An Introduction To Electrical Power And Energy
Why Do You Need To Know About Electrical Energy?
What Is Electrical Power?
Power and Energy In Electrical Devices
Resistors
Batteries

Problems

You are at:  Basic Concepts - Quantities - Power & Energy
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Why Do You Need To Know About Electrical Energy?

        A power station is a place where other forms of energy - coal, gas, potential energy in water and nuclear energy - are turned into electrical energy for transmission to places that use electrical energy.  Electrical engineering is concerned with transmission and ultilization of two things - energy and information.  Here, in this lesson, we are going to focus on power and energy.  In this lesson you will want to learn the following.

   Given an electrical circuit or device
   Be able to compute instantaneous rate of energy use (power).
   Be able to compute how much energy is used over a period of time.
   Be able to compute how much energy is stored in an electrical storage device like a battery or a capacitor.

What Is Electrical Power?

        Electrical power is conceptually simple.  Consider a device that has a voltage across it and a current flowing through it.  That situation is shown in the diagram at the right.         That's what power is - the rate at which energy is expended.  The rest of the story includes these points.
Power in Electrical Devices

        A resistor is one device for which you can compute power dissipation.

But there's more to the story.         At different times, these two results - which are equivalent - can be used - whichever is appropriate.  Besides being a useful result tthese are also  illuminating results (And that's not a reference to the fact that a typical light bulb is a resistor that dissipates power/energy.).
Problems

P1.   You have a 1KW resistor, and there is 25 volts across the resistor.  Determine the power (in watts) that the resistor dissipates.

Enter your answer in the box below, then click the button to submit your answer.  You will get a grade on a 0 (completely wrong) to 100 (perfectly accurate answer) scale.

        You have a 1KW resistor, and there is 25 volts across the resistor.  Determine the power (in watts) that the resistor dissipates.


Your grade is:


P2. You have a 25 watt light bulb that operates with 12.6 volts across it.  Determine the resistance of the light bulb.

Enter your answer in the box below, then click the button to submit your answer.

Your grade is:


Power In Batteries

       Batteries are ubiquitous components.  They are in TV remotes, cell phones and things like that.  But, batteries also appear in places you don't expect them to be.  For example, you can turn this computer off.  When you turn it back on it remembers things and recalculates things like the time.  Now, you expect that for things that can be stored on a hard drive.  You don't expect it for the time.

         When you turn this computer off and later turn it back on it will have the right date and time.  How does it do that?  If you think about it (and don't do that for too long!) you have to believe that there is a battery somewhere inside the computer and that when you turn the computer off that battery runs some sort of little clock hidden inside the computer.  You can't see the clock and you wouldn't even know it's there, but you can probably see the time now on the task bar of this computer - and it's probably close to being right!

        Batteries are used to solve many problems.

        You use batteries - whether you want to or not, and whether you know it or not!  You need to be able to compute some of the quantities involved.  Here is a simple circuit where a resistor is connected to a battery.  We know some salient facts about this circuit.         Now, we need to look at a circuit diagram for this situation.  That circuit diagram is shown to the right of the picture below.

Power flow into the battery or any other device - is the product, VI, when          We have reproduced the diagram from above here to emphasize how the voltage and current polarities are defined.  Notice that the current arrow in our earlier definition points toward the "+" sign on the device.

        In the battery-resistor circuit below, the current arrow is directed out of the positive ("+") terminal of the battery.  That means the power delivered to the battery must be computed by (note the minus sign!):

P = - VI

What does this mean?  Let's look at a numerical example.  Let's assume the battery voltage is 12 volts and the resistor is 24 ohms.  That means the current is 0.5 amps, i.e.:

Ir = 12v/24W = 0.5a

In other words, the power flowing into the battery is:

P = - Vb Ir = - 12 * 0.5 = - 6w

How much energy is stored in a battery?         That might sound like a lot, but an interested student might want to compare that amount of energy with the energy stored in a gallon of gasoline.

Problems
Power/Energy Problem Basic6P01
Power/Energy Problem Basic6P02 - Grandfather Clock
Power/Energy Problem Basic6P03 - Town Power
Power/Energy Problem Basic6P04 - Light Bulb
Power/Energy Problem Basic6P05
Power/Energy Problem Basic6P06 - Resistor
Power/Energy Problem Basic6P07 - Which resistor is warmer?
Power/Energy Problem Basic6P08 - AC Power


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