Physics

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Section 1 WHAT IS HAPPENING IN THE WIRES?

INTRODUCTION Electricity is usually invisible. Except for lightning and sparks, you never see it in daily life. However, light bulbs and a magnetic compass can show you when something electrical is happening. By observing their behavior and making a few assumptions, you can begin forming ideas about electricity. This type of thinking is called “building a model”. INVESTIGATION ONE: WHAT IS NEEDED TO LIGHT A BULB? 1.1 Activity: Lighting bulbs in a loop Insert three D-cells into the battery holder (as in Figure 1.1), and insert two ROUND bulbs (not long bulbs) into a pair of sockets. Use three wires to connect the sockets to each other and to the two “terminals” of the battery holder: 1) the spring inside the case near the red spot, and 2) the metal post on the outside of the case near the blue spot. The bulbs should light and be of similar brightness. The battery, bulbs and wires now form a “closed loop”.

Black Plastic Battery Case
Blue Spot on Case

Connect wire to Spring Wire Round Bulb in Socket

Wire

Wire
Figure 1.1 BASIC CLOSED LOOP

Round Bulb in Socket

1. “Break” the loop by disconnecting a wire from one end of the battery holder; then reconnect the loop. Do you see both bulbs light at exactly the same time? Do you believe that both bulbs actually light at the same time? Do both bulbs appear to go out at exactly the same time? (We will return to this question later; for now, just report your observations as well as possible.)

2. Reconnect the wire to the battery, and then disconnect a different wire somewhere else in the loop. Try doing this in several places. Be sure that you have only one break in the loop at a time. Is there any place where you can break the loop and one or both of the bulbs will still stay lit? PASCO scientific Student Manual 1

3. Unhook any wire and then bring it back as close as you can to where it was connected — without actually making contact. Do this slowly and carefully, watching the space between the wire and the contact point. Do the bulbs light? Do you think actual contact is needed for the bulbs to light continuously?

INVESTIGATION TWO: IS ANYTHING HAPPENING IN THE WIRES? 1.2 Activity: Using the compass to investigate a closed loop The magnetic compass in your kit can be used to detect electrical activity in the wires during bulb lighting. Read and follow these instructions very carefully: 1. Place the compass on the table top, as far away as possible from any metal parts. Tape the compass to the table — masking tape works best. (Place a rolled piece of masking tape below the compass.) Note that the compass is not connected to any wire. It is a detector for what is happening in the wires. 2. Stretch the loop out as far as possible; keep the battery as far from the compass as you can. (The steel case of the D-cells may have become magnetized and will interfere with the compass reading.) 3. Disconnect the loop somewhere. Place a wire, which is attached to the battery on top of the compass (Figure 1.2a), and align this wire parallel to the needle of the compass and directly over the needle.

Figure 1.2a COMPASS TAPED IN PLACE

When you have assembled the loop in Figure 1.2a, connect and disconnect a wire several times while you observe the compass needle. It’s a good idea for one person to hold the wire on top of the compass firmly while another connects and disconnects the loop. 1. Does the compass needle deflect clockwise or counter-clockwise when you connect the wire to close the loop? What happens to the compass needle when the battery is disconnected to ‘break’ the loop? Close Loop: Break Loop: Clockwise Clockwise Counter-clockwise (Circle one) Counter-clockwise (Circle one)

2. Is there any evidence that something is happening in the wire over the compass during the time the loop is broken? What is the evidence, for or against? PASCO scientific Student Manual 2

Do not move the compass. Break the loop...
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