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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.

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Presentation on theme: "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."— Presentation transcript:

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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. 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! So, what will work? Shock Value2

3 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 Value3

4 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 Value4

5 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. Shock Value5 Now we can say that we have a complete circuit.

6 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 is an example of a “SOURCE.” A source is a device that converts some form of energy into electrical energy. + - Shock Value6 A generator converts mechanical energy into EE. A battery converts chemical energy into electrical energy (EE). A solar (photovoltaic) cell converts light (radiant) energy into EE.

7 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 is an example of a “LOAD.” A load is a device that converts electrical energy into some other form of energy. + - Shock Value7 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 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. A motor converts EE into mechanical energy. A heater converts EE into thermal energy (heat).

8 SCHEMATIC SYMBOLS Shock Value8 Battery Light Bulb Switch (open) Resistor Ammeter Voltmeter Wire Junction (wires connected) Switch (closed)

9 Shock Value9 Switch (closed) Switch (open) Photo of set-up. My sketches are terrible. Schematic of the same circuit. What does a switch do? How does it do it? A closed switch completes the circuit or turns the bulb on and an open switch breaks the circuit or turns the bulb off.

10 Using an Ammeter Shock Value10 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.

11 Using an Ammeter Shock Value11 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 now 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. But, years ago we did not know about electrons! So what then?

12 Shock Value12 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. 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.

13 Shock Value13 For the rest of this presentation I will use electron flow for current.

14 Using an Ammeter Shock Value14 Does it matter where we put an ammeter in a circuit? What happens to the current as it passes through the light bulb? 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. 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. Now wire it and see for yourself.

15 Shock Value15 Hopefully, by now it is obvious that the current into the bulb is the same as the current out of the bulb. So, it does not matter where you put the ammeter or the switch in a series circuit. The current everywhere is the same!

16 Shock Value16 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. 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.

17 Multi-scale ammeter. Shock Value17 -- If the other wire is connected to the “50” terminal, you read the 50 mA scale. 50 If the other wire is connected to the “500” terminal, you read the 500 mA scale. 500 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!

18 Voltage or Potential Difference Shock Value18 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.

19 Voltage or Potential Difference Shock Value19 Touch the red (+) probe of the voltmeter to the positive terminal of the battery. Orienting a voltmeter is a little more challenging than an ammeter. 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. 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. WHY?

20 Shock Value20 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. 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.

21 Two Bulbs in Series Shock Value21 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, V S. 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.

22 Two Bulbs in Series Shock Value22 What happens when you unscrew Bulb #1? Bulb # 2 goes out. The ammeter indicates that the current, I,is zero. The circuit is broken! Bulb # 2 goes out. The ammeter indicates that the current, I,is zero. The circuit is broken! What happens when you unscrew Bulb #2? 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. Bulb # 1 goes out. The ammeter indicates that the current, I,is zero. The circuit is broken! Bulb # 1 goes out. The ammeter indicates that the current, I,is zero. The circuit is broken!

23 Two Bulbs in Series Shock Value23 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? 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.

24 Two Bulbs in Series Shock Value24 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? 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 14x = 120 or x = I will need nine bulbs. I had a bunch of 14 volt bulbs and wired them and it does work. 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.

25 Two Bulbs in Series Shock Value25 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? 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? ΣV GAIN = ΣV Loss 120 V = V 1 + V 2 + … + V V = 100V V = 1.20 volts ΣV GAIN = ΣV Loss 120 V = V 1 + V 2 + … + V V = 100V V = 1.20 volts ΣV GAIN = ΣV Loss The voltage drop across each bulb is 1.2 volts ΣV GAIN = ΣV Loss The voltage drop across each bulb is 1.2 volts

26 Two Bulbs in Series Shock Value26 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? 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. The voltage gain is doubled, the current increases, and the bulbs return to their former brightness when one bulb was wired to one battery.

27 Two Bulbs in Parallel Shock Value27 What is different when you wire two bulbs in parallel? You might want to try this with one battery first. Both bulbs are bright. V 1 = V 2 = V S Both bulbs are bright. V 1 = V 2 = V S Trace the path taken by the current. The current leaves the battery and it all goes through the ammeter illustrated.

28 Two Bulbs in Parallel Shock Value28 What happens when you unscrew bulb #1? Bulb #1 goes out. Bulb #2 stays lit about the same. The current drops (roughly in half). V S = V 2 and stays about the same as before bulb #1 was unscrewed. Bulb #1 goes out. Bulb #2 stays lit about the same. The current drops (roughly in half). V S = 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). V S = V 1 and stays about the same as before bulb #1 was unscrewed.. Bulb #2 goes out. Bulb #1 stays lit about the same. The current drops (roughly in half). V S = V 1 and stays about the same as before bulb #1 was unscrewed..

29 How is a home wired? Shock Value29 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!

30 Resistance Shock Value30 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, V CW, will be very small. At most V CW = 0.02 V Copper wire is a good conductor so it is said to have little resistance. The voltage drop across the light bulb, V B, will be close to V S.

31 Resistance Shock Value31 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. V NW becomes sizable usually more than V B. The bulb is barely lit or may not glow at all. V S = V NW + V B. The current in the ammeter goes down quite a bit. V NW becomes sizable usually more than V B. The bulb is barely lit or may not glow at all. V S = V NW + V B.

32 Resistance Shock Value32 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. R = V/I and its unit is called an ohm, abbreviated Ω. Named after Georg Simon Ohm.

33 Resistance Shock Value33 As you shorten the nichrome wire you will notice that: 1)The current increases. 2)V NW decreases. 3)V B increases. 4)The bulb finally starts to glow or gets brighter. As you shorten the nichrome wire you will notice that: 1)The current increases. 2)V NW decreases. 3)V B 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.

34 Resistance Shock Value34 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.

35 Resistance Shock Value35 The symbol for a rheostat is: The schematic of the circuit is:

36 Short Circuit Shock Value36 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. the current, I, increases. Bulb #1 goes out. Bulb #2 gets brighter. V S stays the same or increases slightly. The wire gave a low resistance path around Bulb #1. V S stays the same or increases slightly. The wire gave a low resistance path around Bulb #1.

37 Short Circuit Shock Value37 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. the current, I, increases a lot. Bulbs #1 & #2 go out. V S 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. V S 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.

38 Shock Value38 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. the current, I, increases a lot. Bulbs #1 & #2 go out. V S 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. V S 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.

39 Shock Value39 THE END!


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