Experiment 11.3: Current and Resistance

Investigate properties of current and resistance using a simple circuit with aluminum foil.

Student Information

Purpose

To investigate properties of current and resistance in a simple circuit.

Background Information

The heat you feel in this experiment comes from the electrons moving in the foil. As soon as you touch the aluminum foil to both ends of the battery, electrons flow through the aluminum from the negative side of the battery to the positive side. The electrons accelerate under the electromagnetic force of the battery and, as a result, start moving quickly.

As electrons flow through the aluminum, they begin colliding with the electrons in some of the aluminum atoms. They also collide with impurities in the metal. These collisions produce heat, and you feel that heat in your fingers. So the heat you feel in the experiment is a result of the aluminum resisting the flow of electrons.

Each metal resists electron flow differently; therefore each metal has its own resistance. It turns out that the type of metal is not the only thing that determines resistance. The resistance of a wide piece of metal, for example, is lower than the resistance of a narrow piece made out of the same metal. Think about it this way: which straw would be easier to drink a milkshake through, a thin straw or a thick, wide straw? It is easier to drink a milkshake through a wide straw. In the same way, a thick wire allows the electrons to move through the metal to "spread out" as they move through, reducing congestion and thus the number of collisions.

Two other factors affect resistance:

  1. Length: The longer the metal wire, the larger the resistance. After all, the longer the metal, the more chance the electrons have to collide with things in the metal.
  2. Temperature: As the temperature of the wire increases, more electrons can collide more often. Because electron collisions increase, resistance increases too.

Materials

  • A 1.5 volt battery (Any AA-, C-, or D-cell battery will work. Do not use any battery other than one of those, though, because a higher voltage can make the experiment dangerous)
  • Aluminum foil
  • Scissors
  • Eye protection such as goggles or safety glasses
  • Gloves (optional)

Safety Precautions

⚠️ IMPORTANT SAFETY WARNINGS

  • Always wear eye protection when conducting this experiment.
  • Do not hold the foil for too long, as it can get painful.
  • Only use the specified 1.5V battery. Higher voltages can be dangerous.
  • Do not attempt to recreate this experiment with other power sources.

NOTE OF CAUTION

Before you go on, I want to caution that you could touch the aluminum foil only because both the voltage and the current would be too low to cause you serious harm. Do not do this in any other situation! Since you still have a lot to learn about electricity, you will not know what is safe and what is not. Playing with electricity is dangerous. Don't do anything else with electricity; please keep safe by only letting professionals and experts work with electricity.

Question

What will you feel when aluminum foil connects both ends of a battery?

Hypothesis

Write your prediction of what you will feel when you touch aluminum foil connecting the positive and negative ends of a battery.

Procedure

Step 1

Cut a small strip of aluminum foil about 1.5 times the length of the battery and only about 1 cm wide.

Step 2

Lay the foil across the battery and, using your thumb and forefinger, pinch the foil so that it contacts both ends of the battery, as shown in Figure 11.21.

Figure 11.21 - Aluminum foil connecting both ends of a battery

Figure 11.21

Step 3

Hold the foil there for a few moments. Note what you feel. Do not hold the foil for too long, as it can get painful. Record your observations.

Step 4

Clean up and put all of your materials safely away.

Data Collection

Observations

Record what you felt when the aluminum foil connected both ends of the battery:

Conclusion

Was your prediction correct? In a paragraph, describe what you felt and then explain why, making connections to the text.

Understanding Circuits and Switches

How Electrical Circuits Work

It is important to understand what's going on in an electrical circuit. Many people think that electrical devices "use up" electrons. Thus, you plug your appliance in, and it "eats up" the electricity it pulls from the electrical socket. That's not what happens. The same number of electrons flow out of a toaster as the number that flowed into it. What the toaster does use up, however, is energy. As electrons flow through a circuit, the collisions they experience convert the energy produced by the electromagnetic force into heat.

