All matter is made up of molecules, which in turn, are made up of atoms. Atoms can be broken down into three types of particles namely, electrons, protons and neutrons. Protons and neutrons are at the centre of the atom and form the part called nucleus, with the electrons revolving around the nucleus. Protons and neutrons are tightly bound together and the electrons revolve around the nucleus in different orbits at different distances from the nucleus. The analogy is that of the planets revolving around a star. Electrons can be dislodged from an atom, by application of energy, much easier than the protons and neutrons. Electrons in the outer orbits are easy to dislodge than the electrons in the orbits nearer to the nucleus.
Electrons and protons exhibit a basic property called electric charge. Electrons are
negatively charged and protons are positively charged. Neutrons do not posses any charge i.e. they
are electrically neutral. The electric charge of an electron can be imagined as lines of force pointing towards the electron; similarly, the electric charge of a proton can be imagined as lines of force pointing outwards from the proton. Though electrons and protons differ in mass, the charges possessed by an electron and a proton are equal and opposite. An electron is indicated by a ‘minus’ (-) sign and the proton is indicated by a ‘plus’ (+) sign.
When two electrons are brought close to each other, they tend to push each other away and same is the case with protons. But when an electron and a proton are brought close, they tend to attract each other. This gives the universal law “like charges repel” and “unlike charges attract each other.”
Fig. 1.1 Two protons repelling and an electron and a proton attracting each other.
In some materials, the electrons on the outermost orbit move from one atom to another by even a slight application of energy such as room temperature heat energy. Such electrons move chaotically in the spaces between the atoms, these electrons are called free electrons. Number of free electrons depends upon the material. The relative mobility of free electrons within the material is called conductivity. Materials, like the metals, which have a large number of free electrons, are called conductors. Materials which do not have many free electrons even when a large energy is applied are called insulators. Examples of insulators are wood, glass, wool, paper, pure water. There is one more class of materials which exhibits conductivity property in between that of a conductor and an insulator. These are called semi conductors. Semi conductors are used to manufacture the electronic components that go into an electric circuit board.
Charge is an amount of electrons. Unit of charge is Coulomb (denoted by Q) which is
When there are more electrons on one side than the other in a material, there exists a
potential difference. Electrons tend to move from an area of higher potential (higher concentration)
to an area of lower potential (lower concentration). This is similar to the flow of water. This flow of electrons is called current. Current is the rate of flow of charge, i.e. the number of coulombs flowing, per second. Unit of current is Ampere (amp in short) and is denoted by A. One amp is equal to one coulomb per second. Electricity is the flow of current from one place to another.
Potential difference (p.d.) is also called voltage. Unit of p.d. is volt (denoted by V). One volt is the work done to move unit charge through a unit distance. When talking about power sources, like a battery cell, volt is the potential energy available (work to be done) per unit charge, to move electrons through a conductor.
There are two types of currents namely, direct current (DC) and alternating current (AC). DC flows in only one direction in a circuit; in other words, DC has fixed polarity. A battery or an electric cell is a source of DC. All devices using a battery work on DC. When a circuit is connected to a source of DC, electrons flow from the negative terminal of the battery, through the circuit, into the positive terminal. Batteries and cells come in various voltage ratings e.g. 1.5V, 9V, 12V DC. Batteries used on aircraft are usually 24V DC.
An AC reverses its direction of flow cyclically. There are no negative and positive terminals to an AC power source because, for half a cycle, a terminal becomes negative and during the other half, it becomes positive. Therefore, in a circuit which is connected to an AC source, current flow reverses its direction every half cycle. A complete cycle is 3600, so AC reverses polarity each 1800,
i.e. it interpolates between positive and negative values every 180o. Number of cycles per second is
called frequency. Unit of frequency is Hertz (denoted by Hz). 50Hz means the current does 50 cycles per second. Domestic electric supply at homes is AC. AC voltage supplied to homes in India is 230V, 50 Hz, 1 phase. In some countries like U.S. and Japan, the domestic supply is 110V, 50 Hz (or up to 60 Hz). AC is obtained from AC generators, also called alternators.
