Current is a measure of the amount of charge that passes through an area perpendicular to the flow of charge.
Electric current is defined as the rate of flow of charge per unit time interval.
* It is a scalar quantity.
* Electric current unites are 1c/sec = 1amp = 1 A.
* A current will flow until there is no potential difference.
* The direction of current flow in a wire is opposite the flow of the electrons.
* One ampere is equivalent to one coulomb, which is equal to 6.241 × 1018 electron/sec.
The resistance of an electric conductor is the opposition to the passage of an electric current through that conductor. When electrons move through a material, they undergo many collisions, which hinder the motion. Without such collisions, the electrons would accelerate.
* The SI unit of electrical resistance is the ohm ().
* The resistance(R) of an object is defined as the ratio of voltage(V) across it to current through it (I).
* Therefore Voltage and Current are directly proportional to each other.
* When the resistive force is high, the current will go down if the voltage difference that drives the motion remains the same.
Ohm’s law describes the fundamental law of how the electrical circuits behave. According to Ohm’s law, there is a linear relationship between the voltage drop across a circuit element and the current flowing through it. Therefore the resistance R is viewed as a constant independent of the voltage and the current. In equation form, Ohm’s law is,
V = IR
I, is the current through the conductor in units of amperes
V, is the potential difference measured across the conductor in units of volts
R, is the resistance of the conductor in units of ohms
Ohm’s law states that the R in this relation is constant and it is independent of the current.
The resistance power of conductors of electrical circuits which obey Ohm’s law is called as Ohmic resistance.
Example: The resistance of manganin wire.
* In this the resistance is said to have a constant value.
* In this the plot is a straight line passing through the origin.
* The slope of the line is the resistance R of the device.
Non ohmic resistance
If a device behaves in a way that is not described by Ohm’s law, which means the resistance is not constant but changes in a way that depends on the voltage across it. This type of resistance is called as non ohmic resistance.
Example: Resistance of diode valve, resistance of triode valve.
* These type of devices do not have a constant value of resistance and the resistance is called dynamic resistance because it constantly changing.
* Examples of such devices are tungsten filament, diode, thermistor.
Conductance is an expression of the ease with which electric current flows through a substance. In equations, conductance is symbolized by the uppercase letter G. The standard unit of conductance is the siemens (S), formerly known as the mho.
Conductance (G) = 1/Resistance = 1/R
* Its SI unit is ohm-1.It is also called as mho or siemen.
* The resistance of a conductor is directly proportional to its length and inversely proportional to its cross sectional area of a conductor and R is its resistance then,
R l/A or R = ρ l/A
* Here ρ is a constant of material of conductor. It is called as specific resistance or resistivity. Its SI unit is ohm meter.
Specific conductance or conductivity
Conductivity also the specific conductance, is a measurement of the electrical conductance per unit distance in an electrolytic or aqueous solution.
The reciprocal of resistivity of a conductor is called its conductivity (S).
Conductivity is the ability of a solution, a metal or a gas in brief, all materials to pass an electric current.
* Its SI unit is mho m-1 or seimen / meter.
* In solutions the current is carried by cations and anions.
* In metals it is carried by electrons.
Combination of Resistance
The equivalent resistance of a combination of resistors connected between two points (A and B) is the resistance of that single resistor which produces the same effect as the combination of resistors.
Various resistances can be combined to form a network mainly in two ways, they are
1. Series combination
2. Parallel combination
Rs of resistors connected in series is given by Rs = R1 + R2
For resistors in parallel the equivalent resistance, Rp satisfies the relationship
1/ RP = 1/ R2 + 1/ R3 (Or) RP = R2R3/ R2 + R3
Electric power is the rate at which electric energy is transferred by an electric circuit.
The rate at which electrical energy is consumed in a circuit is called as electric power.
* The SI unit of power is the watt, one joule per second.
* Electric power is usually produced by electric generators.
* It can also be supplied by sources such as electric batteries.
A measure, of electrical energy equivalent to power consumption of one thousand watts for one hour.
1 kilowatt hour = 3.6 × 106 joule
* One kilowatt hour is also called as Board of trade unit.
Ammeter, is an instrument for measuring electric current in amperes in a circuit.
* Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters.
* Electric currents are measured in amperes (A).
* It is connected in series in the circuit.
Voltmeter is an instrument used to measure the potential difference between two points in a circuit.
* Some voltmeters are used in direct current (DC) circuits.
* Some other are designed for alternating current (AC) circuits.
* Specialized voltmeters can measure radio frequency (RF) voltage.
* It is connected in parallel to the circuit.
* The resistance of an ideal voltmeter is infinite.
Electric fuse, is an electric device that can interrupt the flow of electrical current when it is overload.
* It is used in series with an electric appliance.
* It is used to save from being damaged due to high current.
* It is a small conducting wire of alloy of copper, tin and lead having low melting point.
* The pure electric fuse made up of tin.
Galvanometer is an instrument for detecting and measuring small electric currents.
* It can measure current up to 10-6 A.
* These were the first instruments used to detect and measure electric currents.
An electrical conductor joining two points of a circuit, through which more or less of a current may be diverted.
* The term shunt, means to turn away or follow a different path.
* It is a wire of very small resistance.
* By connecting shunt parallel to a galvanometer then, it can be converted into an ammeter.
* By connecting a very high resistance in galvanometer’s series then, it can be converted into a voltmeter.
Transformer is an apparatus for reducing or increasing the voltage of an alternating current.
Transformer is a device which converts low voltage A.C. into high voltage A.C.and high voltage A.C. into low voltage A.C.
* It is based on electromagnetic induction and can be used only in case of alternating current.
* A varying current in the transformer's primary winding creates a varying magnetic flux in the transformer core.
* A varying magnetic field impinging on the transformer's secondary winding.
Generator ( A C Dynamo)
It is a device used to convert mechanical energy into electrical energy for use in an external circuit.
* It works on the principle of electro-magnetic induction.
* Generators provide nearly all of the power for electric power grids.
It is a device which converts electrical energy into mechanical energy. The reverse of this would be the conversion of mechanical energy into electrical energy and is done by an electric generator.
An instrument for converting sound waves into electrical energy variations which may then be amplified, transmitted, or recorded. In 1916, C. Wente developed the first condenser microphone.
* These are used in many applications such as telephones, hearing aids, public address systems for concert halls and public events, motion picture production, live.
* Most microphones today use electromagnetic induction, capacitance change or piezoelectricity to produce an electrical signal from air pressure variations.
* These typically need to be connected to a preamplifier before the signal can be amplified with an audio power amplifier or recorded.