1 Coulomb = 6.242*1018 electrons



ElectronicsLesson-1 Basic Electricity1.1 Current flowIf you connect a dry cell to a torch bulb using two pieces of wire, the bulb will glow. That shows current flowing through the circuit.Fig-1.1The dry cell or battery is made of chemicals and conductors. Flexible wire is normally made of copper and is covered with insulating material like plastic or PVC.Direction of currentElectric current always flows from positive terminal to the negative terminal through the external circuit. But inside the battery, it flows from negative terminal to the positive terminal.You can get a clear idea of the current flow using a LED (Light Emitting Diode)Fig 1.2Conductors & InsulatorsMaterials in which electric current can flow are called conductors, while those that refuse such movement are called nonconductors or insulators.Examples:-Conductors:- any metals, Carbon, Acids, Salts, ionized gasesInsulators:- Glass, Mica, Rubber, Porcelain, Dry wood, Dry air1.2.1 Good & Poor Conductors Conductors are divided into two; good conductors and poor conductors.Good conductors:-Electricity can flow easily through good conductors. It is found to vary with the resistance of the material. A few examples of good conductors are Silver, Copper, Gold, Aluminum etc.Poor Conductors:-Electricity can flow through poor conductors, but not flow easily as in good conductors. A few examples are Nicrom, Manganin, Tungsten and Iron. Batteries in seriesIf we connect a dry cell to a small bulb it will glow, but not very brightly because it uses 1.5 Volt. If we connect two cells in a series, the brightness will increase because the voltage increases to 3 Volts. If we connect a few more cells the filament of the bulb will get destroyed and the current will stop. That means due to the high temperature of the tungsten filament, it melts.Fig 1.31.4 ResistanceFig 1.4According to the diagram in Fig 1.4 if we connect a bulb with three cells, it will glow brightly. If we connect a nicrome wire, “R” as shown in the diagram the brightness of the bulb will decrease. That means the current through the circuit is decreasing. The reason for this phenomenon is defined as the resistance of the nicrome wire. Usually bad conductors have more resistance or low conductivity and good conductors have low resistance or high conductivity.1.5 Electric ChargeEverything physical is built up of atoms. Atoms are very small particles. They are so small that they cannot be seen even through the most powerful microscope. But the atom in turn consists of several different kinds of still smaller particles. Electron and proton are the most important particles of electricity. They have negative and positive electric charges respectively.The important fact about these two opposite kinds of electricity is that they are strongly attracted each other. Also there is a strong force of repulsion between the two charges of the same kind. Opposite charges attract each other with a strong force.An ordinary atom consists of a central core called nucleus, carrying a number of protons. The positive charge on the nucleus is exactly balanced by the negative charges on the electrons, orbiting around the nucleus. However, it is possible for an atom to lose one of its electrons. When that happens the atom has a little less negative charge than it should. That is, it has a net positive charge. Such an atom is said to be ionized, and in this case the atom is a positive ion. If an atom picks up an extra electron, it is called a negative ion. The diagram in Fig-1.5 shows a neutral atom, negative ion and a positive ion. Fig-1.5Atoms and ions are not moving in solid conductors, but they are moving in gases and liquids. The current flows through solid conductors by the movement of electrons.1.5.1 Coulomb (C)The practical unit (SI-unit) of electric charge is "Coulomb". This is equivalent to the many billions of electrons. 1 Coulomb = 6.242*1018 electrons1.6 Electric Current (Ampere) A positive ion will attract any stray electron in the vicinity, including the extra one that may be attached to a nearby negative ion. In this way it is possible for electrons to travel from an atom to atom. The movement of ions or electrons constitutes the electric current. The flow of electric current is measured in Amperes. One "Ampere" is equivalent to the movement of one Coulomb passing a point in the circuit in one second. This is the practical unit (SI-unit) for the electric current, but for the sake of convenience we use smaller units as milliampere (mA) and microampere (?A). 1000 ?A = 1 mA 1000 mA = 1 A1.7 Electro Motive Force ( E.M.F.)The electric current flows due to the electrical pressure or force generated by the source. This is called electromotive force.