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Presentation On Electricity

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Published in: Physics
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This presentation explains about the physics topics, which cover Higher Secondary students of various syllabus like CBSE, Cambridge, Edexcel and IB students. The topics that are explained are electric charges, electric current, AC and DC analysis.

Durai M / Dubai

12 years of teaching experience

Qualification: Masters of Engineering

Teaches: Mathematics, Project Management, Statistics, Electrical Technology, Chemistry, Physics, Maths

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  1. Electricity
  2. • Electrolytes, electric current in gases, ionization, Magnetic field, magnetic induction, magnetic field of a coil, magnetic induction current, • Electromagnetic induction, Faraday law of electromagnetic induction, Lenz law, electromagnetic oscillator, Thompson formula, oscillator resonance. Alternative current, circuits of alternative current with the resistance induction and capacity, rectifiers, amplifiers, power of alternative current, electromotors, transformers. • Mechanical wave motion, interference of the wave motion, Huygens principle, sound and its characteristics, sound intensity, sound velocity, ultrasound, infrasound.Electromagentic wave motion, propagation of electromagnetic wave motion, types of electromagnetic wave motion, electroacoustic transducers. • Light, light refracture, light reflection, refracture index, total reflection, optical systems, lenses, mirrors, microscopes, telescopes wave properties of the light, dispersion, spectral colours, light interference, light diffraction, light polarisation, polarimeter, types of electromagnetic radiation, basic radiometric and photometric quantities.
  3. Electric Currents in Electrolytes • The behavior of electric charges in electrolytes is studied in physical chemistry and electrochemistry. • Here in physics we will review only the Faraday's law of Electrolysis: Definition: The mass m of the substance liberated from an electrolyte solution is proportional to the product Of the current I and the time t of current passage through the electrolyte. where A is the electrochemical equivalent. After rewriting the previous formula, We obtain: it is also evident that m = Nmo N • [kgC'] and Q — NQo Nze where N is the number Of ions transferred between the electrolyte and the electrode, m, is the mass of the indi- vidual ions transferred [kg). Mm is molar mass. i.e. the mass Of one mole Of substance (kg] which is present in ion form. NA is the Avogadrc* constant — the Of particles in one mole of any substance, Qo is the electric charge Of one ion [Cl. e is the elementary charge — I .602x 1009 C, and z is the valence Of the ion (ionized atom or molecule). after substitution A and m —m_ Q zeNA where F = is Faraday's constant 500 C — charge Of I mole Of elementary charges).
  4. Thermoelectric Phenomena • In a junction of two metals, a small electric voltage arises due to different metals. • Similarly a small voltage can be measured across a conductor whose ends are at different temperatures. This is called as Thermoelectric Phenomena • Two junction of metals, which are of different temperature, connected in series, forms a thermocouple. Uses: Thermocouple batteries are used to produce electrical energy and in temperature measurement. Thermoemission is liberation of electrons from very hot metal sufaces. It is used in cathode ray tubes (CRT), X-ray tubes etc,
  5. Magnetism and Electromagnetism • Magnetic Field: • A magnetic field is a vector field that describes the magnetic influence of electric charges in relative motion and magnetized materials. • The effects of magnetic fields are commonly seen in permanent magnets, which pull on magnetic materials (such as iron) and attract or repel other magnets. • Magnetic fields surround and are created by magnetized material and by moving electric charges (electric currents) such as those used in electromagnets. • They exert forces on nearby moving electrical charges and torques on nearby magnets. • In addition, a magnetic field that varies with location exerts a force on magnetic materials. • They exert forces on nearby moving electrical charges and torques on nearby magnets. In addition, a magnetic field that varies with location exerts a force on magnetic materials. • Both the strength and direction of a magnetic field vary with location. As such, it is described mathematically as a vector field.
  6. Forces Acting between two magnetic poles: Force acting between two magnetic poles: Permanent magnets (e.g. magnetised iron rods) consist of north seeking pole = north pole (N) and the south seeking pole = south pole (S). There is a well known rule describing the force acting between the two poles: "Like poles repel - unlike poles attract." "lhe magnitude of the attraction or repulsion force can be calculated according to the formula 44M 2 where mt and ma is the magnetic "strength" of the poles, g is the magnetic permeability [Hm-l — henry per meter]. We can write: where is the relative permeability of the medium [dimensionless], go is the permeability of vacuum (47tx1C7 H.m•l), and d is the distance between the poles [ml. The magnetic field is described by magnetic force lines. Outside the magnet, the force lines are directed from N-pole to S-pole.
