Basic Electricity

Energy, Work, and Power

14 questions By Tony R. Kuphaldt

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Question 7 of 14

Suppose a mass is connected to a winch by means of a cable, and a person turns the winch drum to raise the mass off the ground:

A physicist would likely look at this scenario as an example of energy exchange: the person turning the drum is expending energy, which in turn is being stored in the mass in potential form.

Suppose now that the person stops turning the drum and instead engages a brake mechanism on the drum so that it reverses rotation and slowly allows the mass to return to ground level. Once again, a physicist would view this scenario as an exchange of energy: the mass is now releasing energy, while the brake mechanism is converting that released energy into heat:

In each of the above scenarios, draw arrows depicting directions of two forces: the force that the mass exerts on the drum, and the force that the drum exerts on the mass. Compare these force directions with the direction of motion in each scenario, and explain how these directions relate to the mass and drum alternately acting as energy source and energy load.

Reveal answer

Follow-up question: although it may not be obvious, this question closely relates to the exchange of energy between components in electrical circuits! Explain this analogy.

Notes:
Students typically find the concept of energy flow confusing with regard to electrical components. I try to make this concept clearer by using mechanical analogies, in which force and motion act as analog quantities to voltage and current (or visa-versa).
Question 8 of 14

Draw the direction of current in this circuit, and also identify the polarity of the voltage across the battery and across the resistor. Then, compare the battery’s polarity with the direction of current through it, and the resistor’s polarity with the direction of current through it.

What do you notice about the relationship between voltage polarity and current direction for these two different types of components? Identify the fundamental distinction between these two components that causes them to behave differently.

Reveal answer

Here I show the answer in two different forms: current shown as electron flow (left) and current shown as conventional flow (right).

Whichever notation you choose to follow in your analysis of circuits, the understanding should be the same: the reason voltage polarities across the resistor and battery differ despite the same direction of current through both is the flow of power. The battery acts as a source, while the resistor acts as a load.

Notes:
This type of distinction is very important in the study of physics as well, where one must determine whether a mechanical system is doing work or whether work is being done on it. A clear understanding of the relationship between voltage polarity and current direction for sources and loads is very important for students to have before they study reactive devices such as inductors and capacitors!
Question 9 of 14

When an electric current travels through an electrical resistance, not only will there be a voltage “drop” across that resistance (V = IR), but there will also be energy dissipated by that resistance. We describe the rate of energy dissipation over time as power (P), and we express power in the unit of the watt (W).

Write the equation relating power to voltage and current, for an electrical resistance. Also, describe what physical form this dissipated energy usually takes.

Reveal answer

P = IV

Energy dissipated by an electrical resistance is usually manifest as heat, although sometimes it is also in the form of light.

Notes:
Most people, even if they have no prior experience with electric circuit analysis, have some sense of what a “Watt” is, because so many consumer-grade appliances are rated in watts (light bulbs, heaters, blow driers, etc.). Use these examples as context for your discussion on electric power.