Ohm’s Law Tutorial with easy practice problems | Basic Circuits

Summary

This tutorial delves into Ohm's Law, a fundamental principle in electrical engineering that links voltage, current, and resistance. The presenter explains that Ohm's Law is expressed as V=IR, where V is voltage, I is current, and R is resistance. They clarify the linear relationship between these variables and how changes in one affect the others. The video includes practical examples to demonstrate how to calculate current using Ohm's Law in simple circuits. It also touches on the concepts of resistivity, conductivity, and the significance of Ohm's Law in various electrical applications. The presenter emphasizes the importance of understanding Ohm's Law for future studies in circuit analysis and electrical engineering.

Takeaways

• 🔌 Ohm's Law is a fundamental principle in electrical engineering that relates voltage, current, and resistance in a circuit.
• 🌊 The script uses the analogy of water flow to explain the concept of resistance, which is a force that opposes the flow of current in a circuit.
• ⚡ Ohm's Law is mathematically expressed as V = I * R, where V is voltage, I is current, and R is resistance. This formula can be rearranged to solve for any of the variables.
• 🔄 The relationship between voltage, current, and resistance is linear, meaning changes in one variable will proportionally affect the others.
• 🔍 Resistivity is mentioned as a different concept from resistance, but it is not the focus of the script.
• 📐 Ohm's Law is applicable to resistors, which are considered linear components in most cases for simplification in calculations.
• 🔄 The script emphasizes the importance of understanding the direction of current flow and keeping consistent with the chosen perspective throughout calculations.
• 🌐 The concept of 'ohms' as a unit of measurement for resistance is introduced, along with the concept of conductivity, measured in siemens or mhos.
• ⚠️ Two extreme cases of Ohm's Law are explained: a short circuit (R = 0, resulting in infinite current) and an open circuit (R = ∞, resulting in zero current).
• 📝 Practical examples are provided to demonstrate how Ohm's Law can be applied to simple circuits to calculate current, emphasizing the importance of understanding relative voltage differences rather than absolute values.
• 📚 The script concludes by highlighting the importance of Ohm's Law in further studies of circuits and electrical engineering.

Q & A

What is Ohm's Law and how does it relate to voltage, current, and resistance?

Ohm's Law is a fundamental principle in electrical engineering that states the relationship between voltage (V), current (I), and resistance (R). It is expressed as V = I * R, meaning the voltage across a resistor is equal to the current flowing through it times the resistance of the resistor.

What is the significance of Ohm's Law in circuit analysis?

Ohm's Law is significant because it provides a linear relationship between voltage, current, and resistance, which is a foundational concept used in all levels of electrical engineering and circuit analysis. It allows for the calculation of unknown quantities in a circuit when two of the three are known.

Can Ohm's Law be rearranged to solve for different variables?

Yes, Ohm's Law can be rearranged to solve for any of the three variables (voltage, current, or resistance). For example, it can be rearranged to I = V/R to find the current, or R = V/I to find the resistance.

What is the difference between resistivity and resistance mentioned in the script?

Resistivity is a material property that quantifies how strongly a given material opposes the flow of electric current. Resistance, on the other hand, is the opposition to the flow of electric current in a specific component or part of a circuit, which depends on the resistivity of the material, its dimensions, and shape.

What are the two extreme cases of Ohm's Law mentioned in the script?

The two extreme cases of Ohm's Law mentioned are when resistance is zero (short circuit) and when resistance is infinite (open circuit). In the short circuit case, the current would be infinite if the voltage is not zero. In the open circuit case, the current would be zero regardless of the voltage.

What is the term used for the reciprocal of resistance, and what are its units?

The reciprocal of resistance is called 'conductivity'. Its units can be expressed in mhos or siemens (symbolized by 'S'). For example, if a resistor has 100 ohms of resistance, its conductivity would be 1/100 siemens.

How does the script illustrate the concept of voltage being relative?

The script illustrates the concept of voltage being relative by explaining that the voltage across a resistor is the difference in potential between two points, regardless of the absolute values of those points. This means that the voltage is comparative and depends on the potential difference, not the actual values.

What does the script suggest about the direction of current in a circuit?

The script suggests that the direction of current in a circuit is a matter of perspective and should be consistently maintained once established for the purpose of calculations. It emphasizes that the direction should not be changed midway through calculations to avoid confusion.