What is the advantage of AC over DC? When an electric current flows through a
conductor, heat is developed. Magnitude of DC is constant; it produces more heat than the AC. This means more energy is lost in the form heat. When electricity is transferred over very long distances, large amount of power will be wasted if DC is used, which can be reduced by using AC.
Resistance: Why is heat produced when current flows through a conductor? Because every
material has a basic property called resistance, denoted by R. Resistance can be defined as the opposition offered to the flow of current. Conductors have less resistance and insulators have high resistance. Conductors made of different materials have different resistance. This opposition converts the electric energy into heat energy which is dissipated into the surrounding atmosphere. Resitance of a wire is directly proportional to the length and indirectly proportional to the area
of cross section of the wire. Longer the wire, higher the resistance and thicker the wire, lower the
resistance. Resistance of a material also depends on the temperature (resistance increases with increase in temperature – for most of the materials) and specific resistance of the material.
Resistance is measured in Ohms denoted by the symbol Ω. One ohm is defined as the amount of resistance offered to the flow of one amp of current when there is a constant p.d. of one volt between two points of a conductor. Resistances are widely used in circuits to drop voltages. Physical resistances (or resistors) are components manufactured to have defined amount of resistance.
Ohm’s law states that “the current between two points of a conductor is directly
proportional to the p.d. between those two points”. By introducing the constant factor R, the formula of relationship between volt, current and resistance is:
V = I x R or I = V/R or R = V/I
With this formula, we can calculate one of the components if the other two are known. For example, if a load resistance of 2 ohms is connected across a battery of 24V electromotive force (emf), what is the magnitude of current flowing in the circuit?
R1 R2 R3 V I + _ V R3 R1 R2 V = 24 V, R = 2 Ω, I = ? V/R = I
24V/2 Ω = 12A. Answer is 12 amps of current will flow in the circuit.
Resistance in series: When more than one resistance are connected one after the other in a
circuit, the total resistance is equal to the sum total of individual resistances. The following figure illustrates the idea:
Fig. 1.2 Resistances in series connected across a voltage source.
In the above figure three resistors R1, R2 and R3 are connected in series across a voltage
source V. The dotted arrows indicate the flow of electron current in the circuit. If each resistor is equal to 2 ohms the total resistance will be 6 ohms (since R = R1+R2+R3). If V is 12V, then I would
be 2A. Note that the magnitude of current is the same through all the resistors.
Resistance in parallel: Resistances can be connected in another method that is called
parallel. When resistors are connected in this fashion, the total resistance becomes equal to the sum total of the reciprocals of individual resistors. In other words, the total resistance will be lesser than
the smallest resistor’s resistance. Figure 1.3 illustrates the idea.
Fig. 1.3 Resistances in parallel across a voltage source
In the above figure, three resistors are connected in parallel. If each resistor is 3 ohms, then the total resistance of the combination will be 1 ohm (since 1/R = 1/R1 + 1/R2 + 1/R3). In this
circuit, the current splits and flows through the resistors. The magnitude of current through each resistor depends upon the value of each branch resistance.
Resistors can be fixed or variable. Fixed resistors are of various construction such as wire-wound and carbon resistors. Value of fixed resistors is indicated by colour codes on the surface of the resistors. A variable resistor is indicated as R3 in figure 1.3. Example of variable
resistor is the volume control of a radio. Resistors can be wire-wound and carbon resistors
Power and energy: Electrical work has to be done to move the electrons across a
conductor against the opposition offered due to resistance. Amount of work done is given by the product of EMF (in volts) and the current (in amps). Unit of electrical power is Watt denoted by W. P = EI
Where P = power (in Watts), E = EMF (in Volts) and I = current (in Amps). So, 1V of emf causing 1A of current to flow through 1 Ω produces 1W of power. The above formula can also be expressed in terms of V, I and R.
P = V2/R (because I = V/R) and
P = I2R (because V = IR)
Since Watt is too small a unit for measurements of electricity consumed at homes, a bigger unit, a kilo Watt is used. If one kW power is consumed for 1 hour, it is said that 1 unit (kWh) has been consumed. Electric consumption meters at houses measure and indicate the usage in units, i.e. kWh.
Example: An electric bulb connected across a battery of 12V draws a current of 2A.
