downhill through the rest of the circuit. A common source of emf is the ordinary battery; another is the electric generator found in power plants. Solar cells are sources of emf used both in spacecraft and in pocket calculators. Other less commonly found sources of emf are fuel cells and thermopiles. Biological systems, including the human heart, also function as sources of emf.