How does the script handle negative voltage values in circuit analysis?

The script handles negative voltage values by treating them as relative to an arbitrarily decided zero potential point. It emphasizes that the key is the potential difference between two points, and the actual sign (positive or negative) does not change the analysis as long as the relationship is consistent.

What advice does the script give for solving problems using Ohm's Law?

The script advises to write down the chosen direction of current and the setup of the problem once decided, and to stick with it throughout the calculations. It warns against changing the direction or setup midway, as this can lead to confusion and errors.

Outlines

00:00

🔋 Understanding Ohm's Law

The first paragraph introduces Ohm's Law, explaining the relationship between voltage, current, and resistance. It discusses the concept of resistance, how it affects the flow of current, and presents Ohm's Law as a mathematical equation: Voltage (V) equals Current (I) times Resistance (R). The paragraph also emphasizes the linear relationship between these variables and how this principle is foundational in electrical engineering, crucial for circuit analysis.

05:00

⚡ Short Circuits and Open Circuits

The second paragraph discusses the extremes of Ohm's Law: short circuits and open circuits. It explains that a short circuit occurs when resistance is zero, leading to an infinite current, often causing circuit damage. Conversely, an open circuit happens when resistance is infinite, resulting in zero current flow. These concepts are critical in diagnosing and understanding circuit behavior, with practical examples provided to illustrate these scenarios.

10:02

🔄 Importance of Voltage Potential

In the third paragraph, the focus is on the concept of voltage potential and its relative nature. The example given shows how voltage differences across a resistor affect current flow, using simple circuit setups to demonstrate the point. The paragraph also touches on the importance of consistency in analyzing circuit direction and potential, emphasizing that voltage is relative and must be considered in context.

15:04

💡 Practical Application of Ohm’s Law

The final paragraph wraps up the discussion by reinforcing the application of Ohm's Law in practical circuit analysis. It reiterates that understanding voltage differences and maintaining a consistent approach is key to accurate calculations. The paragraph also encourages further learning through additional examples and resources, while highlighting the enjoyment and importance of circuit analysis in electrical engineering.

Keywords

💡Ohm's Law

Ohm's Law is a fundamental principle in electrical engineering that relates voltage, current, and resistance in a circuit. Defined as V = I * R, where V is voltage, I is current, and R is resistance, it forms the basis for understanding electrical circuits. In the video, Ohm's Law is used to demonstrate how changes in voltage or resistance affect the current flowing through a resistor, highlighting its linear relationship.

💡Voltage

Voltage, often measured in volts, is the electric potential difference between two points in a circuit. It's akin to the pressure that pushes electric charge through a conductor. In the script, voltage is used to illustrate how lifting a glass higher (in a metaphorical sense) results in more current when the resistance is constant.

💡Current

Current, measured in amperes or amps, is the flow of electric charge in a circuit. It's the quantity of electricity passing through a point in a circuit per unit time. The script explains how current changes with variations in voltage or resistance, emphasizing Ohm's Law's role in calculating current.

💡Resistance

Resistance is a property of materials that opposes the flow of electric current, measured in ohms. It's what causes the 'flow' of electricity to be resisted, and it's a key component in Ohm's Law. The script uses the concept of resistance to explain how increasing resistance decreases the current for a given voltage.

💡Resistivity

Resistivity is a material's intrinsic ability to resist the flow of electric current. While the script mentions resistivity, it is not the primary focus, and the presenter chooses to concentrate on resistance for simplicity, noting that resistivity is different but related.

💡Linear Relationship

A linear relationship implies that changes in one variable result in proportional changes in another. In the context of the video, the linear relationship is between voltage, current, and resistance as described by Ohm's Law. If any of these variables change, the others change in direct proportion.

💡Circuit

A circuit is a closed loop that allows electric current to flow. It's composed of various components like resistors, voltage sources, and sometimes nodes. The script uses simple circuits to demonstrate the application of Ohm's Law in calculating current and understanding circuit behavior.

💡Ohms

The ohm is the unit of electrical resistance. It's named after the German physicist Georg Simon Ohm, who formulated Ohm's Law. The script mentions 'ohms' as the unit used to measure resistance in the examples provided.