(a) What is the resistance of the bulb?
Answer: V = IR, R = V/I. Therefore 12V/2A = 6 Ω (b) What is the power consumed in the circuit? Answer: P = VI. Therefore 12V X 2A = 24W
Electric bulbs fitted in houses are graded as per power rating. Normal ratings are 40W, 60W, 100W. Fluorescent lamps (tube lights) are usually of 40W or 60W ratings. Compact Fluorescent Lamps (CFL), which are fast replacing the bulbs and tube lights are rated at 5W, 8W, 11W, etc. A television can be of 80W to 100W rating. An electric ceiling fan may be of 60W or 80W.
Example: A house is fitted with 4 lamps of 60W each and two fans of 80W ratings. If the lamps are
lit for 4 hours and the fans are used for 6 hours daily, (a) What is the power consumed in a month of 30 days? Answer: {(60W X 4 X 4 X 30) + (80W X 2 X 6 X 30)}/1000 = {28800 + 28800}/1000
= 57600/1000 = 57.6 Units (kWh)
(b) If cost of one unit is Rs.2.50/-, what is the monthly electricity charge? Answer: 57.6 X 2.50 = Rs. 144/-
Cells and Batteries: Cells are sources of DC. Cells convert chemical energy into
electrical energy. Cells have positive and negative terminals. When a load is connected across these terminals, chemical reaction takes place inside the cell and electricity is produced.
Primary and Secondary cells: There are two types of cells viz. primary cell and secondary cell. Primary cells can be used only once because the chemical reaction that goes on
inside the cell is irreversible. When the cell is fully discharged, the chemicals inside the cell are spent. Primary cell voltage is 1.5 V.
Secondary cells are reusable. Secondary cells, once discharged, can be charged again. The charging process reverts the chemicals to their original form. Lead-acid, Nickel-Cadmium and Lithium-ion cells are examples of secondary cells. Secondary cell voltage is 2 V.
Cells in series or parallel: Cells can be connected in series or parallel configuration. In
series, +ve terminal of one cell is connected to –ve terminal of the second cell and so on. When cells are connected in series, the total voltage is equal to the sum of individual cell voltages and the current provided by such a combination will be equal to the current rating of one cell. For example, if three cells of 1.5 V 500 mA are connected in series, the combination can provide 4.5 V 500mA.
When cells are connected in parallel, the voltage of the configuration will be equal to the voltage of one cell, but the current rating will be that of sum of all individual cell ratings. For example, if three cells of 1.5 V 500 mA are connected in parallel, the combination can provide 1.5V 1.5 A (500mA X 3 = 1500mA = 1.5A).
Batteries are a number of cells connected together. Batteries give much higher voltage and current than individual cells. Batteries are usually manufactured in 6V, 12V or 24V ratings.
Ampere Hour (AH) is another term used with cells and batteries. It is also called the
rating of the cell/battery. AH means the current-hour rating of a battery. 12 V 15 AH means the battery provides 12 volts emf and can provide 15 amps of current for 1 hour before getting discharged. This also means the battery can provide 7.5 amps for 2 hours, 3.75 amps for 4 hours and so on.
Fig. 1.4 Cells connected in series and parallel across voltmeter
Capacitance: When two conductors (usually metal plates) are placed adjacent to each other,
separated by an insulating medium (such as air, paper, mica), and a DC source is applied to the plates, both the plates get charged to the polarity they are connected to. Even after the DC source is removed, the plates retain this electric charge. This property is called capacitance, denoted by C. Unit of capacitance is Farad. A farad is the charge in coulombs which a capacitor will accept
for the potential across it to change 1 volt. The plates can be neutralised by connecting a wire to
the two plates and the electrons from the negatively charged plate will flow into the positively charged plate until the electron distribution is uniform in both the plates. Such an arrangement of plates and the insulating medium is called a capacitor and the insulating medium is called
dielectric. This way, a capacitor stores electric energy in a circuit. Capacitance of a capacitor is directly proportional to the area of the plates and indirectly proportional to the distance between them. Larger the area of the plates, higher the capacitance and the larger the distance
between the plates, lesser the capacitance. Capacitance also depends upon the material used as dielectric medium.
