Electricity

Electricity is a form of energy that is associated with electric charges that exists inside the atom.

Electric Current:-

· The flow of electric charges constitute an electric current.

· An electric current is measured by the amount of electric charge moving per unit time at any point in the circuit. The conventional symbol for current is ‘I’.

Unit of Electric Current

The SI unit for measuring an electric current is the ampere, which is the flow of electric charge across a surface at the rate of one coulomb per second.

I = q / t

Where I ⇒ current (in Ampere - A)

q ⇒ charge (in coulomb - c)

t ⇒ time taken (in seconds - s)

Conventional Current and Electron Flow:-

· An electric current consisted of moving positive charges, this movement of positive charges is called conventional current.

· It was known that electron flow actually takes place from the negative terminal to the positive terminal of the battery. This movement is known as electron flow.

Conventional current is in the direction opposite to electron flow.

Measurement of Electric Current:-

· Electric current is measured using a device called ammeter.

· The terminals of an ammeter are marked with + and - sign.

· An ammeter must be connected in series in a circuit.

Potential difference (v)

· The potential difference between any two points in the circuit is the amount of energy needed to move one unit of electric charge from one point to the other.

· Electrical charges need energy to push them along a circuit. An electric charge always flows from a point at higher potential to a point at lower potential.

· An electric current can flow only when there is a potential difference (V) or P.D.

· Potential difference between two points is measured by using a device called voltmeter.

Unit of potential difference

The SI unit of potential difference is volt (V).

Electrical Conductivity and Resistivity:-

Resistance (R)

· The higher the resistance in a component, the higher the potential difference needed to move electric charge through the component.

· Resistance of a component is the ratio of the potential difference across it to the current flowing through it,

· The S.I unit of resistance is ohm.

· Greater the ratio of V to I, the greater is the resistance.

Electrical conductivity (σ)

· Electrical conductivity or specific conductance is the measure of a material's ability to conduct an electric current.

· It is commonly represented by the Greek letter σ (sigma).

· The S.I Unit of electrical conductivity is Siemens/meter(S/m)

Electrical resistivity (ρ)

· Electrical resistivity (also known as specific electrical resistance, or volume resistivity) is a fundamental property of a material that quantifies how strongly that material opposes the flow of electric current.

· The SI unit of electrical resistivity is the ohm-metre (Ω.m).

Sources of Electric Current

Electro chemical cells or electric cells

· An electric cell is something that provides electricity to different devices that are not fed directly or easily by the supply of electricity. When electric cells are used, a chemical reaction takes place inside the cells which produces charge in the cell.

· In addition to electro chemical, we use electro thermal source for generating electricity for large scale use. It has two terminals. Types of cell – primary cell and secondary cell

Primary Cell:-

· A dry cell is a type of chemical cell commonly used in the common form batteries for many electrical appliances.

· It is a convenient source of electricity available in portable and compact form.

· Dry cells are normaly used in small devices such as remote control for T.V., torch, camera and toys.

· A dry cell is a portable form of a leclanche cell.

· It cannot be recharged after use.

Construction:-

· It consists of zinc vessel which acts as a negative electrode or anode.

· The vessel contains a moist paste of saw dust saturated with a solution of ammonium chloride and zinc chloride.

· The ammonium chloride acts as an electrolyte. Electrolytes are substances that become ions in solution and acquire the capacity to conduct electricity.

· The purpose of zinc chloride is to maintain the moistness of the paste being highly hygroscopic.

· The carbon rod covered with a brass cap is placed in the middle of the vessel. It acts as positive electrode or cathode.

· It is surrounded by a closely packed mixture of charcoal and manganese dioxide (MnO2) in a muslin bag. Here MnO2 acts as depolarizer.

· The zinc vessel is sealed at the top with pitch or shellac.

· A small hole is provided in it to allow the gases formed by the chemical action to escape.

· The chemical action inside the cell is the same as in leclanche cell.

Secondary Cells

· Secondary cells are used in automobiles and generators.

· The chemical reaction in them can be reversed, hence they can be recharged.

· Lithium cylindrical cells, button cells and alkaline cells are the other types that are in use.

Difference B/W Primary and Secondary Cells:-

Batteries

· Batteries are a collection of one or more cells whose chemical reactions create a flow of electrons in a circuit.

· All batteries are made up of three basic components: an anode (the ‘+’ side), a cathode (the ‘–’ side), and some kind of electrolyte. Electrolyte is a substance that chemically reacts with the anode and cathode.

