1 00:00:00,620 --> 00:00:03,660 Okay, now we're going to talk about Ohm's Law. 2 00:00:04,139 --> 00:00:09,039 Ohm's Law is probably the most fundamental equation in the whole world of electric circuitry. 3 00:00:09,539 --> 00:00:15,220 Anyone who deals with circuits in any way understands and uses Ohm's Law on a regular basis. 4 00:00:15,839 --> 00:00:20,980 And I'm going to explain this in the context of a diagram of a simple circuit here. 5 00:00:21,800 --> 00:00:23,579 And I'll just draw a battery and a light bulb. 6 00:00:23,960 --> 00:00:28,219 Because, once again, we're all familiar with the batteries and light bulbs. 7 00:00:28,219 --> 00:00:29,460 We've seen things like this. 8 00:00:29,460 --> 00:00:50,079 So this is easy to understand. Let's imagine a little light bulb here, and we run a wire from the battery over to the bulb, and then from the bulb back to the other end of the battery, and inside the bulb is the filament, and when the current goes through it, it shines. 9 00:00:50,079 --> 00:00:57,359 whenever you have something like this a circuit element like this light bulb in the circuit there 10 00:00:57,359 --> 00:01:08,379 are three variables to consider there's the voltage and the symbol for the for voltage is v 11 00:01:08,379 --> 00:01:19,810 capital v and voltage is measured in volts so you would say in this case the voltage is 1.5 volts 12 00:01:19,810 --> 00:01:24,810 or if you have the electrical outlet in your home you would say the voltage is 120 volts 13 00:01:24,810 --> 00:01:31,049 unfortunately the symbol for volts is also V so you end up writing things like 14 00:01:31,049 --> 00:01:37,109 this you end up writing things like here V equals 1.5 V which looks like strange 15 00:01:37,109 --> 00:01:41,969 algebra how can V equal one and a half times V that doesn't seem to make sense 16 00:01:41,969 --> 00:01:46,530 but that's not what this says this is not an algebraic algebraic error this 17 00:01:46,530 --> 00:01:52,189 says the voltage the thing is 1.5 volts and this second V over here is the unit 18 00:01:52,189 --> 00:01:55,489 they just happen to have the same letter but it's pretty easy to tell by the 19 00:01:55,489 --> 00:02:01,549 context which is which so you have the voltage in volts you have the resistance 20 00:02:01,549 --> 00:02:11,780 and we call that capital R resistance is exactly what the name implies its 21 00:02:11,780 --> 00:02:16,340 resistance to current flow everything has some resistance well almost 22 00:02:16,340 --> 00:02:21,080 everything almost everything impedes the flow of current in some way and 23 00:02:21,080 --> 00:02:25,159 things that you put in the circuit for example the light bulb has a significant 24 00:02:25,159 --> 00:02:30,020 amount of resistance the wires have very little resistance which is exactly why 25 00:02:30,020 --> 00:02:36,259 we use them in fact the resistance is the of the wires is so small compared to 26 00:02:36,259 --> 00:02:39,400 the resistance of the bulb that we consider the resistance of the wires to 27 00:02:39,400 --> 00:02:43,159 be zero that's really a good approximation it's not quite zero but 28 00:02:43,159 --> 00:02:46,919 it's really close so it's a good approximation to just ignore the 29 00:02:46,919 --> 00:02:50,979 resistance of the wires the resistance to current flow in this circuit 30 00:02:50,979 --> 00:02:55,840 primarily comes from the bulb and anything that you put into the circuit 31 00:02:55,840 --> 00:02:59,139 you could put a light bulb there a heater a toaster anything has some 32 00:02:59,139 --> 00:03:07,599 electrical resistance and resistance is measured in ohms ohms and you recognize 33 00:03:07,599 --> 00:03:11,379 the name ohm there we'll talk about George ohm in just a second resistance 34 00:03:11,379 --> 00:03:17,280 is measured in ohms and the symbol for ohms is