Conductors, Insulators and Current Electricity

1.0Conductors and Insulators

Conductors are the substances in which electricity can flow quite freely.

The substances which allow the electric current to flow through them are called ‘conductors’.

Examples of conductors : 

(1) Metals : Copper (Cu), aluminium (Al), silver (Ag), iron (Fe), etc.  

(2) Non metals : Graphite  

(3) Electrolytic solution : Aqueous solutions of NaCl, HCl, H₂SO₄, etc.

The substances which do not allow the electric current to flow through them are called insulators’.

Examples of insulators : Rubber, glass, wood, plastics, porcelain, pure water, sugar, etc.

2.0Current Electricity

The study of charges in motion and their effects is called ‘current electricity’. 

The rate of flow of electric charge through any cross-section is called ‘electric current’.

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

Where, ‘Q’ is the charge that flows across the area in time ‘t’.

Electric current is a scalar quantity. Though, a direction is associated with electric current, still it is not considered as vector quantity because it does not obey the vector laws but obeys scalar laws of addition.

Unit of Electric Current

The electric current is expressed by a unit called ampere (A), named after the French scientist, Andre-Marie Ampere (1775–1836). Small quantities of current are expressed in milliampere $(1mA=10^{-3}A$) or in microampere ($1\mu A=10^{-6}A$)

SI unit :  Ampere

$1Ampere=1coulomb/sec=1C{s}^{-1}$

Definition of 1 Ampere

1 ampere is the electric current flowing through a conducting wire when 1 coulomb charge flows through it in 1 second.

3.0Comparison Between Water Current and Electric Current

4.0Current Carriers

The charged particles which flow in a particular direction to produce electric current are called ‘current carriers’.

In solid conductors, current carriers are ‘free electrons’. In liquids, current carriers are positive ions and negative ions. In gases, positive ions and electrons are current carriers.

Electric forces in a material cause electric current to flow, just as forces in the water cause water to flow. 

Conventional direction of current 

The conventional direction of electric current is ‘the direction of flow of positive charge’. This means the direction of electric current is ‘opposite to the flow of negative charge’.

Flow of charge through a conductor

(1) When there is no potential difference across a conductor, the directions of motion of free electrons present in it are randomly oriented such that there is no net drift of electrons in a particular direction. Hence, no electric current flows through it. (see figure (a))    

(2) When a potential difference is applied across the conductor, there is an average drift of electrons in a particular direction. Thus, there is a net transfer of electrons across the conductor which results in electric current. [see figure (b)]

One way to have flowing water perform work is shown in figure(a) below. Water flows out of the tank and falls on a paddle wheel, causing it to rotate. A pump then provides a pressure difference that lifts the water back up into the tank. The constant flow of water would stop if the pump stopped working. The flow of water also would stop if one of the pipes broke. Then water no longer could flow in a closed loop, and the paddle wheel would stop rotating. Figure(b) also shows an electric current doing work by lighting a lightbulb. Just as the water current stops flowing if there is no longer a closed loop to flow through, the electric current stops if there is no longer a closed path to follow. A closed path that electric current follows is a circuit. If the circuit in figure (b) is broken by removing the battery, the light bulb, or one of the wires, current will not flow.

5.0Batteries

To keep water flowing continuously in the water circuit in figure(a), a pump is used to provide a pressure difference. In a similar way, to keep an electric current continually flowing in the electric circuit in figure(b), a voltage difference needs to be maintained in the circuit. A battery can provide the voltage difference that is needed to keep current flowing in a circuit. Current flows as long as there is a closed path that connects one battery terminal to the other battery terminal.

6.0Measuring Potential Difference

The device used to measure the potential difference across a current-carrying device (or conductor) is called voltmeter. Voltmeter is connected in parallel to the device across which potential difference is to be measured. (see figure) 

Measuring potential difference across a resistor using a voltmeter.

The voltmeter has a very large resistance, so that it draws very little current from the circuit and hence has very little effect on the voltage being measured. For an ideal voltmeter,  resistance,

$r=\infty$

Measuring Current

The device used to measure the electric current through device (or conductor) is called ammeter. Ammeter is connected in series with the device through which electric current is to be measured. (see figure)

The ammeter has a very small resistance, so that it has very little effect on the current being measured. For an ideal ammeter, resistance, r = 0.

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