(e.m.f.) That causes current flow. It may develop in several ways - the chemical reaction in a cell, the movement of electrons in a solar cell or the magnetic effect in a generator (alternator). The unit of e.m.f. is "Volt". A larger unit is called kilovolt (kV) and smaller units are called millivolt(mV) and microvolt(?V).1.7.1 Potential difference & VoltageThe electrical pressure difference between two points of an electrical circuit is called potential difference. When it is measured by Volts it's called voltage. The units are as same as e.m.f.1.7.2 E.M.F. and Voltage Drop Fig-1.6 E.M.F. of a cell is equal to the voltage between two terminals, when there is no current flow. When there is a current flow or energy consumed by an external circuit, the voltage will be reduced. If the external circuit is disconnected the voltage will increase up to the value of E.M.F. The reduced amount is called the voltage drop. The voltage drop depends on the internal resistance of the battery and the resistance of the external circuit. 1.8 Capacity of a battery ( Ah. or mAh.)Normally large batteries have more capacity. Capacity means how much charge (like energy) can be taken from the battery. If 2 Amp current can take 5 hours continually from a battery, the capacity is little more than 10 Ampere-hour (Ah.)If a battery has a high capacity, it’s internal resistance will be low.1.9 ResistivityIf two conductors of the same size and shape are used with different materials, the amount of current that will flow when a given EMF is applied will be found to vary with the resistance of the material. The lower the resistance, the greater the current for a given value of EMF. Unit of resistance is "Ohm". Smaller units are micro Ohm (?Ω) and milli Ohm (mΩ) and larger units are kilo Ohm (kΩ) and Mega Ohm (MΩ). 1000 ?Ω = 1 mΩ 1000 mΩ = 1 Ω 1000 Ω = 1 kΩ 1000 kΩ = 1 MΩ Regarding the earlier example, if both conductors have the same size and same shape, a property varies with the type of material, it's called resistivity. The resistivity of a material is defined as the resistance of a cube of the material measuring between two opposite faces. But practically this is impossible to measure.The resistance of a wire with uniform cross section is directly proportional to its length and inversely proportional to its cross sectional area.A wire with a certain resistance for a given length will have twice as much resistance for a given length of the wire is doubled. If you consider two wires having same material and same length but the cross sectional area is twice, while doubling the cross sectional area will halve the resistance.The formula related with above factors is:R = ρl / AR = resistance of the wirel = length of the wireA = area of the cross section of the wireρ = specific resistance or resistivity of the material of the wire (ρ is a Greek letter “rho”)Units:- SI-unit is Ω-m. This is useful for calculations. But ?Ω-m is more convenient.If “R” is in Ohms, “l” is in meters and “A” is in square meters, then “ρ” is in Ohm-meter (Ω-m)If “R” is in micro Ohms, “l” is in meters and “A” is in square meters, then “ρ” is in micro Ohm-meter (?Ω-m)Other units are defined same as above: micro Ohm centimeter (?Ω-cm) and Ohm centimeter (Ω-cm)1.9.1 Relative Resistivity One of the best conductors is copper. It is convenient to compare the resistance of the material under consideration with that of a copper conductor of the same size and shape. The ratio of these two is the relative resistivity of the material.In other word the relative resistivity of a material is equal to the resistivity of that material divided by the resistivity of copper.1.9.2 Relative Resistivity of Metals MaterialRelative resistivityResistivity (μΩ-cm) Silver 0.94 1.59 Copper 1.01.7 Gold 1.42.4 Aluminum 1.62.7 Chromium 1.83.1 Tungsten 3.25.4 Zinc 3.45.8 Brass 3.7 - 4.96.3 – 8.3 Cadmium 4.47.5 Nickel 5.18.7 Iron 5.689.7 Platinum 6.511 Steel 7.6 - 12.712.9 – 21.6 Lead 12.821.8 Manganin 25.643.5 Nicrome 58.198.8Example:-What is the resistance of a one meter long, 20 SWG copper wire? (resistivity of copper is 1.7 ??-cm, diameter of 20 swg wire is 0.9144mm)Apply the formula R = ρl / A ρ = 1.7 ??-cm, A = cross section of the wire = π r2 π = 3.14, r = 0.9144/2 = 0.4572 mm = 0.04572 cm, l = 100cmTherefore R = 1.7 x 100 /(3.14 x 0.045722) = 25887 ?? = 0.025887 ?or 25.887 m?EXERCISESWhat is the relationship between Coulomb and Ampere?What is the difference between voltage and electro motive force?Capacity of a 1.5 V dry cell is 1000 mA-h. 300mA current consumed for 30 minutes. What is the remaining capacity of the cell? A 12volts soldering iron element made by using nicrom wire having 0.5 mm diameter. If the resistance of the element is 4 Ohms, what is the length of the nicrom wire. (resistivity of nicrom is 98.8 ??-cm. What is the resistance of 1m of 18 gauge copper wire? (resistivity of copper is 1.7 ??-cm, diameter of 18 SWG is 1.219mm) AnswersOne Ampere is equal to one coulomb passing a point in one second.EMF is the voltage between two terminals of a source, when there is no current flow.850 mA-h79.5 cm11.064 m? ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download