Electric Switch

· Switches help us to start or stop the appliances safely and easily.

Electric Circuit

· We can draw circuit diagrams using these symbols. Symbols for bulbs, cells and switches are shown in figure.

· In a cell, the longer line denotes the positive (+) terminal and the short line denotes the negative (-) terminal. We shall use these symbols to show components in the circuits we draw. Such diagrams are called circuit diagrams.

Types of Electrical Circuits

Series Circuit

· So, similar looking bulb do not always glow equally bright when connected in series.

· The circuit can be broken at several places. For example, between the cell and the bulb, between the two bulbs etc.

· A series circuit is one in which more than 1 bulbs are connected along 1 path through which electricity flows.

Parallel Circuit

· A circuit in which two bulbs are connected in different places.

Similarity and Difference between Series and Parallel Circuit

Conductors and Insulators

· Based on the property of conductance of electricity, substances are classified into two types, namely, Conductors and Insulators (or) bad conductors of electricity.

· The electrons of different types of atoms have different degrees of freedom to move around.

· With some types of materials, such as metals, the outermost electrons in the atoms are loosely bound and they chaotically move in the space between the atoms of that material. Because these virtually unbound electrons are free to leave their respective atoms and float around in the space between adjacent atoms, they are often called as free electrons.

Conductors

· Conductors are the materials whose atoms have electrons that are loosely bound and are free to move through the material.

· A material that is a good conductor gives very little resistance to the flow of charge (electron) on the application of external voltage.

· This flow of charge (electron) is what constitutes an electric current.

· A good conductor has high electrical conductivity.

· More the free electrons, the better the material will conduct for a certain applied voltage.

Insulators

· Those materials which don’t have enough ‘free electrons’ are not good at conducting electricity or we can say that they would be ‘poor conductors’ of electricity and they are called insulators.

· An insulator gives a lot of resistance to the flow of charge (electron).

· During the drift of the electrons in an object when an external voltage is applied, collisions occur between the free electrons and the atoms of the material also affect the movement of charges. These collisions mean that they get scattered. It is a combination of the number of free electrons and how much they are scattered that affects how well the metal conducts electricity.

· The rubber eraser does not allow electric current to pass through it. So rubber is a non-conductor of electricity. Rubber is an insulator.

Most of the metals are good conductors of electricity while most of the non-metals are poor conductors of electricity.

Effects of Electric Current

Heating Effect

· When an electric current passes through a wire, the electrical energy is converted to heat.

· In heating appliances, the heating element is made up of materials with high melting point. An example of such a material is nichrome.

· The heating effect of electric current has many practical applications. The electric bulb, geyser, iron box, immersible water heater are based on this effect. These appliances have heating coils of high resistance.

Factors affecting Heating Effect of current

Electric Fuse

· Electric fuse is a safety device which is used in household wiring and in many appliances.

· Electric fuse has a body made of ceramic and two points for connecting the fuse wire.

· The fuse wire melts whenever there is overload of the current in the wire. This breaks the circuit and helps in preventing damage to costly appliances and to the wiring.

· In electrical devices, a glass fuse is often used. This is a small glass tube, in which lies the fuse wire.

MCBs (Miniature Circuit Breaker)

Why MCBs have been replacing electric fuse?

Because whenever the wire of electric fuse fuses, one needs to replace the wire to resume electric supply. Miniature circuit breakers break the circuit automatically. One just needs to switch it on to resume the electric supply.

Magnetic Effect of Electricity

The next effect of electric current is Magnetism.

Application of magnetic effect of electric current –

· Electromagnet Magnetic effect of electric current has been used in making powerful electromagnets.

· Electromagnets are also used to remove splinters of steel or iron in hospitals dealing with eye injuries.

· Electro magnets are used in many appliances that we use in our day to day life, namely, electric bell, cranes and telephone.

Telephone

· In telephones, a changing magnetic effect causes a thin sheet of metal (diaphragm) to vibrate.

· The diaphragm is made up a metal that can be attracted to magnets.

1. The diaphragm is attached to spring that is fixed to the earpiece.

2. When a current flows through the wires, the soft – iron bar becomes an electromagnet.

3. The diaphragm becomes attracted to the electromagnet.

4. As the person on the other end of the line speaks, his voice cause the current in the circuit to change. This causes the diaphragm in the earpiece to vibrate, producing sound.

Chemical Effects of Electricity

Chemical reactions happens, when electricity passes through various conducting liquids. This is known as chemical effects of electricity.