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    Laura_hottie's Avatar
    Laura_hottie Posts: 1, Reputation: 1
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    #1

    Oct 13, 2009, 06:04 PM
    Electricity
    1)A 88-V supply is connected to a resistor causing 38 mA of current to flow. What power is being dissipated in the resistor?

    2)What is the current through a device that is dissipating 88 W of heat when a 35-V source is applied across it?


    3a) Calculate the maximum current a 510 W, 1/2 watt resistor can have flowing through it safely. ANSWER IN mA)

    3b) What is the maximum e.m.f. which may be applied to the resistor above without causing overheating? ANSWER IN (VOLTS)
    Perito's Avatar
    Perito Posts: 3,139, Reputation: 150
    Ultra Member
    		  
    #2

    Oct 13, 2009, 07:40 PM
    1) A 88-V supply is connected to a resistor causing 38 mA of current to flow. What power is being dissipated in the resistor?

    2)What is the current through a device that is dissipating 88 W of heat when a 35-V source is applied across it?


    3a) Calculate the maximum current a 510 W, 1/2 watt resistor can have flowing through it safely. ANSWER IN mA)

    3b) What is the maximum e.m.f. which may be applied to the resistor above without causing overheating? ANSWER IN (VOLTS)
    1. Power in resistive DC circuits is given by P=VA where V is the voltage and A is the current. Using that and Ohm's law, power can be given by either of these formulas:

    (Formula 1)

    (Formula 2)

    You want formula 2. If you want power in watts, voltage must be in volts, current must be in amperes, and resistance must be in ohms.

    2. You have a 1/2 watt resistor. I'm not sure what you mean by a 510 W, 1/2 watt resistor. I'll presume this a 510 Ω (Ω=ohm) resistor. Then, plug the resistance into formula 1. P will be 1/2 watt, so solve for I, the current.

    3. EMF is a synonym for voltage. You have resistance and power. Plug into formula 2 and solve for the voltage.
    KISS's Avatar
    KISS Posts: 12,510, Reputation: 839
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    #3

    Oct 13, 2009, 07:47 PM

    Ohms law is valid for P(Watts), V(Volts) and I (Amperes)

    A mA (milliamp) = how much of an amp?
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