cycle 5 developing our ideas activity 3: electric circuits...

CYCLE 5 Developing Our Ideas

ACTIVITY 3: Electric Circuits and Current

© 2005 PET 5-23

Purpose

In Cycles 1 and 2 you learned that there are two ways that scientists describe interactions: either in terms of energy transfer and energy changes, or in terms of forces. When thinking about an electric circuit interaction, there are also two ways of describing it. In the previous activity you learned how to describe the interaction in terms of the transfer of electrical energy from the battery (or other source) to the receiver. In this activity you will learn of another way of describing what happens in an electric circuit, and that is in terms of electric current.

How can we describe an electric circuit interaction in terms of electric current?

Does Electricity Flow One-Way or Two-Ways in the Circuit?

Two students both believed there is a flow of electricity in a simple circuit consisting of a battery and bulb. But they differed in thinking about whether the electricity flows one way or two ways.

Kristen says: “I think electricity flows out of each end of the battery, meets at the bulb, and causes the bulb to light.”

Amara says: “I think electricity flows in one direction around the circuit. It comes out of one end of the battery, goes through the bulb, and then into the other end of the battery.”

Cycle 5: Modified by Dr. Armen Kocharian

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Let us refer to Kristen’s model as the “two flow” model, and to Amara’s model as the “one-flow” model.

Do you agree with the one-flow or two-flow model, or with both? What is your reasoning?

To provide some evidence that might help settle the issue, you should perform the following experiment. Experiment #1: How do the two ends of the battery need to be connected to the two sides of the bulb? The evidence from Activity 1 suggests that one side of the bulb needs to be connected to the positive end of the battery, and the other side of the bulb needs to be connected to the negative end of the battery. But, do the two sides of the bulb need to be connected to the positive and negative ends of the same battery? Consider the following arrangement:

Do you think the bulb in the above arrangement will light? Explain your reasons.

Activity 3: Electric Circuits and Current

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Get two batteries, two hook-up wires and a bulb in a socket. Hook up the arrangement shown above.

Does the bulb glow? In Activity 2, Experiment #2, step 2, you hooked a battery to the motor/fan, then figured out how to make the fan blades rotate in the opposite direction. You may wish to go back and look at what you did in that experiment.

Based on your observations in the experiment with the two separate batteries, and the experiment from Activity 2 with the motor/fan, what would you now claim about the one-flow versus two-flow model? Why?

Participate in a whole class discussion.

Beginning to Think About Electric Current

Back in Cycle 2 you learned that the rate of change of speed of an object (also known as acceleration) depends on two factors: the strength of the unbalanced force acting on it and the mass of the object. Although an electric circuit is a much more complicated system than just a single object acted on by a force, it is helpful to think about an electric circuit in a somewhat similar way. Scientists believe that inside batteries, wires and all electric devices there are particles called electric charges. During an electric circuit interaction between a battery and a bulb the battery exerts a push (force) on these electric charges, causing them to move around the circuit, from one end of the battery, through the wires and filament of the bulb and into the other end of the battery. (Notice this implies that scientists have a one-flow model of electricity.) The rate of flow of electric charges is called the electric current.


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The greater the rate of flow of electric charges, the greater the value of the electric current. So, in comparing what happens in an electric circuit with a battery and bulb to what happens when a force is exerted on an object, the push provided by the battery is analogous to the force acting on the object. The value of the electric current in the circuit is analogous to the rate of change of speed of the object. The property of a bulb that is analogous to the mass of the object is called the resistance of the bulb. In the remainder of this activity you will perform experiments to develop and test ideas about the relationships between the strength of the push provided by the battery, the amount of resistance of the bulb, and the value of the electric current in an electric circuit.

Collecting and Interpreting Evidence

In the next experiment you will study the relationship between the strength of the push from the battery and the value of the electric current. To help you think about and make sense of this relationship, you will first be introduced to a simple model of the electric circuit. Experiment #2: How does the value of the electric current depend on the strength of the battery push? You will need:

• Straw • Sheet of 8.5 x 11 inch paper • Two pieces of tape • Stand to mount paper on table

A simple model of an electric circuit involves blowing air through a straw. Each member of your team will need a regular straw. In using this model we will assume your mouth and lungs are analogous to the battery. The air inside your mouth, lungs and straw is

analogous to the electric charges inside the battery, bulb and wires of the circuit. The straw is analogous to the bulb. (More specifically, the straw is analogous to the filament of the bulb. You observed in Activity 1 that only the filament part of the bulb glows.)


