Homework Helpers: Physics

6 Electric Current and Circuits

When I was about 9 years old, my older (and much smarter) brother said, “Anyone who can”t make a flashlight by the time that they are 9 years old must not be very smart.” I remember feeling embarrassed because I didn”t know how to make a flashlight, and I wondered if that meant that I was mentally deficient. I realize now that it wasn”t that I was stupid; it was that my brother was exceptional (he went on to earn a PhD in electrical engineering), and intelligence, like everything in physics, is relative. He did have a point, however. Flashlights are fairly easy to construct. They are slightly harder to understand. I hope that by the time you finish reading this chapter, you will be able to construct a flashlight, and understand how it, and other simple circuits work. Just don”t use this knowledge to make your younger brother or sister feel inadequate.

The bulb of a flashlight will shine if there is a proper electrical current flowing through it. Electrical current is a measure of the rate of flow of charge through an object. It should be easy to remember if you think of how electrical current is analogous to water current.

If you measured the rate of water current, you would end up with units of volume over units of time. For example, a water current of 45 liters/minute would mean that 45 liters of water flow by a given point every minute. When we measure electrical current, we get units of charge over units of time. For example, an electrical current of 3.0 coulombs/second would mean that 3 coulombs of charge pass by a given area every second. The ampere is an SI base unit that is equivalent to an electrical current of 1 coulomb/second.

Example 1

How much charge in coulombs passes through a cross section of a particular wire every second if the current in the wire is 3.0 A? How many electrons (q of an e^– = 1.60 × 10^–19 C) would charge be equivalent to?

To see how many electrons this charge would be equivalent to, we simply divide by the elementary charge (e), the charge on one electron.

What did I mean in the first paragraph of this lesson when I said that a bulb required a “proper” current to light up? If the current is too small—that is, if not enough charge goes by per unit of time—the bulb will not light up. If the current is too great, meaning the rate of charge flow is greater than the bulb can handle, the bulb will break.

You may have heard of direct current (DC) and alternating current (AC), but not really understood what the terms mean. For example, in some situations, water flows in one direction only. If you turn on your sink, the water flows downward. You can watch the running water for hours and never see the water flow upward, or back into the pipe. This example is analogous to the type of electrical current called direct current. In direct current, the net charge travels in one general direction, although the individual charges don”t move in straight lines. This could also be analogous to a river. The individual water molecules in a river move in random directions, but the net flow of water will always be downstream.

Alternating current would be more like the water you see at the beach. As you stand near the shore, the water from a wave moves up the beach to cover you feet, and then it recedes back. In alternating current, the net charge changes direction over and over as the net force on them is continuously reversed.

When you look at a river, it is easy to see the direction that the water flows. In what direction do charges in a wire, or charges in a solution, flow? It is important to remember that the current can represent any types of charges in motion. In solutions containing electrolytes, the dissolved ions would be the charge carriers. In wires, the electrons are the charge carriers. However, when people use the term electric current, they are almost always referring to what is known as conventional current. Conventional current is always in the direction of the motion of positive charge. In a simple flashlight, conventional current is in the opposite direction as the flow of electrons.

Now that we have gone over the concept of current, let”s discuss the components that you will need to construct a flashlight. You will need to take a couple of simple items and set them up in what is called a circuit. An electrical circuit is an arrangement of electrical components that provide a continuous pathway for electricity to flow through. Some circuits, such as our flashlight, are very simple. Others are much more complex. The first thing that you need in order to construct your flashlight is a source of potential difference, such as a battery. The two terminals of the battery represent sites of varying electric potential energy. The electrons “fall” from the negative terminal, through the components of the circuit, back to the positive terminal. The second thing that you need is a bulb. As the electrons travel through the filament of the bulb, some of the kinetic energy that they posses is transferred into heat and light. The final component is a conductor, such as wire, to attach the battery to the bulb.

Figure 6.1

A battery, a bulb, and some wire are all it takes to construct a simple flashlight. When you get past the terminology involved and the abstract concept of electron flow, a flashlight really is easy to make. In the remaining lessons in this chapter, I will explain more about the concepts involved in circuit design.

Lesson 6–1 Review

1. _______________ is electric current that flows in one direction.

2. A wire carries a current of 2.0 A. How much charge passes by a point in 55.0 seconds?

3. 175 C of charge pass a point in a wire in 35 s. What is the current in the wire?

4. If the direction of the flow of electrons in a wire loop is clockwise, what is the direction of the conventional current?