  7. Magnetic Flux and Flux density The magnetic flux passing through the area A is defined as the total number ofmagnetic force lines passing through an areaA. The magnetic flux density Bata point isthemagneticfluxinaunitareaplacedatright.anglestothemagnetic force lines, where magnetic flux [Wb - weber]. The magnetic flux density is sometimes magnetic induction or simply B-vectot
  8. Magnetizing Force (Magnetizing strength, intensity of magnetic field) Magnetising force H is regarded as the cause of the total magnetic flux density B in the medium of permeability g. We can write: B (T (tes1a)J where g = is magnetic permeability, H is magnetising force [Am-I — ampere per meter]. According to the value Of relative permeability, different materials can be divided into three groups: diamagnetics: gr S 1 — B is slightly lowered, if compared with the vacuum paramagnetics•. g r 2 1 — B is slightly increased ferromagnetics: g, » 1 — B is many times increased In ferromagnetic materials, the atoms can be arranged into so-called magnetic domains. in which the vectors Of magnetic flux density are spontaneously oriented in the same direction. During magnetisation and demag- netisation processes, these domains behave as small individual magnets. Therefore. When oriented in the same direction during magnetisation, the magnetic fields of the domains add up — we can obtain a strong permanent magnet from originally non-magnetic material (typically steel). The Curie temperature is the temperature at which the magnetisation of a material is removed by thermal movement (magnetic domains lose their order) — 770 oc for iron.
  9. Magnetic Force Exerted on a current carrying conductor If a straight Wire carrying current I is positioned in a homogeneous magnetic field With magnetic flux density B making an angle with the vector B (see Fig. 16b), the force acting on it can be calculated according to the formula: F = Blisina [NJ, where is the length of the wire (conductor) exposed to the magnetic field. With respect to the last formula. a magnetic flux density 1 T exists if the force exerted on a straight wire of 1 m length is 1 N when the wire carries a current of 1 A and is placed at right-angles to the direction of the magnetic How to deterrnine the direction Of the force exerted? We can use a simple rule: A lign one of the externally applied magnetic lines of force to be parallel with and touching one Of the magnetic lines of force produced by the current. The direction from this common point to the conductor gives the direction of force on the conductor. See also Fig. 16(c). Fig. Vector by a — if the the page. B (b) A conductor a field. A to field — Of the
  10. Magnetic Force between two parallel conductors The force per unit length on wire B (carrying current 1B) due to the magnetic field produced by the current IA in the wire A is given by: 2nd where d is the perpendicular distance between the wires. Ifthe currents have the same direction the wires are attracted. In case they have opposite direction they are repelled. Definition of I ampere in SI: A current of I A flows in one infinite staight wire ifan equal current in a similar bire placed in parallel I meter away in a vacuum produces a mutual force ofg/21t N per I meter oflength (i.e. 2xl(P N.rnl, since go = 410<107 N.m•l).
  11. Electro magnetic Induction Electromagnetic induction is the production Of an electric voltage (and hence a current) in conductors due to a changing magnetic field. It is described by the Faradays laws: I.A change of the magnetic flux linked with a conductor induces an electromotive force (voltage) in the conductor, 2. The magnitude Ofthc induced electromotive force is proportional to the rate Ofchange ofmagneticflux linkage The "linkage" is the number of force lines intersected by the moving conductor or the product of the magnetic flux (b and number of turns in a solenoid, etc. For the induced voltage U we have: At where AO.N is the change of magnetic flux linkage, At the time of change, and N the number of turns. Lenz's law: The direction of the induced current in a conductor caused by a changing magneticflux is such that its own magneticfield opposes the change in magnetic flux.
  12. Voltage induced in a solenoid It is given by the formula: Al At where L is the self-inductance of the solenoid and [H — henry] 1 In the last formula, Nisthenumberofsolenoidtums,listhelengthofthesolenoid area ofthesolenoid [mil, and gisthepermeabllityofthemediuminsidethesolenoid,
