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Understanding Resistivity and Electric Power in Circuits

Learn about resistivity, power generation, AC current, and circuit safety measures. Explore how resistivity affects wire resistance, power delivery, and preventing overheating. Discover AC power characteristics and the importance of RMS values in electrical circuits.

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Understanding Resistivity and Electric Power in Circuits

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Presentation Transcript

  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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