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 Resistance R, of a wire is proportional to its length, and inversely proportional to its cross- sectional area:
Resistivity Resistivity is a material property The reciprocal of resistivity is conductivity Resistivity is temperature dependent
Electric Power Electric Power delivered to the load is: or SI: Watts (W) 1W = 1 Volt-Amp
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:
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.
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.
Circuit Breakers Circuit breakers (CB)use a bi-metallic strip to open the circuit
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.
Alternating Current DC Current is constant with time. I 0 AC Current varies periodically with time
AC Power Since the current is squared, power is always positive
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.
Average Power