the Greek letter Omega this is 35 00:03:17,280 --> 00:03:21,659 the last letter of the Greek alphabet and it looks kind of like a horseshoe 36 00:03:21,659 --> 00:03:25,259 like that so you might say something like this you might say resistance 37 00:03:25,259 --> 00:03:32,280 equals six ohms or the resistance equals a thousand ohms that's just how that 38 00:03:32,280 --> 00:03:41,590 symbol is used and then the third thing to consider is the the current and the 39 00:03:41,590 --> 00:03:47,530 symbol for current is I the letter C is used for other things so that's not the 40 00:03:47,530 --> 00:03:52,349 most convenient or intuitive symbol, but just roll with that. Just take that as a fact. Current is 41 00:03:52,349 --> 00:03:59,770 commonly represented by the letter I, and it's measured in amps, and the symbol for amps is A, 42 00:04:00,009 --> 00:04:05,069 which is kind of nice. That makes sense. Sometimes you hear the term, instead of amps, you hear 43 00:04:05,069 --> 00:04:13,210 amperes, and that's the correct term. This is named after a person, Andre Ampere, a French 44 00:04:13,210 --> 00:04:18,069 physicist who did a lot of work with electrical theory at the time this was 45 00:04:18,069 --> 00:04:21,550 being discovered and amps is just short for amperes and you hear it said 46 00:04:21,550 --> 00:04:27,310 both ways amps or amperes so someone might write for example the current is 47 00:04:27,310 --> 00:04:34,209 5 amps or they might write the current is 20 amps that's just how the symbol I 48 00:04:34,209 --> 00:04:38,889 is used and the A is used in this context I is the current and it's 49 00:04:38,889 --> 00:04:46,149 measured in amps amps is the unit now here's Ohm's law these three things the 50 00:04:46,149 --> 00:04:51,610 voltage the resistance and the current are all related by a nice simple 51 00:04:51,610 --> 00:05:03,459 mathematical equation this is it V equals I times R V is the voltage I is 52 00:05:03,459 --> 00:05:15,500 the current and R is the resistance and this equation is known as Ohm's law and 53 00:05:15,500 --> 00:05:23,410 and it's not really a law in the sense that it that it's universally applicable 54 00:05:23,410 --> 00:05:24,689 like other laws of physics 55 00:05:24,689 --> 00:05:29,529 but it does work incredibly well for most conductors across a wide range of 56 00:05:29,529 --> 00:05:30,790 temperatures and conditions 57 00:05:30,790 --> 00:05:35,589 so this does accurately describe the way things behave in the real world 58 00:05:35,589 --> 00:05:39,069 the voltage flowing through something is always equal to the current 59 00:05:39,069 --> 00:05:42,490 times the resistance now here's a picture of George Ohm 60 00:05:42,490 --> 00:05:47,050 this guy was a schoolteacher and he was conducting electrical experiments 61 00:05:47,050 --> 00:05:50,490 and this was soon after the electric battery had been invented 62 00:05:50,490 --> 00:05:53,870 and before the electric battery people were only able to 63 00:05:53,870 --> 00:05:57,490 do experiments with static electric charges and they had these little devices 64 00:05:57,490 --> 00:05:58,709 called Leiden jars 65 00:05:58,709 --> 00:06:02,350 that they could use to store up some pretty big static charges 66 00:06:02,350 --> 00:06:05,649 but it would discharge real quickly they couldn't do they couldn't do 67 00:06:05,649 --> 00:06:07,870 experiments with any sustained current flow 68 00:06:07,870 --> 00:06:11,069 once the battery was invented and that was in the year 1800 69 00:06:11,069 --> 00:06:15,529 that allowed scientists and physicists to begin to experiment 70 00:06:15,529 --> 00:06:19,970 with the continuous flow of current and Ohm discovered this in his experiments 71 00:06:19,970 --> 00:06:23,029 this fundamental relationship between the voltage and the current and the 72 00:06:23,029 --> 