  13. Alternating Currents: Alternating current (AC) is used for energiSing various electrical devices. It is also efficient to use for long- distance delivery Of electrical energy. •1b describe alternating Currents or voltages, we can use sirnilar quantities as u sed in the description Of sirnple harmonic motion: The cycle is one cornplete waveform. period T Is] is the time duration of one cycle. Frequency f KHz = ] is the number Of cycles per second. Amplitude is the maxixnum value of positive or negative current (or voltage). Instantaneous values Of alternat- ing current and voltage are: = rpsin ( •p) [Al and where It and U, are the instantaneous values of current or voltage, I, and U are the arnplitudes of current or volt- age (their peak values). (phi) is the phase angle which defines the values Of I or U in tune t O. Note; Peak-to- peak values peak values multiplied by 2. Average value Of alternating current or voltage is zero, because their variation With tirne is sinusoidal. Effective (root rnean square — RMS) current and voltage: •Ihe effective voltage (current) is that value of constant direct voltage (current) which would produce the same expenditure Of electrical energy in the circuit as the alternating voltage (current). The following equations where Ip and Up are the arnplitudes Of current or voltage (their peak values).
  14. Electrical Power, Resistance and Impedence: Electrical power of alternating current: p = Udef = RI} = [W = VA = watt volt-ampere) The previously mentioned Ohn-fi law is valid for AC only when using effective values of voltage and current. Reactance and impedance: Reactance can be described as "resistance to the flow of electric charge in AC circuits". Impedance is the total reactance. In general, the reactance and impedance Z depend on the frequency of the alternating current Reactance of a capacitor in an AC circuit: [Q], where Xc is the reactance ofa capacitor of capacity C [F], and fis the frequency [Hz) ofthe alternating current. The higher the frequency of an alternating current, the smaller is the reactance of a capacitor.
  15. AC Transformers This device is used for transformation of electric alternating voltages from high to low values and vice versa. A transformer consists of two solenoids with common core, i.e. primary (p, input) winding, the secondary (s, output) and the soft iron core - see Fig. 20. windhg "p" OUTPUT whding "s" Fib 20. The AC transformer. The iron core is drawn in grey colour. During transformation, a part of the electric energy is lost, mainly by conversion into heat. Therefore, ideal (eficiency = 100%) and practical transformers can be distinguished. The following formulas are used for some practical calculations on transformers:
  16. and where U is the voltage across the winding I is the current through the winding and n is the number of turns in the winding. Thus, in the transformer shown in the Fig, 20, the voltage is reduced to one half and cur- rent increased (n = 4, n, 2). The transformer effciencyis $ven by. .100 There are "copper" and "iron" energy losses in the transformer. he first ones are due to Joule's heat in copper winding wires, the second are due to the magnetization and demagnetization processes in the iron core.
  17. Measuring instruments: The construction of instruments for measuring electric currents, voltages and other electric quantities are bee yond this textbooks scope. However, it is necessary to know that Voltmeters are characterised by very high infrinsic resistance. They must be connected in parül to a "resis- tor" of which the voltage difference is to be measured, The measuring range ofa voltmeter can be enluged by "series resistors". Ampermeters (ammeters) are characterised by very low iæsic resistance. They must be connected in se- lies into a circuit The measuring range of an ammeter can be enlarged by "shunts" - small resistors added in parallel
  18. What is the kinetic energy of an electron moving in circular trajectory with radius r = 10 cm between the two poles ofan electromagnet which produces a homogeneous magneticßux density of 1 "IT. The plane of the circular motion is perpendicular to the magnetic force lines. Ike mass of the electron is 9.11x1@3' kg. Solution: The kinetic energy of a moving body is given by the formula: Ek = —mv2 2 We have to calculate the electron velocity at first. The magnetic force of the magnetic field exerted on the electron must be equal to the centripetal force (the electron is on a circular path): F = Bevsin@ = Bev = m — (sine = 1) After rewriting: Ber thus: Ek = —mv2 2 Result: After substitution and calculation we obtain 1.408x1ff'6 J. (This is only an approximate value. For correct calculation, relativistic correction of electron mass would be necessary since the electron moves with velocity Of km.s•'.)
  19. Problems to solve: 18 An "infinite" very thin wire isplaced in vacuum, and carries an electric current of 10 A. Vilat is the distance at which the value ofthe density Bis equal to IT? 19, What is the reactance ofa capacitor Ofcapacity 1 PIF in a circuit ofalternating electric current offrequency I kHz? 20, What is the voltage induced in a single wire loop (one tum ofa solenoid) ofdiameter 10cm after switching on a homogeneous magneticfield (B: 0.1 T) in I ms, Assume that the B value increases linearly during this tima The wire loop is placed in vacuum, 21, What is the peak-to-peak value ofthe commonly used mains voltage (Uc- 230 V).

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