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Blow through the straw and observe the effect at the other end. By feeling the moving air pushing against your hand you get a sense of the rate of flow of the air, which we will call the air current. The air current in the straw is analogous to the electric current in the circuit. However, this analogy is not perfect, in part because in a circuit the wire doesn’t end, but instead is connected to the other end of the battery. But even though this analogy (like all analogies) is not perfect, we can still use the model to help us make sense of some properties of electric current and circuits. We will call this the blowing-through-straw model for an electric circuit.

Blow harder and harder through the straw. Does the value of the air current increase, decrease or remain constant? It will be useful in experiments to follow to have some simple and qualitative way to measure the value of the air current. Take an 8.5-inch x 11-inch piece of paper and draw a large dot in its center. Then tape the upper left and right corners of the paper to the rod so the paper hangs freely. See picture on right.


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Hold the straw horizontally with its end almost touching the large dot.

Blow gently and quickly. Then blow harder and harder. Another member of your group should observe each time about how far the paper moves away from its initial position.

What seems to be the general relationship between how hard you blow through the straw and how far the bottom of the paper moves?

The harder you blow through the straw, the higher is the value of the air current (the rate at which air moves through the straw). The hanging paper can be used as a very simple air current meter, allowing you to get a qualitative measure of the rate at which air moves through the straw (and comes out the other end). Do not put away the air current meter. You will use it again later in the activity.


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Now consider the electric circuit. A single battery cell provides a constant push on the electric charges in the circuit. The strength of the battery push (analogous to how hard you blow through the straw) is determined by the voltage of the battery. The greater the battery voltage, the greater the strength of the battery push on the electric charges. Each D-cell battery has a voltage of 1.5 volts. If you combine two D-cells together (in the right way), the total voltage is 3.0 volts; three together gives a total voltage of 4.5 volts, and so on.

What do you predict would be the relationship, if any, between the strength of the battery push and the value of the electric current in a circuit? Why do you think so? To test your prediction, you will use a computer simulator. The EM Devices simulator allows you to set up circuits with batteries and bulbs. The simulator also has a device to measure the rate at which electric charges move through the circuit; that is, a device to measure electric current. The device is called an ammeter. (It is analogous to your air current meter, although it works quite differently.) The units for electric current are amperes, or more commonly milli-amperes (mA), which are thousandths of an ampere. To the right is a picture from the simulator of a simple electric circuit consisting of a battery, ammeter and bulb. The table below summarizes the analogous relationships between the electric circuit and the blowing-through-straw model.


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Analogy between Electric Circuit and Blowing-through-straw Model

Electric Circuit Blowing-through-straw Model

Battery Lungs and mouth Strength of battery push (measured in volts)

Strength of blowing through straw

Light bulb filament Straw Electric charges in battery, bulb, ammeter and connecting wires

Air in lungs, mouth, straw and hitting paper

Electric current is rate of flow of electric charges

Air current is the rate of flow of air

Ammeter measures value of electric current

Air current meter (hanging paper) measures value of air current

Value of electric current measured in amps (milla-amps)

Value of air current measured by how far end of paper moves from its original position

Open the EM Devices simulator by going to the PET simulator index page and click on Cycle 5 Activity 3 Setup #1. Then read How To Use the EM Devices Simulator at the end of this activity. This will tell you how to set up a circuit. After reading the instructions, continue below.

Start with one battery, close the switch, and record the value of the electric current in Table 1. Fill in the value of the battery voltage. Then add a second battery, a third battery, and a fourth battery. (Make sure the batteries are connected together with the minus side of one battery connected to the plus side of the next battery.) In each case record the ammeter reading in Table 1, fill in the battery voltage, and also note what happens to the brightness of the bulb.

Table 1: Battery Voltage versus Electric Current

Number of Batteries Battery Voltage (volts) Electric Current (mA)

1

2

3

4


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What is the relationship between the battery voltage (strength of battery push) and the value of the electric current?

Does this relationship confirm or disconfirm your prediction?

What is the relationship between the value of the electric current and the brightness of the bulb? This observed relationship between the electric current value and bulb brightness suggests that the brightness of the bulb can be used as a measure of the value of the electric current. However, it is only a qualitative measure, depending on the observer’s judgments of brightness. The ammeter provides a quantitative measure (using measured numbers rather than subjective judgments) and, for that reason, is generally more useful when studying electric circuits. As you have seen, it is the filament wire in the bulb that actually glows. So far you have always used identical bulbs, with identical filament wires. What would happen if the filament wire changed in some way? Would the value of the electric current in the circuit change, or remain the same? The purpose of the next experiment is to help you answer that question. Experiment #3: How does the value of the electric current in a circuit depend on the properties of the bulb filament? You will need the following materials: • 2 coffee stirrers • Regular straw • Air current meter • Scissors


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Consider two properties of a filament wire that could be different from one bulb to the next: its length and thickness. Before doing an experiment to investigate what happens in electric circuits you will first work with straws and stirrers and apply the blowing-through-straw model to make predictions about electric circuits. First, cut the regular straw so it is the same length as the full-length stirrer.

