Electricity and Magnetism
Investigation 1 – Concept Day
Electricity and Magnetism: Investigation 1
In this Investigation, we wish to introduce you to Ohm’s Law. We also want to prepare you with this essential background concept for Investigation 1 lab, in which you will perform experiments that will demonstrate the relationship of Ohm’s Law variables: Voltage, Current, and Resistance.
However, we will begin by discussing/reviewing basic components of electrical circuits and stress the movement of electrons (e–) in the process. If you keep the movement of electrons in mind during these Investigations, you should be able to appreciate that electricity is kinetic energy, the energy of movement. It is the electrons and, more importantly, the electromagnetic wave flowing with the electrons, that move through metal conductors in electrical circuits. This is important because one can look at an electrical circuit and see no obvious movement at all. Therefore, kinetic energy does not come to mind. The electromagnetic wave caused by the motions of electrons in a circuit does, in fact, move at incredible speeds – actually, close to the speed of light (299,792,458 m/s). That is 670,616,629.3844 miles per hour, so don’t think you could ever run from a wall switch to a lamp and beat the electricity there!
Finally, we want you to visualize that electrons move through circuits by transferring charge from one atom to the next in wires.
In this slide, four key terms are addressed.
Circuit: An uninterrupted path between the terminals of a power source. In lab, the power source will be a 1.5-volt, D-cell battery.
Voltage: Measurement of electrical potential between two points. Measured in Volts (V). Students will use a multimeter to measure voltage in Lab.
Current: The amount of electric charge that passes a given point in a specified period of time. Measured in Amps (A). Students will use a multimeter to measure current in Lab.
Resistance: the decrease in the flow of electrons and current due to a change in the diameter or material of a wire. Measured in Ohms (Ω).
While we do not address Ohm’s Law on this slide, notice that each component of the law is defined here.
In this slide, there are two main points to be made. First, the orientation and polarity of the battery should be noted. We will discuss how a battery works in more detail in Investigation 2 Concepts, but for now, you should think of the negative terminal as where electrons within the battery are concentrated, essentially “waiting” for a completed circuit to be formed so that they can begin moving toward the positive terminal of the battery, through the circuit. The graphic in this slide shows the course of electrons flowing through the circuit. You may remember the direction of electron flow by thinking in terms of “opposites attract”. An analogy would be two bar magnets in which like-poles repel each other whereas opposite poles attract each other. Electrons will always move to a positive terminal and away from a negative terminal in a complete circuit. The battery itself is constructed so that electrons can not move through the battery directly from the negative to the positive terminal, but must travel through the completed circuit to get there. It is almost like water moving through a pipe or hose and we will discuss this analogy in more detail shortly. Early “electricians” thought of electricity as some kind of mysterious fluid and the term “current” is quite understandable.
The second point important point on this slide is that the direction of the current is, by convention in the opposite direction of the electron flow. This sometimes-confusing nomenclature goes back to Benjamin Franklin naming the poles of the battery as positive and negative in an arbitrary manner. He defined the “flow” of electrical current as being from the positive terminal towards the negative terminal of a battery through a circuit. This began a convention that is still in place today. This was, of course, some one-hundred years before the electron was discovered. Therefore, Franklin was not wrong or mistaken in his original terminology. In any case, relaying this story may help you remember the direction of both electron flow and current flow.
You may have seen this slide before in the Atomic Structure CELL. In that instance, we discussed the properties of metals that gave them electrical conductivity. That is, metals like copper have outer electrons in their orbits that are relatively free to be “bumped” from one atom to the next. Thus, one electron may jump to an adjacent copper atom and displace an electron from it so that it is freed to jump to the next atom and so on down the wire. This movement of electrons from atom to atom is what propagates electron flow and electromagnetic waves.
As shown, in this example the electron current is from the left to the right. The positive terminal of the battery would therefore be located to the right. On the other hand, the CURRENT in this example moves from right to left, towards the negative terminal of the battery.
In this slide, we simply add the concept of voltage and amperage to the graphic from slide ELECT-1-3.
- Voltage is the electrical potential between two points in the circuit.
- Amperage is the amount of charge that passes a given point in the circuit in a specific period of time.
An analogy between a garden hose and an electric circuit is often made and is presented in the next two slides.
