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CHAPTER-26

CHAPTER-26. Current and Resistance. Ch 26-2 Electric Current. Electric Current: Motion of conduction electrons under the effect of an E field in the conductor Fig (a) loop of wire in electrostatic equilibrium, E=0 no current

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CHAPTER-26

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  1. CHAPTER-26 Current and Resistance

  2. Ch 26-2 Electric Current • Electric Current: Motion of conduction electrons under the effect of an E field in the conductor • Fig (a) loop of wire in electrostatic equilibrium, E=0 no current • Fig (b) loop connected to a battery- E  0 in the loop electrons move in the direction opposite to current i

  3. Ch 26-2 Electric Current

  4. Ch 26-2 Electric Current • Current is scalar quantity • Junction point: • a point where a current split into two or more currents or two or more currents merge into one current • Current entering the junction is equal to current leaving the junction : i0=i1+i2 • Current Density J: a vector quantity; magnitude of J given by current i per unit cross section area A of a conductor then: • J=i/A ; i= J.dA= JdA cos i = JdA= JdA= JA • Direction of J : parallel to i

  5. Ch 26-3 Current Density • Random speed: Speed of electrons in random motion in the absence of an E-field electrons with zero net motion random speeds  106 m/s • Drift Speed Vd : in the presence of an E-field electrons move randomly with net motion in the direction opposite to E field • Drift speed Vd  10-5 - 10-4 m/s J=(ne)vd • where ne is carrier charge density: +ve for positive carrier and -ve of electrons

  6. Ch 26-4 Resistance and Resistivity • Resistance R : ratio of applied voltage V across a conductor to the current resulting through the conductor R= V/i • Unit of resistance Ohm (): 1  = 1V/1A; i=V/R if we consider electric field E in a conductor then we deal with J and resistivity  instead of i and R respectively : J= E / ; = E/J; E= J • Calculating Resistance from Resistivity = E/J=(V/l)/(i/A)=(V/i)(A/l);  = R A/l; R= A/l • Variation of resistance with Temperature: • =T-0= 0  (T-T0), where  is temperature coefficient of resistivity.

  7. Ch 26-5 Ohm’s Law • Ohm’s law- An assertion: Current through a device directly proportional to the potential difference across the device

  8. Ch 26-5 Power in Electric Circuit • A resistor connected across points a and b in the circuit. Battery maintains a potential difference of V between its terminal and a current i in the circuit. • The amount of charge dq moved through a and b in time dt is dq= idt • Since charge moves from +ve to –ve terminal, its potential energy U decreases by U=dqV=i dtV. • Power P associated with this energy dissipation is • P=dU/dt =iV=i2R=V2/R

  9. Thank you

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