Since both the straw and stirrer have the same length, what is the main difference between the two?

Blow through the straw, and then blow through the stirrer. Which one seems more difficult to blow air through?

Next, cut one of the coffee stirrers into a 1/3-length piece and a 2/3-length piece. Put aside the 2/3-length piece.

Blow through the 1/3-length stirrer, and then blow through the full-length stirrer. Which one seems more difficult to blow air through?

Scientists would use the term resistance to refer to the property of the straw or stirrer that determines how difficult it is to blow air through it. Fill in the following: The longer the stirrer/straw, the __________________ (greater, smaller) its resistance to the flow of air though it. The thinner the stirrer/straw, the __________________ (greater, smaller) its resistance to the flow of air though it. Next, make sure the air current meter is set up.

Hold the 1/3-length stirrer with its end very close to the dot on the paper, blow quickly through it, and notice how far the bottom of the paper moves


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away. Then hold the full-length stirrer with its end the same distance from the dot, and blow equally hard and as quickly. In which case does the paper move away further?

Repeat using the full-length straw. When you blow equally quick and hard through the same length stirrer and straw, in which case does the paper move further away?

The distance the paper moves is related to the value of the air current. Fill in the following:

For the same blowing strength, the longer the stirrer/straw, the __________________ (higher, lower) the value of the air current. For the same blowing strength, the thinner the stirrer/straw, the __________________ (higher, lower) the value of the air current.

For the same blowing strength, what seems to be the relationship between the amount of resistance of the stirrer/straw and the value of the air current in a stirrer/straw?

Check your answer to the above question with another group. If there are differences in your answer, together you should repeat one or more of the previous observations. Having worked with the straws/stirrers, you are now in a position to use the blowing-through-straw model to help you think about actual bulbs and batteries. The filaments of different types of bulbs generally have different lengths and/or thicknesses. Scientists use the term resistance to refer to the property of a wire that determines how difficult it is for electric charges to move through it. The amount of resistance depends (in part) on its length and thickness. In analogy with the stirrers/straws:


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• The longer the filament, the greater its resistance to the flow of electric charges though it.

• The thinner the filament, the greater its resistance to the flow of electric

charges though it.

For the same battery voltage, predict the relationship between the amount of resistance of the bulb filament and the value of the electric current in a circuit? How does the blowing-through-straw model guide your prediction? You can use the EM Devices simulator to test your prediction. Go to the PET simulator index page and click on Cycle 5 Activity 3 Setup #2. In the simulator window you will see the two identical circuits already setup. Each circuit includes a battery, ammeter and bulb. The battery voltage (1.5 volts) is the same in both cases.

If you double click on the upper bulb, you will open its property window. Each bulb starts with an amount of resistance equal to 30 ohms. (The ohm is the unit of resistance.) Change the amount of resistance of the upper bulb to 16 ohms. Then click OK. In the two circuits the upper bulb has a resistance of 16 ohms, the lower bulb a resistance of 30 ohms.

(Since the resistance of the bulb filament depends on its length and thickness, changing the amount of resistance would normally require changing its length and/or its thickness. The simulator, however, doesn’t allow you to change length and/or thickness, only the total amount of resistance.)

Turn on the simulator. How does the brightness of the 16-ohm bulb compare with the brightness of the 30-ohm bulb?


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In the following table, for the two values of bulb resistance record the corresponding values of the electric current.

Table 2: Electric Current versus Bulb Filament Resistance

Bulb Filament Resistance (ohms)

Measured Electric Current (mA)

16

30

Try several other values for the resistance and record them in Table 2. For each new value, observe what happens to the bulb brightness and record the measured values of the electric current. (If the electric current gets too high the bulb burns out. In that case, delete the burned out bulb and replace it with a new bulb from the palette to the right of the setup area.)

For the same battery voltage, as the resistance of the bulb filament increases, does the value of the electric current in the circuit increase, decrease, or remain essentially constant? How does this result compare with your prediction?

As the resistance of the bulb filament increases, what happens to the brightness of the bulb?


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Does the bulb always glow, or are there values of electric current for which the bulb does not seem not to glow noticeably?

