EdBrorein

What is Power and Energy? Part 1

Blog Post created by EdBrorein Employee on Feb 21, 2017

Energy is becoming an increasingly valuable commodity as the world keeps finding ways to consume it at a faster rate than coming up with ways to produce it. Even if we were able to produce energy in unlimited abundance, its production and consumption leaves an indelible mark on the world. A key part of addressing this demand is making smarter and more efficient use of the energy we produce. It’s great to see how technologies are evolving in a number of industries to do this and that we at Keysight are taking part in it to help with solutions for the automotive and energy sectors.

 

So this leads me to what I intend to write about today: My physics lecture on what power and energy are. While power and energy are pretty fundamental concepts and many do understand what they are, I sometimes encounter folks mistakenly using one in place of the other. They are indeed closely related but still distinctly different things.

 

Let me start with energy.  It is probably best to look at it in the classical mechanical sense for a particle in motion. Its kinetic energy is described by the equation:

 

Ek = ½ mv2

 

Where Ek is the energy of a particle, m is its mass, and v is its velocity. As long as this particle in motion is not acted on, its energy remains unchanged. But what if it is acted on by an external force? That leads us to what is defined as work. Mechanical work is a force acting over a displacement or distance. If this force is in the same direction as the displacement the work is defined as positive. Energy is added to the particle. If the force is opposite to the displacement then the work is negative. The energy of the particle is reduced. Work is expressed as:

 

W = Ek2 – Ek1

 

Where Ek1 is the energy of the particle before it is acted on and Ek2 Is the energy of the particle after it has been acted on by a force. Work is a measureable change in energy of that particle.

 

This leads to potential energy. In the mechanical world potential energy can be described as what I will call a recoverable force applied against a displacement. Most typically it would be a mass or weight lifted a height against gravity. It can also be a force used to stretch or compress a spring over a distance. For gravity the potential energy is by:

 

Ep = mgy

 

Where Ep is the potential energy of the particle, m is its mass, g is gravity, and y is the height of the particle above a set reference point. Note that weight is the product of mass and gravity. Work added or detracted correspondingly is lifting or lowering this particle over vertical distance, against gravity.

 

 

 

With electrical things work and energy is one and the same as with mechanical things. It is stated that energy cannot be created or destroyed, only converted from one form to another. Light energy can be converted to electrical energy with a solar cell. Electrical energy can be converted to mechanical energy with an electrical motor, and so on. These processes are not 100% efficient and a good portion of the original energy also gets converted to heat energy.  A common measure of energy is joules, which is 1 watt-second. You probably are most familiar with this when you pay your electrical utility bill, which is based on the amount of kilowatt-hours of electrical energy you consumed since your previous billing.

 

 

Like mechanical systems, energy can be stored in electrical systems, in particular in the reactive components; the inductors and capacitors. Energy in an inductor is given by:

 

E = ½ LI2

 

Where E is the energy in joules, L is the inductance in Henrys, and I is the current in amps. An inductor stores its energy in its magnetic field. Similarly energy in a capacitor is given by:

 

E = ½ CV2

 

Where E Is the energy in joules, C is the capacitance in Farads, and V is the electric potential in volts. A capacitor stores its energy in its electric field.

 

Hopefully this gives you a little more appreciation about what energy (and work) is.  Look for my upcoming second part when I tie it all together with power!

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