ELECTRIC CIRCUITSWNA Physics

Experiment 1.1 Obtain one battery, one bulb, and one wire. Connect these

in as many ways as you can. Sketch each arrangement in your notebook. On one side of the page, list arrangements in which the bulb lights. On the other side of the page, list arrangements in which the bulb does not light.

  You should have sketches of at least four different

arrangements that light the bulb. How are they similar? How are they different from arrangements in which the bulb fails to light?

  State what requirements must be met in order for a bulb to

light.

Exercise 1.2 An arrangement of a bulb, a battery, and a

wire that allows the bulb to light is said of be a closed electric circuit. The terms complete circuit or just circuit are also used. The word circuit was originally used to mean a circular route or course.

Why is a circuit an especially suitable name for an arrangement of bulb, battery, and wire in which the bulb is lit?

Exercise 1.3

Examine a flashlight. Make a sketch of the flashlight showing the circuit that exists when the bulb is lit. How many wires are used to make this circuit?

Exercise 1.4

Write an operational definition of an electric circuit.

Experiment 1.5

Using a bulb, battery, and two wires, set up an electric circuit in which the bulb is lit. Does it matter which part of the bulb is connected to the end of the battery with the plus sign in it?

Conductors and Insulators

Substances can be divided into categories based on their effect on an electric circuit. In the following experiment, we classify some common substances.

CAUTION! Do not leave any circuit connected

longer than necessary to observe bulb brightness. Leaving a circuit connected for too long can ruin the battery (and is not environmentally friendly).

Experiment 1.6

Use a battery, a bulb, and two wires to make a circuit in which the bulb lights. Obtain objects made out of several different materials such as copper, paper, steel, porcelain, plastic, glass, aluminum, rubber, and pencil lead. Insert one of these objects into the circuit.

  Does the bulb continue to glow

brightly or does it dim or go out?

More Experiment 1.6

Classify your materials into different categories according to their effect on the bulb. Make a list of the objects in each category.

  What do most objects that let the bulb light

have in common?   An object that allows the bulb to continue glowing

brightly is called a conductor. An object that makes the bulb go out is called an insulator. Some objects, like pencil lead, fall between the two categories.

Exercise 1.7

Suppose you have a closed box from which two wires protrude. Explain how to use a battery and a bulb to find out whether there is an electrical connection between the two wires inside the box.

 

Experiment 1.8 Carefully examine a bulb. Use a magnifying

glass if possible. You may also find it helpful to look at a broken bulb. Make a careful sketch of the inside of the bulb.

  Use your test circuit from Experiment 1.6 to

determine whether each part of the bulb is a conductor or an insulator and label it on your sketch. Describe what you believe is the purpose of each part.

  Note the two wires coming up from inside the

base of the bulb. Use your test circuit to determine where in the base each of these wires originates.

Experiment 1.9

A. For convenience, light bulbs are usually placed in sockets. Carefully examine a socket. Identify the conducting and insulating parts, and label these on a careful sketch of the socket. Use the method you invented in Exercise 1.8 to determine which of the conducting parts are connected to one another and show the connections in your sketch.

  If there is more than one type of socket available,

repeat this experiment for each type. Identify which parts of one type of socket correspond with which parts of other types.

More Experiment 1.9

B. Repeat the experiment for a battery holder and switch. Try the switch both open and closed. What do you thing is the function of the switch?

More Experiment 1.9

C. Using a battery, a holder, a bulb, a socket, and two wires, set up a circuit that lights the bulb. Trace the path of conductors around the circuit. Draw a sketch of the circuit in which you show in detail the conducting path through the socket.

Circuit Diagrams

We will start with simple circuits, combining one battery and one bulb.

Circuit Diagrams

Battery

Bulb

Wire

Switch

Circuit Diagrams

How can we tell if the wires are electrically connected?

Circuit Diagrams show electrical connections, NOT physical layout.

Circuit Diagrams

Carefully view the four circuits at the back of the class.

Draw a simple picture of each circuit.

Draw a circuit diagram for each circuit.

Compare diagrams with your group.

Electric Current Model

Scientific Model: A set of rules that applies to a particular system that

makes it possible to explain and predict the behavior of that system.

As we learn more, we can change our model.

What are our assumptions?

Exercise 2.2 A. In section 1, you found that a

complete circuit was necessary for a bulb to light.Does this observation suggest that the flow

in an electric circuit is one way or round trip? Explain.

What does your answer above suggest is a major difference between the flow in an electric circuit and the flow of water in a river?