💡Conductivity

Conductivity is the reciprocal of resistance and is a measure of a material's ability to conduct an electric current. It's mentioned in the script as 'mho' or 'siemens', but the presenter notes it's rarely used in practice compared to resistance measured in ohms.

💡Short Circuit

A short circuit occurs when the resistance in a circuit is extremely low, effectively bypassing the intended circuit path. The script describes it as a scenario where the current becomes infinitely high if the resistance (R) approaches zero according to Ohm's Law (V/R = I).

💡Open Circuit

An open circuit is a break in the path of an electrical circuit, resulting in no current flow. In the script, it's explained as the opposite of a short circuit, where the resistance is infinite, and thus the current (I) becomes zero according to Ohm's Law.

Highlights

Introduction to Ohm's Law and its relation to voltage, current, and resistance.

Explanation of the difference between resistivity and resistance.

Ohm's Law formula V = IR, and its rearrangements to solve for different variables.

Intuitive understanding of Ohm's Law using the water flow analogy.

Linear relationship between voltage, current, and resistance in resistors.

Practical application of Ohm's Law in circuit analysis.

How to use Ohm's Law to calculate current with given voltage and resistance.

The significance of Ohm's Law in electrical engineering and its wide applicability.

Introduction to the concept of ohms as a unit of measurement for resistance.

Explanation of conductivity as the reciprocal of resistance.

Two extreme cases of Ohm's Law: short circuit and open circuit.

Practical examples of applying Ohm's Law in simple circuits.

The importance of maintaining the direction of current in circuit calculations.

Understanding the concept of voltage potential and its relativity in circuit analysis.

The impact of negative voltages and their role in circuit analysis.

Final thoughts on Ohm's Law and its foundational role in further circuit analysis.

Transcripts

[Music]

