Science Olympiad Division B Shock Value Use a

Science Olympiad Division B Shock Value

Use a single “D” battery, a single bare wire and a light bulb. Find four different ways to light the light bulb using only a battery, one wire and the bulb. Quite often one thinks that they must run electricity from the battery to the bulb. They try from either end of the battery. The bulb won’t light! If the wire is HOT the bulb won’t light. Disconnect the wire from the battery and try something else. So, what will work? Shock Value 2

Use a single “D” battery, a single bare wire and a light bulb. Find four different ways to light the light bulb using only a battery, one wire and the bulb. There are two positions of the bulb at each end of the battery. The wire must go from a different part of the bulb to the other end of the battery. Shock Value 3

Obviously, there are two important parts of the battery. The positive terminal and the negative terminal. These terminals are at opposite ends of the battery. - + If you look closely at the bulb, there is a filament that glows when it is properly wired. Many filaments are coiled. You will also notice that one end of the filament connects to the tip of the bulb and the other to the threads. Shock Value 4

In order to light the bulb with two wires and the bulb away from the battery, one wire must go from one terminal of the battery (+) to the tip of the bulb. The other wire must go from the other terminal of the battery (-) to the threads of the bulb. It does not matter which wire goes to the tip and which goes to the thread as long as one end of the battery is connected to one part of the bulb and the other terminal is connected to the other part of the bulb. - + Now we can say that we have a complete circuit. Shock Value 5

How would one define a complete circuit, which we will call just a circuit? A pathway for charge to flow from one terminal of the battery, through the light bulb and back to the other terminal of the battery. A battery is an example of a “SOURCE. ” A source is a device that converts some form of energy into electrical energy. A battery converts chemical energy into electrical energy (EE). - + A generator converts mechanical energy into EE. A solar (photovoltaic) cell converts light (radiant) energy into EE. Shock Value 6

A light bulb is an example of a “LOAD. ” A load is a device that converts electrical energy into some other form of energy. A light bulb converts EE into thermal (heat) energy and light (radiant) energy. Incandescent bulbs need a lot of heat to have the filament glow and give off a lot of light. Other types of bulbs, CFL’s, LED’s, etc. are more effective in converting EE into light with little or no heat. Which do you think require less electricity? - + A motor converts EE into mechanical energy. A heater converts EE into thermal energy (heat). Shock Value 7

SCHEMATIC SYMBOLS Battery Ammeter A Light Bulb Voltmeter V Resistor Wire Switch (open) Junction (wires connected) Switch (closed) Shock Value 8

Photo of set-up. My sketches are terrible. Schematic of the same circuit. What does a switch do? How does it do it? Switch (closed) A closed switch completes the circuit or turns the bulb on and an open switch breaks the circuit or turns the bulb off. Shock Value Switch (open) 9

Using an Ammeter An ammeter measures the current flowing through the circuit. Current is the flow of charge through the wires. In physics the unit for charge is a coulomb. The symbol for current is an “I” and the unit is an ampere which is abbreviated amp or A. An amp is a coulomb per second. An ammeter is wired in series in a circuit. That means that you have to break the circuit to insert the ammeter. I would recommend an analog ammeter. When my students used the ammeter on the multimeter, they usually made a mistake and blew the fuse. It is easy to blow the fuse and not too easy to replace it. Shock Value 10

Using an Ammeter A series circuit means that there is only one pathway for the current to flow. If you look at the circuit, the current must pass through the ammeter and then the bulb and then the switch. This leads to an interesting question. What is the direction of the current? We know that electrons move in a metal wire. So electrons flow out of the negative terminal of the battery, go through the ammeter, the light bulb, and the switch and return to the positive terminal of the battery. Shock Value e- A But, years ago we did not know about electrons! So what then? 11

Benjamin Franklin is credited with the idea of positive and negative charge. Positive meant an abundance of electrical charge. So it made sense to him that positive charge flowed from an abundance of charge (+) to and absence of charge (-). If you look in a physics book it will say that current is the flow of positive charge from the positive terminal of the battery, through the switch, the light bulb, and the ammeter and return to the negative terminal of the battery. + A Many electronics books use the flow of electrons for current. Make sure that you look to see which way current is defined. Also, you may want to clarify this with the event supervisor. Shock Value 12

For the rest of this presentation I will use electron flow for current. e- A Shock Value 13

Using an Ammeter Does it matter where we put an ammeter in a circuit? What happens to the current as it passes through the light bulb? Let’s put the ammeter on the opposite side of the circuit. What will be different about the current? a) The current will be less. b) The current will be the same. c) The current will be greater. Make your prediction. Shock Value A Now wire it and see for yourself. 14

Hopefully, by now it is obvious that the current into the bulb is the same as the current out of the bulb. A A So, it does not matter where you put the ammeter or the switch in a series circuit. The current everywhere is the same! Shock Value 15

If the values for the current are not the same, make sure that you have good connections in the circuit. To convince you I put two ammeters in this circuit; one before the bulb and one after the bulb. Shock Value 16

