CURRENT ELECTRICITY - II

SALIENT FEATURES Faraday s laws of electrolysis Magnetic effects of electricity Electro magnetic induction CURRENT ELECTRICITY - II FARADAY S LAWS OF ELECTROYLYSIS ELECTROLYSIS The process of decomposition of a chemical compound in a solution when an electric current passes through it is called electrolysis. ELECTROLYTIC CELL The vessel which contains an electrolyte and allows electrolysis to take place is called a voltameter or electrolytic cell ELECTROLYTE The substance or a solution which undergoes electrolysis is known as electrolyte. MECHANISM OF ELECTROLYSIS Let us discuss the mechanism of electrolysis by taking the electrolysis of CuSO 4. The dissociation of CuSO 4 molecule takes place according to the chemical reaction. CuSO 4 Cu 2+ 2 + SO 4 The Cu + ions are attracted by the cathode and deposited on it. The sulphate ions are attracted by the anode and reacts with the copper (Anode) and forms Copper Sulphate (CuSO 4 ) which goes into the solution. Thus, 1) The concentration of electrolyte remains unaffected during electrolysis. 2) The flow of the electrons in the external circuit, from anode to cathode, constitutes an electric current. 3) The number of electrons extracted at the cathode is equal to the number of electrons supplied at the anode. 4) Since copper atoms are deposited on the cathode, the mass of cathode increases and the mass of anode decreases by an equal amount. FARADAY S FIRST LAW OF ELECTROLYSIS The Faraday s first law of electrolysis states that the mass (m) of the ions liberated from an electrolyte is directly proportional to the strength of the current (i) and the time (t) for which the current passes. According to First law, m α i, m α t m α it m = Zit. Z is known as Electro Chemical Equivalance (ece). ELECTRO CHEMICAL EQUIVALANCE The electro chemical equivalence (e c e) of an element is defined as the mass of its ions liberated at the electrode when one coulomb of electricity is passed through the electrolyte. Z = m/it Ece can be measured in grams/coulomb E.C.E OF SOME ELEMENTS Element ECE (Z) gm/coulomb Copper 0.0003294

Silver 0.00118 Zinc 0.0003387 Oxygen 0.0000829 Hydrogen 0.0000105 FARADAY S SECOND LAW OF ELECTROLYSIS Faraday s second law of electrolysis states that when the same quantity of electricity passes through different electrolytes, the masses of ions liberated at the respective electrodes are proportional to their chemical equivalents (or) equivalent weights (E). Equivalent weight The ratio of atomic weight (A) of an element to its valency (V) is defined as Chemical equivalent or equivalent weight (E) m 1 / E 1 = m 2 / E 2 = m 3 / E 3 VERIFICATION OF FARADAY S SECOND LAW OF ELECTROLYSIS Take the three voltmeters viz CuSO 4, AgNO 3, and ZnSO 4 and connect them in series as shown in the figure. In all the three voltmeters, let the electrodes A and C be made up of copper only. Before we start the experiment, determine the weights of the individual cathodes in each of the voltmeters. Adjust the rheostat such that a current of (i) about 1A is read in the ammeter. Since three voltmeters are connected in series, same current passes through all the respective electrolytes. Allow the current to pass for about half-an-hour time. At the end, determine the final weights of the individual cathodes. The differences between the corresponding final and initial weights give the masses m 1, m 2, and m 3 of copper, Silver and Zinc deposited on the respective cathodes. Calculate the ratio of m 1 / E 1, m 2 /E 2, m 3 / E 3, where E 1, E 2, E 3 are the chemical equivalents of copper, silver and zinc metals respectively. You will find that the value of each of the ratio is same. This verifies the Faraday s second law of electrolysis. APPLICATIONS OF ELECTROLSIS 1) Metallurgy:

