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Current Electricity Basics

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Current Electricity Basics - Lesson Summary

Electricity is one of the oldest branches of science without which we cannot just imagine ourselves in the current world. The current flow from a high potential area to a low potential area is termed the conventional current whereas the flow of electrons constitute the electron current and is in a direction opposite to that of the conventional current.

Flowing water constitutes water current in rivers. Similarly, if the electric charge flows through a conductor we say that there is an electric current in the conductor. A continuous and closed path of an electric current is called an electric circuit.
 
In a torch light, the cells provide flow of charges or an electric current through the torch bulb to glow. The torch gives light only when its switch is on. If the circuit is broken anywhere (or the witch of the torch is turned off), the current stops flowing and the bulb does not glow.
 
In circuits using metallic wires, electrons constitute the flow of charges. As the electrons ere not known (not discovered) at the time of the observation of the electric phenomenon. So, electric current was considered to be the flow of positive charges and the direction of flow of positive charges was taken to be the direction of electric current.
 
Conventionally, in an electric circuit the direction of electric current is taken as opposite to the direction of the flow of electrons, which are negative charges. Electric current is the rate of flow of charges.
 
If a net charge Q flows across any cross-section of a conductor in time t, then the current I through the cross-section is I =  Q t .
 
The SI unit of electric charge is coulomb (C), which is equivalent to the charge contained in nearly 6 × 10 18 electrons. (We know that an electron possesses a negative charge of 1.6 × 10 –19 C.) The electric current is expressed by a unit called ampere (A), named after the French scientist, Andre-Marie Ampere (1775–1836). One ampere is constituted by the flow of one coulomb of charge per second, i.e., 1 A = 1 C/1 s. The SI unit of electric current is ampere.
 
Small quantities of current are expressed in milliampere (1 mA = 10 –3 A) or in microampere (1 μA = 10 –6 A). An instrument called ammeter measures electric current in a circuit.
 
An ammeter is always connected in series in a circuit through which the current is to be measured. In a typical electric circuit comprising a cell, an electric bulb, an ammeter and a plug key, the electric current flows from the positive terminal of the cell to the negative terminal of the cell through the bulb and ammeter.
 
Charges do not flow in a copper wire by themselves, just as water in a perfectly horizontal tube does not flow. Water flows in a tube only if there is a pressure difference between the two ends of a tube. Similarly electrons move along the conductor only if there is a difference of electric pressure, known as the potential difference.
 
For flow of charges in a conducting metallic wire, the gravity, of course, has no role to play.
 
The difference of potential may be produced by a battery, consisting of one or more electric cells. The chemical action within a cell generates the potential difference across the terminals of the cell, even when no current is drawn from it. When the cell is connected to a conducting circuit element, the potential difference sets the charges in motion in the conductor and produces an electric current. In order to maintain the current in a given electric circuit, the cell has to expend its chemical energy stored in it.
 
The electric potential difference between two points in an electric circuit carrying some current is the work done to move a unit positive charge from one point to the other: Potential difference (V) between two points = Work done (W)/Charge (Q),
i.e.,  V =  W Q  
 
The SI unit of electric potential difference is volt (V), named after the Italian physicist Alessandro Volta. One volt is the potential difference between two points in a current carrying conductor when 1 joule of work is done to move a charge of 1 coulomb from one point to the other. 1 volt = 1 joule/1 coulomb; 1 V = 1 J/ C.
 
The potential difference is measured using a voltmeter. The voltmeter is always connected in parallel across the points between which the potential difference is to be measured.
 
A schematic diagram in which different components of the circuit are represented by symbols is called a electric circuit diagram.
 
A closed loop through which charge can continuously move is referred to as a closed circuit. If the loop is not complete then charge stops flowing and such a circuit is called an open circuit.
 
In 1827, a German physicist Georg Simon Ohm (1787–1854) found out the relationship between the current I, flowing in a metallic wire and the potential difference across its terminals. He stated that the electric current flowing through a metallic wire is directly proportional to the potential difference V, across its ends provided its temperature remains the same. This is called Ohm’s law. In other words, V/I = constant = R, or V = IR, where R is a constant for the given metallic wire at a given temperature and is called its resistance.
 
As per electricity, we have two categories of materials, namely conductors and insulators. All of the conductors do not conduct electricity the same way. Some of them offer a restriction to the flow of charge and are referred to as resistors. The restriction to the flow of charge is electrical resistance and depends on the physical dimensions and temperature of the conductor.

The resistance (R) of a conductor varies directly with its length (l) and inversely with its area of cross-section (A). The mathematical expression is   R = ρl/A,  where ‘r’ is the constant called resistivity or specific resistance of the material which depends on the nature and temperature of the material. Resistivity is measured in ohm-metre.

At a given temperature, the current through a conductor is directly proportional to the potential difference across its ends and is known as Ohm’s law.
                           Ohmic Conductors                      Non-Ohmic Conductors 1. Conductors that obey Ohm’s Law are called Ohmic conductors.

2. In Ohmic conductors, current is proportional to voltage.

3. Magnitude of current remains unchanged when current or voltage is reversed in Ohmic conductors.

4. In Ohmic conductors, temperature affects current and resistance.

1. Conductors which do not obey Ohm’s Law are called Non-Ohmic conductors.

2. In Non-Ohmic conductors, current is not proportional to voltage.

3. Magnitude of current changes when current or voltage is reversed in Non-Ohmic conductors.

4. In Non-Ohmic conductors, different factors affect current and resistance.



Factors affecting the resistance of a conductor :
(1) The material of wire :
The resistance of a wire depends on the number of collisions which the electrons moving through it suffer with the other electrons and with the fixed positive ions of the wire. In different materials, the concentration of electrons and the arrangement of atoms are different, therefore, the resistance of wires of same length, same area of cross section, but of different materials differ depending on their material. Good conductors having higher concentration of free electrons (such as metals) offer less resistance.
(2) The length of wire : The number of collisions suffered by the moving electrons will be more if they have to travel a longer distance in wire, therefore, a long wire offers more resistance than a short wire (i.e, resistance ∝ length of wire)
(3) The area of cross section of wire : In a thick wire, electrons get a larger area of cross section to flow as compared to a thin wire, therefore, a thick wire offers a less resistance ((i.e, resistance ∝ 1/area of cross section)
(4)The temperature of wire: If the temperature of wire increases, ions in it vibrate more violently. As a result, the number of collisions increases and hence the resistance of wire increases with the increase in its temperature.

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