elsarnagawy, mash 207, 312 energy and power 1. elsarnagawy, mash 207, 312 objectives define energy...

Elsarnagawy, MASH 207, 312

Energy and Power

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Objectives

• Define energy and power

• Calculate power in a circuit

• Properly select resistors based on power considerations

• Explain energy conversion and voltage drop

• Discuss power supplies and their characteristics

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DC Electric PowerDC Electric Power

• The electric The electric power in in wattswatts associated with a associated with a complete electric circuit or a circuit component complete electric circuit or a circuit component represents the represents the rate at which energy is converted from rate at which energy is converted from the electrical energy of the moving charges to some the electrical energy of the moving charges to some other formother form,, e.g., heat, mechanical energy, or energy e.g., heat, mechanical energy, or energy stored in electric fields or magnetic fields. stored in electric fields or magnetic fields.

• For a resistor in a DC circuit the power is given by the For a resistor in a DC circuit the power is given by the product of applied product of applied voltage and the and the electric current::

Power = Voltage x CurrentPower = Voltage x Current

P = VIP = VI

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Power

Power is the rate energy is “used” (actually converted to heat or another form). Power is measured in watts (or kilowatts). Notice that rate always involves time.

One watt = one joule/second

Three equations for power in circuits that are collectively known as Watt’s law are:

P IV 2RP I2V

PR

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Power in an Electric Circuit

There is always a certain amount of power in an electric circuit, and it is dependant on the amount of resistance and the amount of current, expressed as:

P = I2R

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Equivalent Expressions for Power

• Using Ohm’s law, and substituting V for IR:

P = VI

• Using Ohm’s law, and substituting I for V/R:

P = V2/R

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Watt’s Law

• The power relationships are known as Watt’s law.

• Current (I) must be in amperes,

• Voltage (V) must be in volts,

• Resistance (R) must be in ohms.

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Power Dissipated in Resistor

Convenient expressions for the power dissipated in a resistor can be obtained by the use of Ohm's Law.

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What power is dissipated in a 27 resistor is the current is 0.135 A?

2

2(0.135 A) 27

0.49 W

P I R

Power

Given that you know the resistance and current, substitute the values into P =I 2R.

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What power is dissipated by a heater that draws 12 A of current from a 120 V supply?

Power

12 A 120 V

1440 W

P IV

The most direct solution is to substitute into P = IV.

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What power is dissipated in a 100 resistor with 5 V across it?

Power

2

25 V

0.25 W100

VP

R

The most direct solution is to substitute into .2V

PR

Small resistors operating in low voltage systems need to be sized for the anticipated power.

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Energy

Energy, W, is the ability to do work and is measured in joules. One joule is the work done when a force of one Newton is applied through a distance of one meter.

1 n

1 m

The symbol for energy, W, represents work, but should not be confused with the unit for power, the watt, W.

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Kilowatt-hour

• The kilowatt-hour (kWh) is frequently used as a unit of energy. One kWh is used when one thousand watts is used for one hour.

• Power utilized over a period of time represents energy consumption.

W = P t

• Energy can also be expressed as watt-seconds (Ws), watt-hour (Wh).

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Energy

The kilowatt-hour (kWh) is a much larger unit of energy than the joule. There are 3.6 x 106 J in a kWh. The kWh is convenient for electrical appliances.

What is the energy used in operating a 1200 W heater for 20 minutes?

1200 W = 1.2 kW

20 min = 1/3 h

1.2 kW X 1/3 h =0.4 kWh

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Energy and Power

Energy is the ability to do work; and power is the rate at which energy is used.

Power = energy/time

P = W / t

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Units of Energy and Power

• Energy is measured in joules (J)

• Power is measured in watts (W)

• By definition:

One watt is the amount of power when one joule of energy used in one second.

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Amounts of Power

• Amounts of power less than one watt are expressed as milliwatts (mW), microwatts (W), and even picowatts (pW).

• Electrical utilities and transmitting stations may use large amounts of power, expressed as kilowatts (kW), and megawatts (MW).

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Heat produced by Current

• When there is current through resistance, the collisions of the electrons produce heat, as a result of the conversion of electrical energy.

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Resistor Power Rating

Resistor power rating is not related to ohmic value but rather is determined by the physical composition , size and shape of the resistor.

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Resistor Power Rating

• Power rating of a resistor is the maximum amount of power that a resistor can dissipate without being damaged by excessive heat buildup.

• Power rating is directly related to surface area.

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Metal-film Resistors

Metal-film resistors have standard power ratings of 1/8 W, 1/4 W, 1/2 W, and 1 W.

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Selecting the Proper Power Rating

• A resistor used in a circuit must have a power rating in excess of what it will have to handle.

• Ideally, a rating that is approximately twice the actual power should be used when possible.

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Resistor Failures

• When excessive power is applied to a resistor, the resistor will overheat.

• The resistor will burn open, or its resistance value will be greatly altered.

• Overheated resistors may be charred (burnt), or the surface color may change.

• Resistors suspected of being damaged should be removed from the circuit and checked with an ohmmeter.

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Energy Conversion and Voltage Drop in Resistance

• As electrons flow through resistors, some of their energy is given up as heat.

• The same number of electrons entering a resistor will exit it, only their energy will be less, so the voltage exiting a resistor is less than the voltage entering the resistor.

• There is a voltage drop across a resistor.

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Power Supplies

A power supply is a device that provides power to a load.

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Power Supplies

• A battery is a dc power supply that converts chemical energy into electrical energy.

• Electronic power supplies generally convert 110 VAC (alternating current) from a wall outlet into a regulated dc (direct current) at a level suitable for electronic components.

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Ampere-hour Ratings of Batteries

• Because of their limited source of chemical energy, batteries have a certain capacity that limits the amount of time over which they can produce a given power level.

• The ampere-hour rating determines the length of time that a battery can deliver a certain amount of current to a load at the rated voltage.

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Power Supply Efficiency

An important characteristic of electronic power supplies is efficiency, which is the ratio of output power to input power.

Efficiency = Pout/Pin

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Power Loss

The output power of an electronic power supply is always less that the input power, because some of the input power is used to operate the power supply circuitry.

POUT = PIN - PLOSS

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Efficiency of Power Supplies

High efficiency means that little power is lost and there is a higher proportion of output power for a given input power.

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Summary

• The power rating in watts of a resistor determines the maximum power that it can handle safely.

• A resistor should have a power rating higher than the maximum power that it is expected to handle in the circuit.

• Power rating is not related to resistance value.• A resistor normally opens when it burns out.• Energy is the ability to do work and is equal to

power multiplied by time.

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Summary

• The kilowatt-hour equals one thousand watts used for one hour or any other combination of watts and hours that has a product of one.

• A power supply is an energy source used to operate electrical and electronic devices.

• A battery is one type of power supply that converts chemical energy into electrical energy.

• An electronic power supply converts commercial energy into regulated dc at various voltages levels.

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Summary

• The output power of a supply is the output voltage times the load current.

• A load is a device that draws current from the power supply.

• The capacity of a battery is measured in ampere-hours (Ah).

• One ampere-hour equals one ampere used for one hour, or any combinations of amperes and hours that has a product of one.

• A circuit with high efficiency wastes less power than one with a lower efficiency.

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elsarnagawy, mash 207, 312 energy and power 1. elsarnagawy, mash 207, 312 objectives define energy...

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