5 Multi-scale ammeter. Here you have to look at where the wires are connected. There is always one wire connected to the black (-) terminal. 500 50 If the other wire is connected to the “ 50” terminal, you read the 50 m. A scale. 5 500 50 - If the other wire is connected to the “ 500” terminal, you read the 500 m. A scale. If the other wire is connected to the “ 5” terminal, you read the 5 A scale. Only one scale can be used at a time! Shock Value 17

Voltage or Potential Difference So, the current does not change when it passes through a light bulb, then what does cause the bulb to light? To answer this there is another property that we need to measure. A voltmeter measures the voltage (potential) gain at the battery (source) or the voltage (potential) drop or loss at the light bulb (load). Look back at sources and loads. A voltmeter is wired in parallel to the circuit. You do not need to break the circuit. All you have to do is touch the red (+) probe of the voltmeter to the positive terminal of the battery and the black (-) probe of the voltmeter to the negative terminal of the battery. If done properly the reading should be positive. If you get a negative value you either connected the probes backwards on the voltmeter or to the battery. A parallel circuit has multiple paths. You are actually diverting a very tiny current through the voltmeter in order to get a reading. You will observe series and parallel circuits for light bulbs later. The symbol for voltage is a capital V and the unit is a volt, V. Shock Value 18

Voltage or Potential Difference Orienting a voltmeter is a little more challenging than an ammeter. Touch the red (+) probe of the voltmeter to the positive terminal of the battery. Touch the black (-) probe of the voltmeter to the negative terminal of the battery. If done properly the reading should be positive. If you get a negative value you either connected the probes backwards on the voltmeter or to the battery. WHY? If you move the probes together around the circuit to the light bulb you should have the probes connected in the following orientation and the reading should be negative. The value should be close to the reading at the battery, except it is negative. Shock Value 19

Remember that electrons flow around the circuit from the negative terminal of the battery through the loads along the way and back to the positive terminal of the battery. The size of the current never changes. e- A One way to illustrate this is to say that the electrons pick up energy at the battery (positive voltage or voltage gain) and gives up its energy at the light bulb (negative voltage or voltage loss). You may find that the voltage loss at the light bulb is a little bit less than the voltage gain at the battery. This is due to losses in the ammeter, switch, wires; poor connections; or a combination of both. Keep in mind that energy is conserved. So as one goes around the circuit in a complete loop, the voltage gains must be equal to the voltage losses. Shock Value 20

Two Bulbs in Series You will probably find that the sum of the voltage losses at the bulbs, V 1 + V 2, is close in value to the voltage gain at the battery, VS. Remember there can be other losses. You will notice that the bulbs are very dim or maybe out, but there is still current registered on the ammeter. If you are lucky the bulbs are identical and they are both the same brightness. Chances are they are not and one is dim and the other is dimmer or out. Note: In any complete loop around the circuit from the negative terminal of the battery, through the loads, to the positive terminal of the battery, and back to the negative terminal, that the sum of the voltage gains equal the sum of the voltage losses. Shock Value 21

Two Bulbs in Series What happens when you unscrew Bulb #1? Bulb # 2 goes out. The ammeter indicates that the current, I , is zero. The circuit is broken! What happens when you unscrew Bulb #2? Bulb # 1 goes out. The ammeter indicates that the current, I , is zero. The circuit is broken! Some of the older Christmas lights were wired in series and when one burned out they all went out. This drove many a person crazy trying to find out which one blew out. Shock Value 22

Two Bulbs in Series 4. "Christmas Lights" are often wired in series just like the circuit you just tested. a) What happens when one of the lights burns out? Why? As we just saw, as one bulb goes out (the filament breaks, which is just like unscrewing a bulb) the circuit is broken and there is no pathway for the current and all the bulbs go out. The challenge then is to figure out which one went out and replace it. Shock Value 23

Two Bulbs in Series 4. "Christmas Lights" are often wired in series just like the circuit you just tested. b) We can make a string of lights in series just like the "Christmas Lights. " If you were given light bulbs that are designed to operate with a voltage of 14 volts, how many would you have to wire in series before it is connected to a 120 -volt outlet? Remember that the sum of the voltage gains equals the sum of the voltage losses. If the bulb is designed for 14 volts then 14 x = 120 or x = 8. 57. I will need nine bulbs. I had a bunch of 14 volt bulbs and wired them and it does work. CAUTION! I would not recommend trying this unless you really know what you are doing. And DEFINITELY, DO NOT LET A STUDENT DO THIS! A better suggestion: If you have an old 80 or 100 bulb string. Cut off a socket and strip the wires and then wire it to a 1. 5 volt AA, C or D battery and watch it light up. Shock Value 24

Two Bulbs in Series 4. "Christmas Lights" are often wired in series just like the circuit you just tested. c) If you have a one hundred bulb string set of lights that are wired in series, what is the voltage drop across each bulb? ΣVGAIN = ΣVLoss 120 V = V 1 + V 2 + … + V 100 120 V = 100 V V = 1. 20 volts ΣVGAIN = ΣVLoss The voltage drop across each bulb is 1. 2 volts Shock Value 25