The analogy between a circuit and a garden hose may help you understand the concepts of voltage, current, and resistance. From the very beginning, the comparison of the flow of a liquid to electricity has been made. Thus, we have inherited such terms as “flow” of electricity and “current”. Early researchers such as Benjamin Franklin and others in fact thought of electricity as some form of fluid.
In this slide:
- voltage is represented as the spigot that increases water pressure. Whatever happens later, it will ultimately be affected by how much pressure is put on the hose at the spigot. Increasing the number of batteries in a circuit would be analogous to opening up the water spigot further.
- Electrical current, on the other hand, is analogous to the actual amount of water flowing through the hose. We will see on the following slide that current can be changed even if we do not change the water pressure (voltage).
In this slide, the analogy between an electrical circuit and a garden hoses is extended to include an important third factor, resistance.
As shown, resistance can be added to the flow of water through the hose (it’s current) by placing a weight on the hose itself. We could, for example, step on the hose or bend and pinch it to restrict the hose’s diameter. You can probably understand intuitively that placing resistance on the hose will likely decrease the amount of water flowing through it. Ohm’s Law simply states this mathematically and gives us an easy look at the relationship between current, voltage, and resistance:
If you step on the hose you will increase resistance (R) in the denominator. If the voltage (V) is unchanged (that is, we do not touch the spigot) the current (I, flow of water through the hose) must decrease! It is that simple. This is a perfect example of how mathematics makes science more easily understandable. You will soon see that, like any other algebraic expression or formula, we can rearrange it to solve for any of the three variables if we know the other two. We will use Ohm’s Law in several of its forms during these Investigations.
Note: It should be kept in mind that, even without adding resistance to the hose, the diameter of the hose itself causes resistance. Thus, a larger diameter hose would provide less resistance than a small diameter hose. The same is true of wires in an electric circuit. Thin wires cause more resistance than thick wires. As we will see in Investigation 2, this is one reason that the filaments in light bulbs are so thin. They create so much resistance that they become intensely hot and glow.
Note: Another analogy that may help at this point is that of blowing through a straw. Which straw would offer more resistance, a very thick or very thin one? What if you pinched the straw, would more or less air flow through it?
In this slide, we present the system of schematic drawing of electric circuits. At the top left, we repeat the definition of voltage and current to once again emphasize that voltage is measured in volts (V) and current in amps (A).
In the schematic drawing, the battery, which supplies the voltage by providing a flow of not water but electrons, is represented by a break in the circuit with a short and long perpendicular line. By convention, the shorter line indicates the negative pole of the battery while the longer line represents the positive pole as shown in the illustration.
The jagged line at the bottom of the schematic represents an electrical resistor. If we go back to the garden hose analogy, this is where we would step on the hose. As shown, resistance (R) causes a decrease in the flow of electrons through the circuit and is measured in Ohms (Ω, the Greek letter omega).
Note: Notice in this schematic, that the current runs through the circuit from the positive to the negative pole of the battery, even though the electrons in the battery move through the circuit in the opposite direction. We discussed this somewhat confusing convention earlier. At this point, we are best to simply think in terms of current moving through the circuit from the positive to negative battery terminals as not to confuse ourselves.
Finally, the upper right of the slide indicates that each of the parameters we have discussed thus far; voltage (V), current (I), and resistance (R) can be measured with a multimeter. You will use this instrument in Lab.
This and the next slide show how you will determine the amperage and voltage measurements in the lab.
A multimeter-wiring photograph is shown in this slide, as is its inclusion in the schematic of the circuit to measure amperage (A).
The wiring configuration may be a bit complicated for you to follow in your SDR, so this slide can be very helpful for you in preparing for the lab.
A Multimeter-wiring photograph appears in the upper right, as does its inclusion to measure voltage in the schematic drawing.
The wiring configuration may be a bit complicated for you to follow in your SDR. You may wish to use this slide in preparing for Investigation 1 lab.
This final slide shows the schematic representation and actual setup for the trials in which voltage is changed in Investigation 1 Lab.
In this slide, one, two, and three batteries are connected in series (in a linear row) in a circuit containing one resistor. The schematics depict this with the insertion of additional battery symbols.