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1 CURRENT ELECTRICITY CHAPTER 13 CURRENT ELECTRICITY Qs. Define Charge and Current. CHARGE Definition Flow of electron is known as Charge. It is denoted by Q. Unit Its unit is Coulomb. 1 Coulomb = 10(-6) μcoulomb 1 coulomb = 10 (-3) mili coulomb 1 coulomb = 10(-9) neno coulomb CURRENT Definition The flow of charge per unit time is known as Current. It is denoted by I.

2 Unit The unit of current is coulomb/sec or Ampere. AMPERE If one coulomb charge passes through the conductor in 1 second then the current is 1 Ampere. Mathematical Form Mathematically, I = Q/t Qs. State and Explain Ohm s Law. OHM S LAW Introduction A German scientist George Simon Ohm studied the relationship between voltage, current and resistance. On the basis of his experimental results, he proposed a law which is known as Ohm s Law. Statement Ohm s Law to metallic conductors can be stated as The current through a conductor is directly proportional to the potential difference

3 between the ends of the conductor provided that physical conditions are kept constant. It can also be stated as The ratio between voltage and current remains constant, if the physical conditions are kept constant. Mathematical Form Mathematically, V I V = IR R = V/I Where R is the constant of proportionality known as resistance of the conductor. Its unit is volt per ampere (Volt/Ampere) or Ohm (Ω). Ohm (Ω) If 1 ampere current passes through the conductor due to 1 volt potential difference then the resistance of conductor is 1 Ohm. Resistance Opposition offered in the flow of current. Graphical Representation. When graph is plotted between current and potential differences then straight line is obtained.

4 Limitations of the Law Ohm s Law is valid only for metallic resistance at a given temperature and for steady currents. Qs. Define the term Resistivity or Coefficient of Resistor. RESISTIVITY OR COEFFICIENT OF RESISTOR Definition It is the resistance of a unit conductor whose cross-sectional area is 1 sqm. Unit Its unit is Ohm meter Mathematical Form The resistance of any conductor depends upon the following factors. 1. Length of the conductor 2. Cross-sectional area of the conductor. 3. Material of the conductor. The resistance of the conductor is directly proportional to the length of the conductor and inversely proportional to the crosssectional area. Mathematically,

5 R L (I) R α 1/A (II) Combining eq (I) and (II) R α L/A => R = ρl/a Where ρ is the constant of proportionality known as Resistivity or Coefficient of resistance. ρ = RA/L Qs. Explain the effect of temperature on resistance or temperature coefficient of resistance. EFFECT OF TEMPERATURE ON RESISTANCE It is observed that if we increase the temperature then resistance of a conductor will increase. Consideration Let Ro be the initial resistance of a conductor at 4 C. If we increase the temperature from t1 C to t2 C, then resistance will increase. This increment in resistance is denoted by ΔR. The increment in resistance depends upon the following two factors.

6 1. Original Resistance (Ro) 2. Difference in temperature Δt. Mathematical Verification The increment in resistance is directly proportional to the original resistance and temperature difference. Mathematically, ΔR Ro (I) ΔR Δt (II) Combining eq (I) and eq (II) we get ΔR RoΔt => ΔR = αroδt Where α is the temperature coefficient of resistance. It is defined as It is the increment in resistance per unit resistance per degree rise in temperature. Its unit is 1/ C or C. If RT is the total resistance, then RT = Ro + ΔR => RT = Ro + αro Δt => RT = Ro (1 + αδt) As we know that resistance is directly proportional to resistivity therefore, ρt = ρo (1 + αδt) Qs. Define the term Power Decipation in Resistor.

7 POWER DECIPATION IN RESISTORS Definition When current flows in a conductor then a part of electrical energy appears in the form of heat energy which is known as Power Decipation in Resistor. Units Its unit is Joule per second (J/s). Most commonly used unit is Kwh. 1 Kwh = 36 x 10(5) Joules Mathematical Form Since, P = Electrical Work / Time Electrical Work = QV (I) This electrical work produces heat energy in the resistor. P = QV / t P = Q / t. V But, I = Q / t P = VI From Ohm s Law V = IR P = IIR P = I2R

8 OR, P = 12R2 / R => P = V2 / R As we know that, Energy = Power x time => E = P x t => E = Vit => E = I2Rt And, E = V2 / R. t Qs. Define and explain Electromotive Force. ELECTROMOTIVE FORCE Definition It is the terminal voltage difference when no current draws from a cell or a battery. OR Work done per coulomb on the charges. It is denoted by E. Unit Electromotive force or simply e.m.f is a scalar

9 quantity it has the same dimension as that of voltage, therefore its unit is volt. Explanation When an electric current passes through a resistor, it dissipates energy, which is transformed into heat energy. Thus to sustain a current in conductor some source of energy is needed so that it could continuously supply power equal to that which is dissipated as heat in the resistor. The strength of this source is called Electromotive Force. Consideration Let consider a simple circuit in which a resistor R is connected by leads of negligible resistance to the terminals of a battery. The battery is made up of some electrolyte and electrode for the production of e.m.f and hence when this current flows from battery, it encounters some resistance by the electrolyte present in two electrodes. This resistance is known as internal resistance r of the battery. Mathematical Form According to Ohm s Law V = IR

10 I = V / R Or, I = E / R + r Where E is e.m.f and r is internal resistance => E = IR + Ir E = V + Ir