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# L4 resistance

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### L4 resistance

1. 1. Resistive Circuits
2. 2. ELE 101 / 102 Dept of E & E Engg, MIT Manipal 2 Resistors Resistance Property which opposes the flow of current R = V/I Unit – Ohms (Ω) Power P = VI = V2 /R = I2 R Conductance Reciprocal of resistance Symbol: G Unit – Siemens (S) R V I
3. 3. ELE 101 / 102 Dept of E & E Engg, MIT Manipal 3 Problem - 1
4. 4. ELE 101 / 102 Dept of E & E Engg, MIT Manipal 4 Problem - 2
5. 5. ELE 101 / 102 Dept of E & E Engg, MIT Manipal 5 Resistivity & Temperature Coefficient ( ) ( ) ( )20 10 2 1 0 1001 1 1 Where 1 θα θα α θα + + = += R R RR For a homogeneous material Where = resistivity a = cross sectional area of the Resistor l = length of the Resistor ρ = Temperature coefficient of R at 0o a l R ρ=
6. 6. 0 R R1 R0 R2 θ1 θ2 θ Variation of Resistance of Copper with Temperature ELE 101 / 102 Dept of E & E Engg, MIT Manipal 6 Resistivity & Temperature Coefficient Super Conductivity ! ( )1001 1 θα+= RR
7. 7. ELE 101 / 102 Dept of E & E Engg, MIT Manipal 7 Temperature Coefficients of Resistivity Material Temperat ure Coefficien t °C-1 @ 0° C Aluminum 0.00381 Copper 0.00428 Gold 0.0034 Tungsten 0.0045 Brass 0.001 Manganin 0.00002 Carbon (graphite) -0.00048 Germanium (pure) -0.048 Silicon (pure) -0.075 Material at 0° C Aluminium 2.7 * 10 -8 Copper 1.59 * 10 -8 Tungsten 5.35 *10 -8 Carbon 6500 * 10 -8 Manganin 42 * 10 -8 ρ Resistivity: Resistivity & Temperature Coefficient
8. 8. ELE 101 / 102 Dept of E & E Engg, MIT Manipal 8 Problem- 3 When a potential difference of 10 V is applied to a coil of copper wire of mean temperature 20° C, a current of 1.0 A flows in the coil. After some time the current falls to 0.95 A yet the supply voltage remains unaltered. Determine the mean temperature of the coil given that the temperature coefficient of resistance of copper is 4.28 * 10-3 /°C referred to 0° C. [33.5 o C]
9. 9. ELE 101 / 102 Dept of E & E Engg, MIT Manipal 9 Resistive circuits Resistors in series: Resistors in parallel: R1R2 R3 321 1111 RRRReq ++=
10. 10. ELE 101 / 102 Dept of E & E Engg, MIT Manipal 10 Parallel Resistive circuits Current Division in Parallel Circuits ( ) 1 2 1 21 1 1 1 2 2 2 1 1 2 1 2 1 2 I I I R RR I I I R R R I I R R R I I R R = + + = + = = + = +