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Review. Ohm’s Law: current in a resistive circuit Directly proportional to its applied voltage Inversely proportional to its resistance Also expressed as E = IR and R = E / I Express all quantities in base units of volts, ohms, and amps or utilize the relationship between prefixes.

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Review

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  1. Review Ohm’s Law: current in a resistive circuit Directly proportional to its applied voltage Inversely proportional to its resistance Also expressed as E = IR and R = E/I Express all quantities in base units of volts, ohms, and amps or utilize the relationship between prefixes

  2. Review Open circuit: Current can only exist where there is a conductive path Open circuit: when there is no conductive path so I = 0 Ohm’s Law gives R = E/I = E/0  infinity An open circuit has infinite resistance

  3. Lesson 3: Power, and Energy

  4. Learning Objectives • Describe the relationship between battery capacity, current drain and battery’s useful life. • Calculate the total cost given a rate of energy consumption. • Calculate power supplied/dissipated in a circuit. • Calculate the power efficiency of a circuit.

  5. POWER • In general, the term power is applied to provide an indication of how much work (energy conversion) can be accomplished in a specified amount of time; that is, power is a rate of doing work.

  6. Power Power is defined as the rate of doing work or as the rate of energy transfer. The SI unit of power is the watt (W) or joules per second. The English unit of power is horsepower (hp).

  7. Power in electrical systems We need to express power in terms of voltage and current, recall that Combining them we have

  8. Power in electrical systems Applying Ohm’s law (V = IR and I =V/R) we can also express power as

  9. Power Calculate the power to the heater using all three electrical formulas

  10. Power Power is defined as the rate of doing work or as the rate of energy transfer. The greater the power rating of a light, the more light energy it can produce each second The greater the power rating of a heater, the more heat energy it can produce The greater the power rating of a motor, the more mechanical work it can do per second Power is related to energy. Is the capacity to do work

  11. Example Problem 1 A resistor draws 3 amps from a 12V battery. How much power does the battery deliver to the resistor? I = 3 A E=12V

  12. ENERGY • For power, which is the rate of doing work, to produce an energy conversion of any form, it must be used over a period of time. • The energy (W) lost or gained by any system is therefore determined by:

  13. Energy We can rearrange our formula for power to solve for energy The unit of energy is joules (J), but is also expressed as watt-hours (Wh) or kilowatt-hours (kWh). Cost = Power × time × cost per unit The residential energy cost from BGE is 14.8 cents per kWh.

  14. Energy Cost = Energy × cost per unit or Cost = Power × time × cost per unit

  15. Example Problem 2 Suppose you are at home and use 3 100-W lamps for 3 hours and An Xbox 500W for 2,5 hours. The TV consumes 180 W. At $0.148 per kilowatt-hour, how much will this cost you?

  16. Efficiency In the process of converting energy, energy losses inevitably occur. The measure of output energy (or power) to input energy (or power) is called efficiency. Thermal energy out

  17. Efficiency Poor efficiency in energy transfers results in wasted energy An inefficient piece of equipment generates more heat. As heat must be removed to guarantee a proper function, it means more $$.

  18. Efficiency Efficiency is usually expressed in percent and denoted by the symbol . • Since Pin = Pout + Plosses, efficiency can also be expressed as

  19. Efficiency To find the total efficiency of a system Obtain product of individual efficiencies of all subsystems: Total = 1×2×3 × ∙∙∙

  20. Efficiency Suppose a power amplifier delivers 400 W to its speaker system. If the power loss is 509 W, what is the efficiency?

  21. Example Problem 3 A 120 V dc motor drives a pump through a gearbox. The power output to the pump is 1100 W. Gearbox efficiency is 75%. Power input to the motor is 1600W. What is Overall efficiency? Hp output and efficiency of the motor?

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