Different types of capacitors are paper capacitors, mica capacitors and air capacitors. These capacitors do not have any polarity. Electrolyte capacitors can be designed for large values and these have polarity of plates.
Capacitors in series and parallel: When two or more capacitors are connected in series,
reciprocal of the total capacitance is equal to the sum of the reciprocals of the individual capacitances. This means, the total capacitance of the combination will be lesser than the smallest capacitor in the circuit. This is similar to resistances connected in parallel.
When capacitors are connected in parallel, the total capacitance of the combination will be equal to the sum total of individual capacitances. This is similar to the resistances connected in series. In the figure 1.5, C3 capacitors are variable.
Capacitive reactance: If an AC is applied across the capacitor, due to the changing polarity
of the applied voltage, the energy is transferred from one plate to the other. Higher the frequency of the AC, higher will be the amount of energy passed through the capacitor, whereas, DC is completely blocked by a capacitor. The opposition offered by a capacitor to flow of current, by virtue of its capacitance, is called capacitive reactance denoted by XC. A capacitor exhibits high
reactance (opposition) to low frequencies (remember, DC is zero frequency) and low reactance to higher frequencies. This property of reactance is very useful in constructing filters and oscillators in radio circuits. Unit of capacitive reactance is Ohm.
1.2 Magnetism
Magnetism is the property exhibited by some materials by virtue of which, they attract or repel certain other materials. Though the exact reason for such a property is not known, one explanation offered is the alignment of domains within the material which gives the material magnetic property.
Magnets have two poles namely, north and south poles. When a bar shaped magnet is freely suspended, it aligns itself along the north-south direction. This is because the earth itself behaves like a magnet.
When two magnets are brought nearer, it can be observed that “like poles repel and unlike
poles attract each other”. A magnet is surrounded by lines of force, which originate in the North
pole and terminate at the South pole. Stronger the magnet, higher will be the number of lines of force. Number of lines of force per unit area is known as magnetic flux.
Certain materials can be magnetised by applying magnetising force. The force can be another magnet or electric current. Materials which retain the magnetic property even after the magnetising force is removed are called permanent magnets. Materials which lose the magnetism once the magnetising force is removed are called paramagnetic materials. Ferromagnetic substances are used to make permanent magnets. Electromagnets are magnetic materials wound inside coils of wire; when an electric current is passed through the coil, it induces a magnetic field into the magnetic material and once the current is stopped, the magnetic field reduces to zero.
Magnets are used in electronics to make electric meters, relays, headphones, speakers etc. Magnetic property is used in inductances to store energy.
Inductance: When an electric current is passed through a conductor, a magnetic field is
created around the conductor. If the electric current is fluctuating, the magnetic field around the conductor also changes. A changing magnetic field around a conductor induces a voltage (EMF) in the conductor as a result of changing magnetic flux. This property is known as inductance, denoted by L. Unit of inductance is Henry. One Henry is the inductance required to generate one volt of
induced voltage when the current is changing at a rate of one ampere per second. Inductors
(or inductances) used in electronic circuits are usually in the form of wound coils with or without a core. Inductors can store energy in the electromagnetic form.
Inductances in series and parallel: When inductors are connected in series, the total
inductance is equal to the sum total of individual inductances. This is similar to resistances connected in series. When inductors are connected in parallel, the reciprocal of the total inductance will be equal to the sum of the reciprocals of individual inductor’s inductance. In other words, the total inductance of a parallel combination will be lesser than the smallest value of the inductance in the combination. This is similar to resistances connected in parallel. In figure 1.6, inductor L3 is
variable.
Inductive reactance: Like the capacitive reactance, an inductor, by virtue of its inductance,
offers reactance (opposition) to flow of current through its coils. But the difference is, the reactance offered by an inductor to DC is nil and the reactance increases as the frequency of the input increases. Higher the frequency of the AC applied, higher the inductive reactance. Inductive reactance is denoted by XL and the unit of inductive reactance is Ohm. This property of inductive
Fig. 1.6 Inductances connected in series across a DC source and parallel with an AC source 1.3 Ohm’s law for AC circuits
In an AC circuit, there are three oppositions offered to the flow of current namely, the