Objectives Unit 6: Electricity and Magnetism Identify the factors influencing the electric force between objects. Explain the interaction between charged and uncharged objects. Design, construct, and explain functional electric circuits. Characterize the force between magnetic poles. State the connection between electricity, magnetism, and motion. Activity 1: Electrostatics Purpose This activity shows you how static electric charges (that is, electric charges that are not moving) interact. Materials Balloons, glass or hard rubber rods, bubble soap, ping-pong balls, empty aluminum cans, water faucet, rabbit fur or your hair, field visualizer bottle Procedure Balloon in your hair 1. Predict: What happens if you rub a balloon in your hair? What happens if you bring the same balloon near someone else s hair? 2. Charge a balloon by rubbing it in your hair (rabbit fur or a wool sweater will also work). What happens as you slowly pull the balloon away from your hair (or fur)? 3. What happens if you slowly bring the balloon near your hair again? 4. Now slowly bring the balloon near the hair of someone who didn t rub a balloon in it. What happens? PHYS 1090 55
Balloons and balloons 5. Charge up two balloons. Holding them very lightly, bring them together. How do they behave? Thy rod and thy glass 6. Now charge a glass or hard plastic rod. (It will probably be easiest to use the rabbit fur for this one.) Try to see if you can get it to move other objects, such as soap bubbles, streams of water, empty aluminum cans, or bits of paper. What works? What happens? 7. How do they behave if you allow the objects to touch the rod first? If you use a balloon instead of the rod? Field visualizer 8. Gently shake the visualizer bottle to disperse the fibers in the oil. Place it on a stand to isolate it. Charge a glass, clear plastic, or hard rubber rod or a balloon by rubbing it with fur. Hold the charged object next to the electric field visualizer bottle. In a little while, the fibers suspended in the oil will align with the electric field surrounding the charge. 9. Sketch the pattern of the fibers in the bottle, also indicating the position of the charged object. PHYS 1090 56
10. Generate a different field around the visualizer bottle. You may do this by placing two charged rods in different positions around the bottle, placing opposite charges in different positions around the bottle, placing an uncharged piece of metal under the bottle as a charged rod is held near the bottle, and so on. Sketch the pattern of the fibers in the bottle, also identifying and indicating the positions of the other objects. Questions to consider Of the various objects you rubbed together and brought near each other, give one example of an attraction and one example of a repulsion. How does the force of attraction or repulsion vary with the distance between the objects? What is the appearance of the electric field around a charged object? PHYS 1090 57
Activity 2: Electric Circuits Purpose In this activity you will work with electric charges moving through the components in circuits. Materials Two batteries in holders, wires, two flashlight bulbs, two sockets, two three-way switches Procedure The most basic circuit 1. Predict: How can you connect a battery, a single wire, and a light bulb to light the bulb? (Yes, it is possible!) 2. Using a single wire, one battery, and one light bulb (no socket), arrange them to light the bulb. (Yes, it is possible!) Do a victory dance. 3. Draw the working circuit you created. 4. Explain why the circuit worked. Explain why another arrangement you tried did not. 5. It turns out that there are four ways to make the bulb light. Each circuit has the components connected in a different order. Find the other three ways and draw them. PHYS 1090 58
Stairway Circuit In many rooms and stairways, the lights can be controlled by either of two switches at opposite ends. If the lights are off, flipping either switch turns them on. If the lights are on, flipping either switch turns them off. Flipping one switch does not change the setting of the other one. How do these circuits work? Your task is to find out. The switches used in this type of circuit are three-way switches: each switch has three leads. The lever arm of the switch can take any of three positions: 1. bridging the left and center leads, 2. in between, so that no two leads are bridged, and 3. bridging the right and center leads. Wall switches used for this type of circuit are spring-loaded so that they skip over position 2. Flipping the switch changes between positions 1 and 3. When you use the switches in this activity, never leave them in the open position (2)! Make them behave like the spring-loaded switches in real stairways. 1. To keep things as straightforward as possible, place the battery in a battery holder and screw the light bulb into a socket. Using the light bulb, battery pack, two three-way switches, and as many segments of wire as you need, create a stairway circuit. 2. When you have made a stairway circuit, do a victory dance. Call me over to show off and explain how your circuit works. Sketch your circuit here. Series and parallel circuits Components can be combined in a circuit in two basic ways: in series and in parallel. In series, current flows through one component and then the other before returning to its origin. In parallel, the path forks so that some current flows through one component and the rest through the other before returning to the source. Bulbs in series 1. Build the circuit illustrated. 2. Are the two bulbs as bright as in a simple circuit with one bulb? 3. Unscrew one bulb. What happens to the other? PHYS 1090 59
Bulbs in parallel 1. Build the circuit illustrated. 2. How does the brightness of the bulbs compare to the series circuit? 3. Unscrew one bulb. What happens to the other? Batteries in series 1. Build the circuit illustrated. 2. How does the brightness of the bulb compare to the circuit with one battery? 3. Remove one battery. What happens? Batteries in parallel 1. Build the circuit illustrated. 2. How does the brightness of the bulb compare to the circuit with one battery? 3. Remove one battery. What happens? Now you will use the three-way switches as on-off switches. Connect wires only to the center post and one of the end posts, ignoring the other end post. Switches in series 1. Build the circuit illustrated. 2. What happens when both switches are closed? PHYS 1090 60
3. What happens when one switch is closed and one open? 4. What happens when both switches are open? 5. This is known as an AND circuit. Why? Switches in parallel 1. Build the circuit illustrated. 2. What happens when both switches are closed? 3. What happens when one switch is closed and one open? 4. What happens when both switches are open? 5. This is known as an OR circuit. Why? Questions to consider How did you make the bulb light using one battery and one wire? How did you set up the three-way switches to make a stairway circuit? What does it mean to combine components in series and parallel? If one car headlight burns out, the other stays on. Are they wired in series or parallel? A 6-V lantern battery is actually constructed by combining four 1.5-V cells. Are they combined in series or parallel? PHYS 1090 61