00:06:23,769 resistance 73 00:06:23,769 --> 00:06:28,529 and as as I said before it's one of the most important and most fundamental 74 00:06:28,529 --> 00:06:29,310 equations 75 00:06:29,310 --> 00:06:33,269 in the study of electric circuits now I want to say one more thing about the 76 00:06:33,269 --> 00:06:33,930 equation 77 00:06:33,930 --> 00:06:39,189 Ohm's law which is commonly written like this 78 00:06:39,189 --> 00:06:43,879 V equals IR can be rearranged 79 00:06:43,879 --> 00:06:47,439 algebraically what if I divided both sides 80 00:06:47,439 --> 00:06:52,160 by R and then you see over here on the right side 81 00:06:52,160 --> 00:06:55,560 the R up top and down below will cancel and I'm left with 82 00:06:55,560 --> 00:06:59,600 I equals V over R so let's write it like that 83 00:06:59,600 --> 00:07:07,660 I equals V over R the equation makes a lot of sense 84 00:07:07,660 --> 00:07:11,959 if you think about it this way remember let me write it over here 85 00:07:11,959 --> 00:07:16,019 I equals V over R and this is just 86 00:07:16,019 --> 00:07:20,980 the original equation V equals IR just rearranged algebraically 87 00:07:20,980 --> 00:07:29,569 so I is the current V is the voltage 88 00:07:29,569 --> 00:07:39,560 and R is the resistance so think of 89 00:07:39,560 --> 00:07:43,600 think of voltage remember voltage is what motivates the current flow 90 00:07:43,600 --> 00:07:47,160 you increase the voltage that causes more current to flow 91 00:07:47,160 --> 00:07:50,899 and resistance is that which opposes the current flow 92 00:07:50,899 --> 00:07:53,920 so if you put in something that has higher resistance 93 00:07:53,920 --> 00:07:58,920 That will interfere, that will impede and cause lower current, a lower amount of current to flow. 94 00:07:59,620 --> 00:08:03,579 This fact, these facts show up mathematically in this equation. 95 00:08:03,980 --> 00:08:09,240 You can imagine putting two numbers in here and doing the math, just a division, V divided by R, 96 00:08:09,600 --> 00:08:11,620 and getting out a number for I. 97 00:08:12,000 --> 00:08:12,959 And this makes sense. 98 00:08:12,959 --> 00:08:17,240 If you put in a big number for V right here, then when you do the calculation, 99 00:08:17,420 --> 00:08:19,620 you'll get out a bigger number for I for the current. 100 00:08:19,620 --> 00:08:26,100 and if you put in a big number for the resistance then when you do this calculation you'll be having 101 00:08:26,100 --> 00:08:31,379 a large number be dividing by a large number down here in the denominator so you'll end up with a 102 00:08:31,379 --> 00:08:37,200 smaller value for i so this makes sense increasing if you understand that voltage is what causes 103 00:08:37,200 --> 00:08:41,720 current flow then you understand that increasing the voltage causes more current so putting in a 104 00:08:41,720 --> 00:08:46,299 bigger number for v there gives you a bigger number for your current and if you understand 105 00:08:46,299 --> 00:08:50,340 that resistance is interference with current flow you understand that putting in a bigger number for 106 00:08:50,340 --> 00:08:56,320 r will give you a smaller number for current when you do this calculation so this this equation 107 00:08:56,320 --> 00:09:02,220 makes a lot of sense and it also matches what we see in the real world and that's fundamentally 108 00:09:02,220 --> 00:09:08,460 the test for whether or not something is true in science is does it in fact fit with the actual 109 00:09:08,460 --> 00:09:13,980 data that we see in the real world and in this case it does ohm's law i equals b over r but more 110 00:09:13,980 --> 00:09:18,980 commonly written like this, V equals IR. 111 00:09:18,980 --> 00:09:21,580 And we'll come back next and do some simple example 112 00:09:21,580 --> 00:09:22,980 problems with this equation.