Open and Closed Circuits

For electricity to flow, electrons must travel from one place to another. In the circuits we have discussed so far, they flow from one end of the battery to the other because there is metal linking the two sides of the battery.

In an open circuit, the metal does not link the two sides of the battery because there is a break in the wire. Thus, electrons cannot flow from one side of the battery to the other because they cannot jump over the "gap" in the wire. As a result, there is no current in the circuit.

An open circuit is shown in Figure 11.24 using a switch.

Figure 11.24 - An Open Circuit

Figure 11.24: An Open Circuit

Electrons are unable to flow through the circuit when there is a break in the circuit, as with a switch.

How Switches Control Electricity

Now suppose I took the diagonally pointing piece of wire in Figure 11.23 or the switch in Figure 11.24 and pushed them down so that their ends touched the end of the other metal. What would happen then? As soon as the two ends touched, there would be a complete connection between one end of the battery and the other. Thus, electrons would start to flow from one end of the battery to the other. Once that happened, of course, there would be current flowing through the circuit.

This is the way a switch on an appliance or even a light switch on your wall works. When the switch is open, no current can flow, and the device you are using does not work. When you flip the switch, however, a connection is made, and electrons begin to flow. This allows the device you are powering to operate.

Figure 11.25 - How a Switch Controls Electricity

Figure 11.25: How a Switch Controls Electricity

When the switch is open (top), there is not a complete path from one side of the battery to the other, so current cannot flow. As a result, the light bulb is off. Once the switch is closed (bottom), there is a complete path from one side of the battery to the other, so current can flow. As a result, the light bulb is on.

Series and Parallel Circuits

The idea that a break in an electrical circuit can stop the current flow and thus stop the electrical device or devices in the circuit is quite useful most of the time. However, sometimes it's a real problem.

Consider, for example, the circuit in Figure 11.26. In this circuit, despite the fact that one of the light bulbs is not broken, neither of them will light. Why? Well, a light bulb lights up because current flows through it. In a series circuit, when one light bulb burns out, no light bulbs work because the current cannot flow.

Figure 11.26 - Light Bulbs in a Series Circuit

Figure 11.26: Light Bulbs in a Series Circuit

In a series circuit, when one light bulb burns out, no light bulbs work because the current cannot flow.

Practical Applications

With the knowledge of current, electrical circuits, and resistance that you now have, let's look at how the simplest electrical devices work. For example, consider an electrical heater. This could be a space heater used to warm up a room, a coil on an electric stove, or even the wires on the inside of a toaster. When such a device is turned on, the heater begins to glow, emitting a large amount of heat. This works because the material used to make the heater has a certain amount of electrical resistance. Since there is resistance, there is heat and often light. The light causes the heater to glow, and the heat energy warms the room, cooks the food, or browns the bread.

One way we get electrons to do something useful in an electrical circuit, then, is to use a metal's resistance to convert the kinetic energy of the electrons speeding through the circuit into thermal (heat) and light energy. The type of metal used to do this will influence the effect you get. For example, most metals will resist the flow of electrons in such a way as to produce mostly heat and only a small amount of light. Those metals are used in heaters, stoves, and toasters. Other metals tend to produce more light than heat. Those metals are typically used in the filaments of light bulbs.

Analysis Questions

1. What happened to the temperature of the aluminum foil? Why?

2. What is electrical resistance? How does it relate to what you observed in this experiment?

3. How does the width of the aluminum strip affect the resistance? What would happen if you used a wider strip?

4. What is the relationship between current, resistance, and heat in this experiment?

5. In what direction does the current flow in a circuit? Is this the same direction as electron flow? Explain.

6. What is the difference between an open circuit and a closed circuit? How does a switch work to control the flow of electricity?

7. Why do all light bulbs in a series circuit go out when one burns out?

Extension Activities

  • Try using different widths of aluminum foil and observe any differences in how quickly the foil heats up.
  • Research how this simple experiment relates to how fuses work in electrical systems.
  • Investigate how different materials have different resistances to electrical current.