Activity 3: Magnetic Forces Purpose In this activity you will observe how magnets interact with each other and with other materials. Materials Bar magnets, magnets of other shapes, string, flexible magnetic strip, magnetic field visualizer, white paper, iron filings Procedure (Adapted in part from Electricity and Magnetism: Stop Faking It! Finally understanding science so you can teach it. William C. Robertson. Arlington, VA: National Science Teachers Association, 2005, pp 38 39.) 1. Predict: What sticks to magnets? 2. Predict: How do two magnets interact with each other? Tie a string around the center of one of the bar magnets so you can hold the magnet by the string while the magnet is free to rotate. It s tough to get the magnet to balance, so be patient. Once you have the magnet hanging freely, bring one end of the other bar magnet near it. What happens? 4. Bring the opposite end of the second magnet near the hanging magnet. What happens? 5. Take a couple of the magnets of other shapes and see if they behave the same as your bar magnets. (You don t have to hang one of them from a string, but you can if that makes things clearer.) They won t behave exactly the same simply because of their shape, but you should still notice both attraction and repulsion. Describe or sketch the magnets and the pattern of attraction and repulsion. PHYS 1090 62
6. You undoubtedly know that magnets don t interact just with other magnets, but with some other materials as well. How else would you display the kids artwork on the fridge? Even though you already know that magnets stick to refrigerators, take a few moments and find out what other materials are attracted to magnets. And if you think it's as simple as separating things into metals and non-metals, think again. Try attracting a nickel or a penny with a magnet. What seems to be the rule about materials being attracted by a magnet? 7. While you re messing about with these different materials, see whether it makes a difference which end of a bar magnet you point toward a paper clip or a refrigerator. Does one end attract and the other repel, or do both do the same thing? 8. Place a bar magnet on a field visualizer. Gently shake the visualizer to allow its little magnets to align in their equilibrium positions. Note the pattern they make. Reorient the bar magnet and repeat. What do the little magnets do in the presence of a large magnet? 9. Place a magnet under a sheet of paper and sprinkle a small amount of iron filings evenly on the paper. Filings should be on the paper in locations away from the magnet as well as directly atop it. Sketch the pattern that they form. PHYS 1090 63
10. Repeat with another magnet. (Definitely try this with a flexible refrigerator magnet!) Questions to consider What is the pattern of attraction and repulsion of magnets? How does the magnetic force vary with distance? Did you find any magnets with only one pole? What is the shape of the field around a bar magnet? Refrigerator magnet? PHYS 1090 64
Activity 4: Electromagnetism Purpose You will observe the connection between electricity and magnetism. Materials, procedure, etc. Rectangular current loop, magnetic compasses, swinging wire apparatus, wire coil, galvanometer, batteries or low-voltage DC source, hand generator Rectangular Current Loop This apparatus uses a number of parallel wires to effectively multiply the current and any resulting magnetic effects. A conducting wire winds several times around a rectangular frame and a direct current runs through the energized coil. Near the center of any edge of the rectangle, the apparatus acts like a single large electric current moving in a straight path. 1. Set several magnetic compasses on one of the platforms. Allow the needles to settle into their north-south alignment. Sketch the compasses including their alignment (so that I can tell which way the north-seeking poles of the compasses point). 2. Find which of the terminals of the battery is positive and which is negative. Electric current travels from positive to negative. Connect the wires of the apparatus to the terminals of a battery so that current flows in the wires through the platform. What do the compass needles do? 3. When the compass needles settle down with the current still flowing, sketch the apparatus below. Include the location and alignment of the compasses and the direction of the electric current. 4. Reverse the polarity: connect the wires of the apparatus to the opposite terminals of the battery. Since electric current travels from positive to negative, the current in the PHYS 1090 65
wire through the platform is now flowing in the opposite direction. Sketch the compasses and their alignment below, also indicating the direction of the current. Swinging Wire This apparatus contains a wire loop free to swing. The free end of the loop hangs between the poles of a powerful U magnet. 1. Briefly connect the leads of the apparatus to the battery so that a current momentarily flows through the swinging wire. What does the wire do? 2. Find which pole of the magnet is north and which is south, and which way the current (symbol I ) flows through the wire. Set up all four indicated combinations of magnet and current polarity and find which way the wire swings in each combination. Record the direction that the wire moves on the diagram. 3. Remove the magnet. Now what does the wire swing do when a current runs through it? Magnet and Coil This apparatus involves a coil of wire electrically connected to a galvanometer, which measures electric current. A bar magnet is also provided. 1. How can you make the galvanometer needle deflect? PHYS 1090 66
2. How can you influence the magnitude of the galvanometer deflection? How can you influence its direction? 3. What happens when you hold the magnet still inside the coil? 4. What happens when the coil wires are reversed to the galvanometer? Hand generator 1. Close the switch so that the light bulb makes a complete circuit with the generator. Crank the generator so that the bulb lights. What do you feel? 2. Open the switch, breaking the circuit. Crank the generator at least as fast as before. What do you feel now? 3. Explain the difference. Questions to consider Which direction did the compass needles around the straight section of currentcarrying wire point? How did the current-carrying loop of wire respond to being in a magnetic field? When did a magnet near a coil cause deflection of the galvanometer needle? Which activities used motion to create an electric current? Which activities used an electric current to create motion? Which is harder: cranking an isolated generator, or cranking a generator that powers a light bulb? PHYS 1090 67