Basic Electricity

Energy, Work, and Power

14 questions By Tony R. Kuphaldt

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Question 4 of 14

Shown here is a schematic diagram for a simple battery-powered flashlight:

What could be modified about the circuit or its components to make the flashlight produce more light when turned on?

Reveal answer

Somehow, the power dissipated by the light bulb must be increased. Perhaps the most obvious way to increase power dissipation is to use a battery with a greater voltage output, thus giving greater bulb current and greater power. However, this is not the only option! Think of another way the flashlight’s output may be increased.

Notes:
The “obvious” solution is a direct application of Ohm’s Law. Other solutions may not be so direct, but they will all relate back to Ohm’s Law somehow.
Question 5 of 14

There are two basic Ohm’s Law equations: one relating voltage, current, and resistance; and the other relating voltage, current, and power (the latter equation is sometimes known as Joule’s Law rather than Ohm’s Law):

E = I R

P = I E

In electronics textbooks and reference books, you will find twelve different variations of these two equations, one solving for each variable in terms of a unique pair of two other variables. However, you need not memorize all twelve equations if you have the ability to algebraically manipulate the two simple equations shown above.

Demonstrate how algebra is used to derive the ten “other” forms of the two Ohm’s Law / Joule’s Law equations shown here.

Reveal answer

I won’t show you how to do the algebraic manipulations, but I will show you the ten other equations. First, those equations that may be derived strictly from E = I R:

I = E
R

R = E
I

Next, those equations that may be derived strictly from P = I E:

I = P
E

E = P
I

Next, those equations that may be derived by using algebraic substitution between the original two equations given in the question:

P = I² R

P = E²
R

And finally, those equations which may be derived from manipulating the last two power equations:

R = P
I²

I = √

P
R

E = √

P R

R = E²
P

Notes:
Algebra is an extremely important tool in many technical fields. One nice thing about the study of electronics is that it provides a relatively simple context in which fundamental algebraic principles may be learned (or at least illuminated).

The same may be said for calculus concepts as well: basic principles of derivative and integral (with respect to time) may be easily applied to capacitor and inductor circuits, providing students with an accessible context in which these otherwise abstract concepts may be grasped. But calculus is a topic for later worksheet questions . . .
Question 6 of 14

Suppose someone mechanically couples an electric motor to an electric generator, then electrically couples the two devices together in an effort to make a perpetual-motion machine:

Why won’t this assembly spin forever, once started?

Reveal answer

This will not work because neither the motor nor the generator is 100% efficient.

Notes:
The easy answer to this question is “the Law of Conservation of Energy (or the Second Law of Thermodynamics) forbids it,” but citing such a “Law” really doesn’t explain why perpetual motion machines are doomed to failure. It is important for students to realize that reality is not bound to the physical “Laws” scientists set; rather, what we call “Laws” are actually just descriptions of regularities seen in nature. It is important to emphasize critical thinking in a question like this, for it is no more intellectually mature to deny the possibility of an event based on dogmatic adherence to a Law than it is to naively believe that anything is possible.