today we're going to talk about

ohm's law so in the last tutorial we

talked about voltage

current power and energy well we talked

about a glass of water in the flow and

all that sort of stuff

but the flow isn't always free sometimes

there's something that makes it so this

the flow is resisted and that is called

resistance

and today ohm's law is the way we're

going to relate voltage and

current because they are definitely

related now i'm just going to point out

that there is resistivity and resistance

and right now i'm just going to ignore

resistivity but it is something

different and so

when we're talking about this say

resistance so we have a very simple law

that compares

voltage current and resistance and that

is called ohm's law

so ohm's law is basically v

voltage equals the current i

times resistance and that can very

easily mathematically be switched so you

have

voltage over the resistance that v looks

terrible as always equals current

and you know honestly we could do

voltage over

current equals resistance you can move

this thing around however you want

but i typically like to see it in this

way it's a

pretty way for me but honestly the

voltage over resistance equals current

it's probably the best intuitive way to

understand this

and as you can see here so if you have a

voltage you

lift that glass higher if you have the

same amount of resistance you're going

to have more current

but if you have the same voltage and you

increase the resistance

you're going to decrease the current so

one of the things about this is you'll

notice this is a very linear

relationship

if you change any of these the other

things are going to change

linearly and this is not the case with a

lot of components but with resistors we

typically treat them as linear

and we don't really worry about those

other cases in the vast majority of time

so voltage over resistance equals

current

this

is so huge it's going to be used in

basically everything every

circus class from here until forever and

then a lot of the things in electrical

engineering still refer to this even if

it's not

circus related so this is an incredibly

important thing

and also very straightforward and you're

going to learn a lot about it but let's

go over it a little bit more

so if you are given a circuit let's just

put a node right here

a resistor and then another node

we already have everything we need to

solve something so let's say

this is going to be 100 volts and this

is

0 volts so now we have 100 volts across

this resistor and then let's say this

resistor is

100 ohms

we can use ohm's law to very easily say

okay we have the voltage and we have the

resistance so we'll want to use this

format

we'll say 100 volts over

100 ohms equals

one amp okay that seems super simple and

it is super simple

it gets much more complicated but that

is the essence

of how you can use ohm's law to solve

for the current or if you happen to know

if you happen to know what the current

is

then you can do the same thing i'm

saying oh i know i have one amp through

here and there's 100 volts across it so

whatever i need so you can use any of

these you just need to have two

and then you'll figure out the other one

so this is pretty simple

and you notice i use the word ohms and

it's

wow that is and i apologize my

handwriting is so

so bad but this is a greek omega let's

see if it can look a little bit better

no that just looks like some sort of

farming implement now

but that is what that symbol is that

means ohms and that is the measurement

of resistance now there's actually

something out there that

is called a mo and it's an upside down

oh my goodness that's really going to

stretch my ability

and that is 1 over

r and that is called conductivity and

that can be mohs or

siemens whatever you want so 100 ohms

would be one over 100

siemens and that's just an s

or you can also do one over 100

moes and you're not really going to see

that

that often at least i haven't if you do

well great i'm glad that i've introduced

it but

you might hear that on occasion and

that's the only reason i wanted to

introduce it to you

but in my career and in my time in

college i

very very rarely saw that so if you do

that you can totally change ohm's law to

again

follow those rules but now that i've put

that out there we are going to

completely

ignore this ever again so enjoy

so before we get into the samples i just

want to give two cases of the extremes

of ohm's law

so if you have v equals ir imagine

your r is zero so you have

v over r

which equals zero what does that come

out to be

it comes out to be infinity and that is

called a short circuit

and that's something what happens if you

take a battery and you just put a wire

from one end to the other

that wire is essentially zero resistance

so that battery outputs as much

current as it possibly can until it

melts and everything goes away

in most cases you don't want that some

cases you might

i think when you are doing welding they

try and get that is

that resistance as low as possible so

that they can get as much heat there

as possible but this in general is

something

we want to avoid because it usually

means your circuit's about to blow up

now let's say we have the opposite

problem voltage

over infinite resistance

okay there we go then what is our

current going to be

it's going to be 0 get the exact

opposite and this is called an open

circuit and sometimes this is what you

want sometimes this isn't what you want

sometimes when you have a circuit and

you're testing things and you

are not getting any current it's like oh

okay that means that i have an

open somewhere in this and if it's on

your circuit board and you can't see

where it's

broken that's a problem but at least you

know from there being a complete

lack of current that is is an open

circuit so those are the two extreme

cases

of ohm's law when resistance is zero and

when resistance is infinity

and how that affects your current i

so with that out of the way let's

actually jump into a couple of examples

so we can see

how this is used practically now we're

not going to be able to solve anything

crazy or anything

truly that interesting until we learn a

couple of more tricks

but at least we'll have a couple of

examples of how to do

ohm's law in very simple circuits

okay so the first circuit that i'm going

to set up here after that super simple

circuit that we had oh man okay i'm just

going to stop complaining about that

and keep on going is we are going to

have 15 volts

here 5 volts here

and 200 ohms right there

so when you're dealing with voltage

potential and it's that voltage across

something and that's what's important

so when i'm holding the glass here it

doesn't matter

that i'm actually in the basement right

now because all i care about is from

here

to here now from here to here was

sitting on the roof of my house

or on the roof of our office that

distance is still the same

so voltage potential can move like this

and as long as that distance between the

two points is the same