Two Bulbs in Series So, what can we do to make both bulbs bright again? If we now put two batteries in series, connecting the negative terminal of the second with the positive terminal of the first, what happens? The voltage gain is doubled, the current increases, and the bulbs return to their former brightness when one bulb was wired to one battery. AGAIN Note: In any complete loop around the circuit from the negative terminal of the battery, through the loads, to the positive terminal of the battery, and back to the negative terminal, that the sum of the voltage gains equal the sum of the voltage losses. There is still only one path for current to travel through the circuit. Shock Value 26

Two Bulbs in Parallel What is different when you wire two bulbs in parallel? e- You might want to try this with one battery first. Both bulbs are bright. V 1 = V 2 = VS Trace the path taken by the current. The current leaves the battery and it all goes through the ammeter illustrated. The current comes to a junction and splits, some going through Bulb #1 and the other going through Bulb #2. The current rejoins at the next junction goes through the switch and returns to the battery. Note that there are two pathways between the junctions, Parallel circuit. Shock Value 27

Two Bulbs in Parallel What happens when you unscrew bulb #1? Bulb #1 goes out. Bulb #2 stays lit about the same. The current drops (roughly in half). VS = V 2 and stays about the same as before bulb #1 was unscrewed. What happens when you screw Bulb #1 back in and unscrew Bulb #2? Bulb #2 goes out. Bulb #1 stays lit about the same. The current drops (roughly in half). VS = V 1 and stays about the same as before bulb #1 was unscrewed. . Shock Value 28

How is a home wired? Using what you observed in series and parallel circuits, do you think that your house in wired in series or parallel? Explain your answer. Let’s look at the options. If your house were wired in series, what would happen if one light bulb blew out or was turned off? All the other lights and appliances would go out! That’s no good! If your house were wired in parallel, what would happen if one light bulb blew out or was turned off? All the other lights and appliances would stay on! That’s good! Shock Value 29

Resistance When you place a meter long piece of copper wire in the circuit, there should be no significant change in the current. The voltage drop across the copper wire, VCW, will be very small. At most VCW = 0. 02 V The voltage drop across the light bulb, VB, will be close to VS. Copper wire is a good conductor so it is said to have little resistance. Shock Value 30

Resistance When you replace the copper wire with a meter long piece of nichrome wire, what is different? It works best if you have 28 or 30 gage nichrome wire. You may be able to get a piece from your physics department in the high school. The current in the ammeter goes down quite a bit. VNW becomes sizable usually more than VB. The bulb is barely lit or may not glow at all. VS = VNW + VB. Shock Value 31

Resistance Nicrome wire is said to have a lot of resistance. What is resistance? Resistance is the property of the wire the inhibits the flow of charge. Resistance (R) is the ratio of the voltage drop across the device to the current passing through the device. R = V/I and its unit is called an ohm, abbreviated Ω. Named after Georg Simon Ohm. Shock Value 32

Resistance As you shorten the nichrome wire you will notice that: 1) The current increases. 2) VNW decreases. 3) VB increases. 4) The bulb finally starts to glow or gets brighter. What qualities of a given wire affects its resistance? Length, the longer the wire the greater the resistance. Thickness or diameter, the thicker the wire the resistance is less. Shock Value 33

Resistance Nichrome wire will glow and get hot with a great enough voltage and current. Nichrome wire is used to give off heat in a toaster, hair dryer or space heater. This is fun to do. If you go to Radio Shack and get a cheap rheostat and replace the nichrome wire. As you turn the knob the same thing will happen when you changed the length of the nichrome wire. A rheostat acts as a dimmer switch or a volume control by varying the resistance. Shock Value 34

Resistance The symbol for a rheostat is: The schematic of the circuit is: Shock Value A 35

Short Circuit When you do this section, only close the switch long enough to take the required readings. You are KILLING the battery! What happens when you place the wire between the terminals of Bulb #1? the current, I, increases. Bulb #1 goes out. Bulb #2 gets brighter. VS stays the same or increases slightly. The wire gave a low resistance path around Bulb #1. Shock Value 36

Short Circuit When you do this section, only close the switch long enough to take the required readings. You are KILLING the battery! What happens when you place the wire between the terminals shown? the current, I, increases a lot. Bulbs #1 & #2 go out. VS goes to zero or close to it. The wire gave a low resistance path from one terminal of the battery to the other, mostly bypassing the bulbs. Shock Value 37

Short Circuit When you do this section, only close the switch long enough to take the required readings. You are KILLING the battery! What happens when you place the wire between the terminals shown? the current, I, increases a lot. Bulbs #1 & #2 go out. VS goes to zero or close to it. The wire gave a low resistance path from one terminal of the battery to the other, mostly bypassing the bulbs. Shock Value 38

THE END! Shock Value 39