Certain metals like copper, tin, lead, gold, zinc, chromium, nickel etc are purified and extracted by the method of electrolysis. 2) Electro plating: Electroplating is a process of coating a thin film of costlier or less corrodible metal on a base metal by the method of electrolysis. 3) Electro typing Electrotyping is a method of obtaining exact copy of an engraved block containing letters or figures by the method of electrolysis. MAGNETIC EFFECTS OF ELECTRICITY We know that an electric current in a conductor sets up a magnetic field around it. In 1820, the Oersted s experiment gives dependence of the direction of magnetic field on the direction of electric current. Ampere s swimming rule, Amperes right hand rule and Maxwell s cork screw rule all help us to find the direction of magnetic field around the current carrying conductor. MAGNETIC INDUCTION B AT A POINT NEAR A STRAIGHT CURRENT CARRYING CONDUCTOR: The magnetic induction B at a point due to a straight current carrying conductor is inversely proportional to the distance (r) and directly proportional to the strength of the current (i) B α 1/r (1) B α i (2) B = µ 0 i / 2πr (Where µ 0 is the permeability constant) FORCE ON A CURRENT CARRYING CONDUCTOR IN A MAGNETIC FIELD When a current carrying conductor is placed in a strong external magnetic field a force acts on it. The magnetic force results in the deflection or motion of the conductor. This force is given by F = ilb The direction of F can be found by applying Fleming s left hand rule. FLEMING S LEFT HAND RULE Stretch the fingers of your left hand such that the fore finger, the central finger and the thumb are mutually perpendicular with the remaining fingers curled. Then the fore finger indicates the direction of magnetic field, the Central finger indicates the direction of current, and the thumb indicates the direction of the magnetic force or motion the coil. PRINCIPLE ELECTRIC MOTOR Electric motor is a device which converts electrical energy into mechanical energy. CONSTRUCTION

WORKING ABCD is a rectangular coil of insulated copper wire wound on a soft iron core, constitute the Armature. NS is a permanent horse shoe magnet. The coil ABCD is mounted on a shaft symmetrically between the cylindrical concave poles of the magnet. C 1 and C 2 are two metallic half rings and form the commutator. The commutator rotates with the shaft. The wires from battery B are connected to two conducting carbon brushes B 1 and B 2, which are always in contact with either of the half rings. The battery B, the key K, the brushes, the commutator and the coil form an electric circuit. Initially when the coil ABCD is in the horizontal position, the half rings C 1 and C 2 are in contact with brushes B 1 and B 2. When the key K is closed, the current enters the coil through brush B 1 and C 1 the direction of current is along DCBA. And current leaves the coil through C 2 and B 2. Thus top surface of the coil behaves as a south pole and is repelled by the south pole of the magnet. The bottom surface of the coil behaves as North Pole and is attracted by the south pole of the magnet. Hence due to the attractive and the repulsive forces, the coil rotates in the anti-clock direction. The shaft utilizes the mechanical energy thus produced. In an AC motor there is no need of a commutator The RPM (Speed) of a motor depends on 1) Number of turns of the armature (n), 2) Area of the coil (A) 3) The magnitude of current (i) 4) The strength of the magnetic field (B). ELECTRO MAGNETIC INDUCTION

The production of electromotive force in a coil due to changing magnetic flux linked with the coil is called electro magnetic flux. Michael Faraday conducted a series of experiments that led to formulation of Faraday s law of electromagnetic induction. Faraday s law of electromagnetic induction states that the induced emf ( ε ) in a closed circuit (coil) is proportional to the rate at which the magnetic flux ( φ B ) through it changes. LENZ S LAW ε α N (d φ B / dt ) The induced current will appear in such a direction that it opposes the change that produced it. FLEMKNG S RIGHT HAND RULE When the thumb, the fore finger, and the central finger of the right hand are stretched mutually perpendicular to each other and are held such that the Fore finger along the direction of the magnetic field (B) and the thumb is along the direction of Motion (M) of the linear conductor, then the Central finger points along the direction of the induced Current (i) or emf. AC GENERATOR OR AC DYNAMO A dynamo is an electrical device which converts mechanical energy into electrical energy utilizing the phenomenon of electromagnetic induction. Description 1) Armature: The armature ABCD consists of a coil made of insulated copper wire and is wound on a cylindrical soft iron core. The armature is rotated rapidly about a horizontal axis perpendicular to the magnetic field. 2) Permanent Magnet: NS is a permanent horse-shoe magnet. It provides stationary magnetic field.

