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Unit 3 Day 2: Resistivity, Power & AC Current

Unit 3 Day 2: Resistivity, Power & AC Current. Resistivity and its Temperature Dependence Electric Power Heat Generation in Electric Circuits AC Current Average power. Resistivity.

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Unit 3 Day 2: Resistivity, Power & AC Current

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  1. Unit 3 Day 2: Resistivity, Power & AC Current • Resistivity and its Temperature Dependence • Electric Power • Heat Generation in Electric Circuits • AC Current • Average power

  2. Resistivity • Resistance R, of a wire is proportional to its length, and inversely proportional to its cross-sectional area:

  3. Resistivity • Resistivity is a material property • The reciprocal of resistivity is conductivity • Resistivity is temperature dependent

  4. Electric Power • Electric Power delivered to the load is: or • SI: Watts (W) 1W = 1 Volt-Amp

  5. Electric Power • Energy purchased from the power company to use to power our home is: • We purchase energy in large quantities, so it is more convenient to use units of:

  6. Electric Power • When power is delivered to a load, the current thru the wires generate heat (thermal energy) • The power is dissipated in the load as heat because of the resistance of the load • Too much current in a wire or a load will cause it to overheat and may cause fire.

  7. Preventing Circuit Overheating • Lower resistance in the wires by using low resistivity material (ie: Cu or Al), use large diameter wires, and keeping wire lengths as short as possible. • Use current protection devices such as fuses or circuit breakers.

  8. Circuit Breakers • Circuit breakers (CB)use a bi-metallic strip to open the circuit

  9. Household Circuit Management • Connection of too many household appliances to one circuit will cause a CB to trip • If this circuit is protected with a 20A CB, it will trip to prevent overheating of the wires and prevent fire • Extension cords can be a dangerous fire hazard, if they are rated at 10A or less.

  10. Alternating Current • DC Current is constant with time. I0 • AC Current varies periodically with time

  11. AC Power • Since the current is squared, power is always positive

  12. RMS Values • It is difficult to measure peak or average AC Voltage & Current. Therefore a convenient unit of measure was developed called the rms values. • rms is an abbreviation for root-mean-square value • To find the rms value, take the square root of mean value of the squared term.

  13. Average Power

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