Resistance and Thermal Energy

What is the reason why materials like bulb filaments have a certain amount of resistance to the flow of electric charges? Scientists have theories about the origin of resistance that can answer this question, but the theories are much too complicated to describe here. Instead, we will try to think about it in a much simpler way, using a familiar idea. In Cycle 2 you learned that whenever an object slides over a surface, there is a friction force that opposes the motion of the object and tends to slow the object down. In the case of an electric circuit we can imagine that when the battery pushes electric charges through the metal filament of the bulb, a similar thing happens. There are friction-like interactions between the moving electric charges and the atoms that make up the material of the filament wire. These interactions tend to decrease the rate of flow of the electric charges. This is the origin of the resistance of the wire. The actual amount of resistance will depend, in part, on the strength of the friction-like interactions. As you learned in the previous section, the amount of resistance will also depend on the length and thickness of the wire. Why does the bulb glow when there is an electric current in the filament wire? To help answer this question, again consider the analogy of the object sliding over a surface. When the block slides over a surface, mechanical energy is transformed into thermal energy in the surface and this causes the surface to warm up. In the case of the electric circuit, when the charged particles are pushed through the bulb filament (by the battery), electrical energy is transformed into thermal energy of the filament material, and this causes the material to warm up. When the material warms up sufficiently it will begin to glow, and as it becomes even hotter it will glows more brightly. How hot the material becomes, and how brightly it glows, depends both on the value of the electric current and the amount of resistance. However, scientists have found that the brightness depends more on the value of the


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electric current than it does on the amount of resistance. We can represent this brightness-resistance-current idea in the following diagrammatic way:

As the amount of resistance , the brightness of the filament

As the amount of resistance , the brightness of the filament

As the value of electric current , the brightness of the filament

As the value of electric current , the brightness of the filament

Let’s see how your results from Experiments #2 and #3 are consistent with this idea. In Experiment #2 (and Table 1) you found that when the resistance was held constant (same bulb), and you increased the battery voltage the value of the electric current ______________________ (increased, decreased, remained the same), and the brightness of the bulb _________________ (increased, decreased, remained the same).

How are these results consistent with the brightness-resistance-current idea?

In Experiment #3 (and Table 2) you found that when the battery voltage was held constant (one battery), and you increased the resistance of the bulb, the value of the electric current ______________________ (increased, decreased, remained the same), and the brightness of the bulb _________________ (increased, decreased, remained the same).


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How are these results consistent with the brightness-resistance-current idea? As mentioned above, one of the factors that determine the amount of resistance of a wire is the strength of the friction-like interactions. The strength of the friction-like interactions between the moving electric charges and the atoms of the material depends both on the temperature of the material and the type of material. Consider the temperature effect first. For the same type of material, the strength of the interactions, and hence the amount of resistance of the wire, increases when the wire gets hotter. Tungsten filaments are used in bulbs because tungsten can get white-hot without melting. When a bulb is first turned on, the resistance of its filament changes drastically as it heats up from room temperature (low resistance) to its working temperature (high resistance). Next let’s consider an example of how the strength of the friction–like interactions (and hence the resistance) depends on the type of material. The heater elements of most toasters are made from a special nickel-chromium alloy called (not very creatively) "Nichrome". You would rarely find heater elements to be made of copper. The reason is that the structure of copper is very different from the structure of nichrome. The consequence of this difference in structure is that the friction-like interactions occurring when electric charges move through copper is much, much weaker than the friction-like interactions occurring when the electric charges move through nichrome alloy. This means that the amount of resistance of a copper wire is much smaller than the amount of resistance of a nichrome wire of the same length, thickness and temperature. This is why the heater elements of a toaster get hot enough to glow, while the copper power cord hardly warms up at all. In the next experiment, however, you will explore a very special situation where regular copper wires do warm up noticeably.


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Experiment #4: What happens when a copper wire is connected directly to both sides of a battery? You will need:

• Fresh battery (without a battery holder) and bulb (without a bulb holder)

• Three pieces of bare copper wire, each at least 10-cm long. (You will not use the regular plastic covered hook-up wires for this experiment.)

Using the battery and two bare copper wires, make the bulb glow.

While the bulb is glowing, feel the two bare copper wires near where they are in contact with the battery. Also feel the third bare copper wire that is not connected to anything. Do the bare copper wires in the circuit feel warmer, cooler, or about the same temperature as the stand-alone copper wire? Put aside the bulb. Use one piece of bare copper wire and touch its two ends to the two ends of the battery. Keep it there for several seconds. Feel the wire near where it is in contact with the battery and compare its temperature to one of the other stand-alone copper wires. Remove the wire from the battery.