Can you tell from your observations so far the direction of the flow through the circuit?

Exercise 2.2 continued

B. Base your answers to the following questions on the assumptions about the flow in an electric circuit.If 2 identical bulbs are equally bright, what

does this indicate about the electric flow through them?

If one bulb is brighter than another identical bulb what does this indicate about the flow through the brighter bulb?

Flow of Current – Reasoning? Student 1 “When the bulb is lit, there is

a flow from the battery to the bulb. There is also an equal flow from the bulb back to the battery.”

Student 2 “The flow is only from the battery to the bulb. We know this is so, because a battery can light a bulb, but a bulb can’t do anything without a battery.”

Electric Current Model

We cannot see anything flowing We will use the term electric current to

refer to this flow “Current” tells us nothing about the

nature of what flows.

A good scientific model is as simple as possible and includes the fewest features necessary for making correct predictions.

Experiment 2.4 Set up a 2 bulb circuit with the bulbs connected one

after another as shown.

2 bulbs connected one after another are said to be connected in series.

A. Compare the brightness of each of the bulbs with the brightness of an identical bulb in a single-bulb circuit. Remember our assumptions. How does the current

through a bulb in a single-bulb circuit compare with the current through the same bulb when it is connected in series? What does this imply about current through the battery?

Experiment 2.4 con’t. B. Compare the brightness of the two bulbs in

the two-bulb series circuit with each other. What can you conclude from this observation about the amount of current through each bulb?

Pay attention to the large differences you may observe, rather than minor differences that may occur if two “identical” bulbs are, in fact, not quite identical.

(How can you test whether minor differences are due to manufacturing irregularities?|)

Experiment 2.4 con’t. C. On the basis of your observations and the

reasoning you used above, respond to the following questions:

Is current “used up” in the first bulb, or is the amount of the flow the same through both bulbs?

Do you think the order of the bulbs in this circuit might make a difference? Verify your answer by switching the bulbs.

Experiment 2.4 con’t. Do you think the distance between the battery

and the bulb makes a difference? Verify your answer.

Can you tell the direction of flow through the circuit?

How does the amount of the flow through the battery in a single-bulb circuit compare with the flow through the battery in a circuit with two bulbs connected in series?

Exercise 2.5 Student 1: “In this circuit, the flow is from the

battery to the first bulb, where some of the current gets used up. Then the rest flows to the second bulb, where all the remaining current gets used up.”

Student 2: “We know that the current flows back through the battery since we know that we need a complete circuit in order for bulbs to light. If current were used up, there wouldn’t need to be a path back to the battery. Furthermore, the bulbs are equally bright so both must have the same amount of current through them.”

Experiment 2.6 A. Set up a two-bulb circuit with 2 identical

bulbs so that their terminals are attached together as shown.

Two bulbs with their terminals attached together in this way are said to be connected in parallel.

Compare the brightness of each of the bulbs with the brightness of an identical bulb in a single-bulb circuit.

Experiment 2.6 con’t

A con’t: Recall the assumptions we have made in developing our model for electric current. How does the current through a bulb in a single-bulb circuit compare with the current through the same bulb when it is connected in parallel with a second bulb?

Experiment 2.6 con’t B. Compare the brightness of the two

bulbs in the two-bulb parallel circuit with each other. What can you conclude from this observation about the amount of current through each bulb?

Concentrate only on any large differences you may observe, rather than the minor differences that may occur if two “identical” bulbs are, in fact, not quite identical.

Experiment 2.6 con’t C. On the basis of your observations and the

reasoning you used above, respond to the following questions:

What makes the difference? Do you think it is the physical layout of the or the electrical connections? You can investigate this question by comparing what happens:1. When the two bulbs are both on the same side of

the battery and when they are on different sides.2. When each bulb has separate leads to the battery

and when they terminals of the bulbs are connected together and then connected to the battery.

Experiment 2.6 con’t

D. Describe the flow around the entire circuit for the two-bulb parallel circuit. What do your observations of bulb brightness suggest about the way current through the battery divides and recombines at the junctions where the circuit splits into the two parallel branches?Use a diagram or picture to support your

answer.

Experiment 2.6 con’t

E. What can you infer about the relative amounts of current through the battery in a single-bulb circuit and in a circuit in which two identical bulbs are connected in parallel across the battery?

F. Does the amount of current through a battery appear to remain constant or to depend on the number of bulbs in a circuit and how they are connected?

Section 3