3) Slip rings: The ends of armature coil are connected to two different slip rings S 1 and S 2 respectively. The rings are insulated from each other. These rings rotate along with the armature about the same axis as that of the coil. 4) Carbon brushes: Two carbon brushes B 1 and B 2 are always in contact with the slip rings S 1 and S 2 respectively. The other two ends of B 1 and B 2 are connected to an external circuit containing load resistance R WORKING When the armature ABCD is rotated in anti-clock wise direction, the magnetic flux linked with it changes. As a result, current is induced in the coil and flows through the load resistance R. The changes in the magnitude of induced current with the change in the position of armature coil in the magnetic field during one complete cycle i.e. during a rotation through an angle 0 to 2π is shown graphically. Thus rapid rotation of the armature in the dynamo generates current or voltage in the external circuit whose direction alternates in each half-cycle. Hence, such currents are called alternating currents (AC). On the other hand, if half slip rings are arranged and the two ends of the coil are alternately in contact with the slip rings, a direct current (DC) is generated. Then the dynamo is called DC dynamo. SELF INDUCTANCE The production of an induced emf in an isolated coil due to a change in the current in the same coil is called self induction. The self inductance of coil is numerically equal to the ratio between the induced emf and the rate of current in the coil. -ε L = di / dt The unit of self inductance is HENRY MUTUAL INDUCTANCE The production of an induced emf in one coil due to changes in current in another close by coil is known as mutual induction. Mutual inductance of a coil with respect to another coil is numerically equal to the ratio between the induced emf in it and the rate of change of current in the other coil. PRINCIPLE AND WORKING OF A TRANSFORMER A transformer works on the principle of electromagnetic induction using mutual induction. A transformer is an electrical device which either increases or decreases the magnitude of an alternating voltage by utilizing the phenomenon of electro magnetic induction. DESCRIPTION

WORKING The essential parts of a transformer are core, primary coil, and secondary coil. In its general form, a core (C) consists of a soft iron or ferrite material. This core is in the form of a rectangular frame built by placing thin sheets of iron one above the other. These sheets are laminated and therefore are insulated from each other. On one side of the rectangular core, an insulated copper wire is wound to make a coil of turns (n 1 ). If voltage is applied across this coil, it is called a primary (P) coil. On the opposite side of the rectangular core, another copper wire is wound to make a coil of turns (n 2 ). If output voltage is delivered from this coil, it is called a secondary coil (S) In a transformer the primary coil (P) is connected to the source of an AC voltage (V 1 ) The alternating current thus set up in the primary coil (p) produces an alternating magnetic flux in the core. The flux is also linked to the secondary coil. This alternating mutual flux induces an emf (V 2 ) in the secondary coil. This V 2 will be again an AC Voltage. Transformer rule V 1 / V 2 = n 1 / n 2 = i 2 / i 1 Where V 1 Primary Voltage V 2 Secondary Voltage n 1 No. of turns in the primary coil n 2 No. of turns in the secondary coil i 1 Current in the primary coil i 1 Current in the secondary coil. TYPES OF TRANSFORMERS 1) Step up transformer The transformer with number of turns in the secondary coil is higher than those in primary coil is called step up transformer. N 2 > N 1 and V 2 > V 1 2) Step down transformer The transformer with the number of turns in the secondary coil is smaller than those in primary coil is called step down transformer. N 1 > N 2 and V 1 > V 2 POWER TRANSMISSION

Power Generating Step up 220 KV or Step down Station 11KV Transformer 132 KV Transformer (Sub Station) 11KV 11KV 11KV 11KV Local Step down transformer 415 V / 220 V Industries Houses IMPORTANT QUESTIONS 4 MARKS 1) Describe an experiment to verify the Faraday s first law of electrolysis? 2) Describe an experiment to verify the Faraday s second law of electrolysis? (Mar-03, 99) 3) Describe the construction and working of an electric motor? 4) Describe the construction and working of Dynamo? 5) What is the principle of transformer? Describe the transformer? (June-03) 2 MARKS 1 MARK 1) State the Faraday s laws of electrolysis? (Jun-03) 2) What is the difference between AC and DC motors?( Mar-00) 3) State and explain Lenz law? (Jun-05,02, Mar-03,02) 4) What are the factors on which the RPM of a motor depends? 5) Mention the applications of electrolysis? 1) What is the Fleming s right hand rule? (June 01) 2) What is a transformer? On what principle does it work? 3) What is an electric motor? On what principle does it work? 4) Define Electro Chemical Equivalence? 5) What is Self inductance? 6) What is mutual induction? 7) What is electromagnetic induction? 8) What is meant by Step up transformer? (Mar-02, 00, Jun-00)

9) What is meant by Step down transformer? 10) What is the use of Iron core in the transformer?