Does the bare wire connected directly to the two ends of the battery feel warmer or cooler than the stand-alone copper wire, or does it feel about the same temperature? Is this the same result you got above when the bulb was included in the circuit?

Imagine that you hooked up the following two circuits. Predict how the value of the current in the battery-ammeter circuit on the right would compare to the value of the electric current in the battery-ammeter-bulb circuit on the left. Use your observations above, as well as what you read in


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the previous section, to explain your reasoning. (You can assume that although the ammeter reads the value of the electric current in a circuit, its presence does not affect the actual value of the electric current nor how hot the wires might become.)

Open up the same simulator that you used in Experiment #3. Delete everything in the setup window. Construct the two circuits shown above. Turn on the simulator.

Fill in the following values from the simulator.

Value of electric current in the battery-ammeter circuit = __________ Value of electric current in the battery-ammeter-bulb circuit = __________

How do these results compare with what your predictions? Whenever a copper wire makes direct contact with the two sides of a battery, or any source of electricity, the situation is called a short circuit. Even if there is an ammeter in the circuit, the situation is still called a short circuit. The battery-ammeter circuit above is an example of a short circuit. When you


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touched the copper wire directly to the two ends of the battery without including a bulb in the circuit, you made a short circuit. In a short circuit the copper wire can get very hot because the value of the electric current in that wire gets very high. Short circuits in household wiring and appliances can be very dangerous. The wires can get hot enough to set fire to surrounding materials. The function of fuses and circuit breakers is to break a connection in the circuit if the value of the electric current gets too high.

Summarizing Questions

S1. For a constant amount of resistance (same bulb), how does the value of the electric current depend on the size of the battery voltage?

S2. For the same battery voltage, how does the value of the electric current

depend on the amount of resistance of the bulb? S3. How does the length and thickness of a bulb filament wire each influence

the amount of resistance of the wire? Using the blowing-through-straws model, explain why this makes sense.


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S4. In this activity you learned the idea that the brightness of a bulb depends

both on the amount of its resistance and on the value of electric current in it: the more resistance, the brighter the bulb; the more current, the brighter the bulb. However, in Experiment #3 you observed that as the resistance of the bulb was increased, the bulb became dimmer, not brighter! This result seems to contradict the above idea. How can you reconcile this apparent contradiction?

S5. Two students are discussing what they think a battery does that is the

same in different circuits.

Student #1: A battery provides the same amount of voltage. A battery always provides the same strength push on the electric charges in the circuit, and push is voltage. Student #2: A battery provides the same amount of electric current. A battery always pushes the electric charges at the same rate through a circuit, which means that the electric current is always the same.

Do you agree with either student #1 or student #2, or do you have a different interpretation of what a battery does that is the same, regardless of the circuit? Explain your reasoning.


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S6. Consider the following results you observed in Experiment #4. How

might you extend the blowing-through-straw model to explain why it makes sense that the value of the electric current in the battery-ammeter circuit is much higher than its value in the battery-ammeter-bulb circuit.

Participate in a whole class discussion.


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How to Use the EM Devices Simulator

To start, open Cycle 5 Act 3 Setup. You will see a large setup area, and to the right is the tools palette.

1. Place a battery, ammeter and bulb in the setup window by first clicking on each device in the palette, then clicking in the setup area.

2. Practice opening and closing the switch. Click the Select button, and then move the cursor to the top of the handle. Drag the handle down to close the switch. Drag it up to open the switch. Leave the switch in the open (handle up) position.

Switch OPEN

Switch CLOSED

3. For the circuit to work, all the devices must be wired together. To draw a wire between the battery and bulb holder, place the cursor arrow at one end of the battery, then click and start dragging the cursor towards one end of the bulb holder (top picture).


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As you get near, the clip will "flash" a small yellow rectangle (middle picture). At that point, you can let go of the cursor and the wire will connect itself (bottom picture).

4. Wire together the rest of the circuit as shown here.

5. RUN the Simulator by clicking on the Run (Play) button.

6. Close the switch. The bulb should light and the ammeter should show a reading of the value of the electric current in mA. Record the value of the current in the Table in the activity, and note the brightness of the bulb. Then open the switch and STOP the simulator.


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7. To add a second battery (thus increasing the battery voltage from 1.5 volts to 3.0 volts), first click on the wire between the negative end of the battery and the bulb. This will select that wire. Then press on the Delete key on your keyboard to delete the wire.

8. Add another battery (from the palette) and connect it with wires between as shown in the picture to the right. Turn on the simulator, close the switch, and record the value of the electric current in the activity table.

cycle 5 developing our ideas activity 3: electric circuits...

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