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Line 51: − − Difference between A.C. and D.C − Direct current (D.C.) Alternating current (A.C.) − 1. It is the current of constant magnitude. − 2. It flows in one direction in the circuit. − 3. It is obtained from a cell (or battery) 1. It is the current of magnitude varying with time. − 2. It reverses its direction periodically while flowing in a circuit. − 3. It is obtained from A.C. generator and mains. − − Advantages of A.C. over D.C.: − In India, we use 220 volt a.c. in our houses and factories. The use of A.C. is advantageous over D.C. because the voltage of A.C. can be stepped up by the use of a step-up transformer at the power generating station before transmitting it over the long distances. It reduces the loss of electric energy as heat in the transmission line wires. The A.C. is then stepped down to 220 volt by the use of step-down transformers at the successive sub-stations before supplying it to the houses or factories. If D.C. is generated at the power generating station, its voltage cannot be increased for transmission and so due to passage of high current in the transmission line wires; there will be a huge loses of electrical energy as heat in the line wires. − − A.C. Generator D.C. Motor − 1. A generator is a device which converts mechanical energy into electrical energy. − 2. A generator works on the principle of electrro magnetic inductions. − 3. In a generator, the coil is rotated in a magnetic field so as to produce electric current. − 4. A generator makes use of two separate coaxial slip rings. 1. A d.c. motor is a device which converts electrical energy into mechanical energy. − A d.c. motor works on the principle of force acting on a current carrying conductor placed in a magnetic field. − 3. In d.c. motor, the current from d.c. source flows in the coil placed in a magnetic field due to which the coil rotates. − 4. A d.c. motor makes use of two parts of slip ring (i.e., split rings) which act as a commutatar. − − Applications of electromagnetic induction: − Transformer: − In our daily life, we use various electrical applications which require working voltage different form the mains voltage (i.e., 220V) e.g. a door bell needs 6V while a television needs several 1000V. To provide suitable voltage to different applications from the mains, we use transformers with them. Thus, Transformer is a device by which the amplitude of an alternating e.m.f. can be increased or decreased. − A transformer does not affect the frequency of the alternating voltage. The frequency remains unchanged (=50hz) − Principal : A transformer works on the principle of electromagnetic induction and make use of two coils having different number of turns. The alternating e.m.f. to be altered is applied across one coil. Which there is change of magnetic field line due to varying current in this coil the magnetic field line linked with the other coil also changes and so an induced varying current of the same frequency, but of different magnitude flows in the other coil. − Construction : A transformer consists of a rectangular soft iron core made up from the thin laminated sheets of soft iron of T and U shape, placed alternately one above the other and insulated from each other by a paint (or varnish) coating over them as shown in the fig. so as to behave like a simple rectangular core. − The laminated core prevents the loss of energy due to eddy currents in the core. − On one arm of the core, a coil P of insulated copper wire is wound. This coil is connected with the source of alternating e.m.f. (i.e., at the ends of the this coil, the input is given). This is called primary coil. − One the other arm of the core, another coil S of insulated copper wire is wound. The induced alternating e.m.f is obtained across the terminals of this coil (i.e., output is obtained at the ends of this coil). This is called the secondary coil. The ratio of number of turns Ns in secondary coil to the number of turns Np in primary coil (i.e., Ns/Sp) is called the turns ratio. − i.e.,Turns ratio n= (Number of turns in secondary coil Ns)/(Number of turns in primary coil Np) − − The advantage of using a closed core is that it gives a closed path for the magnetic field lines and therefore almost all the magnetic field lines caused due to current in the primary coil, remain linked with the secondary coil (i.e., the flux linkage is nearly perfect) and loss of energy is avoided. The core is made of soft iron so that hysteresis loss of energy in the core is less. − − Working : When the terminals of primary coil are connected to the source of alternating e.m.f., a varying current flows through the primary coil. This varying current produces a varying magnetic field in the core of transformer. Thus the magnetic field lines linked with the secondary coil vary. The change of magnetic field line through the secondary coil induces an emf in it. The induced emf varies in the same manner as the applied emf in the primary coil varies and thus has the same frequency as that of the applied emf. − The magnitude of emf induced in the secondary coil depends on the following two factors. − (i) the ration of the number of turns in the secondary coil to the number of turns in the primary coil (i.e., turns ratio) and − (ii) the magnitude of emf applied in the primary coil − For a transformer, − (emf across the secondary coil (Vs))/(emf across the primary coil (Vp))= (Number of turns in the secondary coil (Ns))/(Number of turns in the primary coil (Np)) − − ((Vs))/( (Vp) )=((Ns))/((Np) )= is called Turns ratio 'n^' of transformer − − The relation between current and voltage of primary and secondary coils of a transformer can be expressed as − ((Vs))/( (Vp))= Ip/Is or IpVp=IsVs − − Current x Voltage of primary coil = current x voltage in secondary coil − Types of transformers : 1) Step up transformer : The transformer used to change a low voltage alternating emf to a high voltage alternating emf (of same frequency) is called step-up transformer. In a step-up transformer, the number of turns in the secondary coil are more than the number of tunrs in the primary coil. i.e., turns ratio n>1 or Ns/Np>1. Vs>Vp, but Is<Ip. More current flows in the primary coil. Therefore one must use thicker wire in the primary coil as compared to that in the secondary coil. − 2) Step down transformer : The transformer used to change a high voltage alternating emf to a low voltage alternating emf (of same frequency) is called step-down transformer. In a step-down transformer, the numbers of turns in the secondary coil are less than the number of tunrs in the primary coil. i.e., turns ratio n<1 or Ns/Np<1. Vs<Vp, but Is>Ip. Here the secondary winding will require thicker wire due to high current. The use of thicker wire reduces its resistance and therefore reduces the loss of energy as heat in the coil. This energy loss due to heat is known as copper loss. − Uses of Transformaers : i) Use of setp-up transforemer : − a) In transmission of electric power at the power generating station to step up the voltage.b) with television c) with wireless sets and − d) with X-ray tubes to provide a high accelerating voltage. − ii) Use of setp-down transforemer : − a) With electric bells, night electric bulbs, mobile phone, computers, etc. − b) At the power sub-stations to step-down the voltage before its distribution to the consumers. − Induction coil : − You know that for discharge tube experiment high voltage is required. How to produce high voltage from a low voltage DC source? An induction coil is a device for obtaining a very high DC voltage starting from a low DC voltage. An induction coil consists of few turns of a primary coil ‘P’, of thick insulated copper wire wound over a soft iron core connected to a battery. S is a secondary coil of large number of turns wound over the primary coil. M is a make and break arrangement. When the DC current flows in the primary, ‘C’ gets magnetised. The iron head H of the switch M is attracted towards C so, the circuit breaks and the current in primary stops. C gets demagnetised and H comes back. The process repeats. During make and break circuit, large emf is induced in the secondary. The voltage depends on the turns ratio. Since the number of turns in secondary is very large, a large voltage in produced in the secondary. − While using it remembers that the coil should be in the north and south direction. That means the needle and the coil should be parallel to each other.
Electromagnetic induction (view source)
Revision as of 16:08, 24 August 2014
, 16:08, 24 August 2014→Notes for teachers
ii) the time in which the magnetic flux changes.
ii) the time in which the magnetic flux changes.
''These are short notes that the teacher wants to share about the concept, any locally relevant information, specific instructions on what kind of methodology used and common misconceptions/mistakes.''
''These are short notes that the teacher wants to share about the concept, any locally relevant information, specific instructions on what kind of methodology used and common misconceptions/mistakes.''