it really doesn't matter so with that in

mind let's look at this and now

we are going to have our v over

r equals i oh that's a one

but now our v is actually 15 volts

minus 5 volts over 200 ohms

which equals 10 volts over 200

ohms so now we're just getting 1 20th

1 over 20 which is about .05 amps

or 50

milliamps so the key thing that i want

to take away from this is it doesn't

matter that this was 15 volts or

5 volts because we could do this exact

same circuit

at 10 volts 0 volts 200 ohms

and then we'd still have 10 volts minus

0

over 200 wow

that got out of control and that's still

going to equal

0.05 amps

so that is what i wanted to do this for

it's just to show

that voltage is all comparative i could

even go the other way around

i could do that as negative 10 volts to

negative 20 volts it's still going to be

the same

thing so voltage is all relative it's

that potential between

or across between two points across

something else

so let's do another one

with my paper okay so two more quick

examples so this first one

i am and notice how i'm keeping these

simple i'm trying to keep the math

simple because to me it's more about the

intuitive understanding

you can use a calculator to get to do

the crazy math but

if you want to be able to look at it and

figure out how to solve it you need to

understand

oh hey that's actually that way i can

flip that or i can move that and it

won't actually affect anything so that's

that's where i'm going with this so

let's actually

assign this 10 volts

and assign this 20 volts

and then we will say this is

100 ohms again just for the fun of it

so now i'm going to set the equation up

and

this is something i always did in

college and i still do quite a bit

i'll just write ohm's law in the corner

just to make sure that i'm not screwing

anything

up so i'll have 10 volts

minus 20 volts

over 100 ohms which equals

negative 10 volts over 100 ohms

which equals negative 1 10

amps or 100

milliamps negative 100 milliamps okay so

negative 100 milliamps what does that

mean

well that means that we looked at this

and the way it's set up you'd imagine

okay glass is up here

but actually this is completely flipped

where the higher potential is down here

so even though we have

established that our current is going

this way in reality it's going

the opposite way but since we

established that the current is going

that way we want to say

negative 100 milliamps and that is an

incredibly important thing to remember

and something i still screw up with

ohm's law

is you need to say i am going to say

that i'm going from this point to this

point my current is going

this direction and i am sticking with

that because if you say oh that's

negative well let me just flip things

around you

highly highly increase the chance of you

messing up your math somewhere so what

you do is you set up the equation you

look at it

you say hey this is the way it's going

to be and then i'm just going to take

the signs as i go because if i take this

and i

switched it and i did all the math

differently and

went back up then it would be okay 20

volts

minus 10 volts over 100 ohms

and then i'm going to get 100 milliamps

but it's going to be in the opposite

direction

so that was the key with this one that i

want you to take

away is that that positive and negative

is

a matter of perspective it's a matter of

which way you're looking at it so

always keep that in mind and make sure

that once you

say i'm going to do this and i highly

recommend writing it down

on the paper as you're doing it once you

say this is how i'm doing it stick with

it

don't switch it don't change your mind

or else you're just going to confuse

yourself

or if you do switch your mind switch

your setup because you realize oh man

that's going to be a complete nightmare

just be very very careful with that okay

let's do

one more all right so this will be our

last example so let's do the same things

we've been doing the ugly resistor

the ohm's law in the corner and then

let's just say

this is negative 15 volts

and this is negative 25 volts

and then let's make this 1000 ohms

just because i'm getting bored with 100

okay so we set up ohm's law and we look

at this and we say okay

what's the difference between those

that's negative 15 volts

again based off of what we just

discussed i'm assuming that our current

is flowing

down so we've got negative 15 volts and

then got a minus that

one but that's minus negative 25 volts

and then over 1 000

ohms well negative 15 minus negative 25

is a positive 10. so that actually

becomes a positive number

you've got 10 over 1000 which equals 1

over 100

which gives us .01

or 10 milliamps

[Music]

okay maybe i should just i'm going to

learn some great things while doing this

about

calm handwriting maybe everything will

make sense

but the point i wanted to bring with

this was again that

different potential it doesn't matter so

even though this is a negative number

and this is a negative number

it's all about the relationship between

the two

it's all about that voltage across

this so just because this is negative 15

and negative 25

in this case because there's nothing

else those numbers are basically

arbitrary again you could go and have

10 and 0 and it acts the exact

same way now if this were part of a

bigger circuit then of course it makes a

huge difference

but negative voltages in general just

mean that they're on the different side

of a

an arbitrarily decided a zero again we

shoot in my basement

all of these circuit bread tutorials are

shot in my basement

so we are beneath the ground i can see

through the window there's that

so if we assume ground is zero

everything we do here is below that zero

but it still functions

if we were on the roof if we are on top

of a skyscraper it more matters about

what the difference is and so i'm going

to establish

that for where i am this point is zero

or i could establish that point being

zero it really doesn't matter that

voltage can be

very very different depending on what

perspective

you have looking at it so that's it

that's ohm's law hopefully i gave you a

good overall view of what it is

gave you a couple of examples and how

you can use it to solve

extremely simple circuits next we're

going to go into a couple more

definitions talking about branches nodes

all that sort of stuff

and then after that we'll be able to

start doing a little bit more

complicated circuit analysis

in which time i'm going to be super

excited because this is a lot of fun for

me i love doing circuit analysis i'm not

good at it

but i enjoy it so hopefully that was

helpful again as always we put a written

tutorial on circuitbred.com link is down

in the description

so if you have any more questions i want

to see a couple of different examples

those are on the written tutorial it's

going to be slightly different than what

we've done here today

so if you have any great questions you

can put them either here in the youtube

comments or over on circuitbred.com in

the comments

and we will catch you in the next one

have a good one

you