Conductors and Insulators | Physics | Khan Academy

Summary

This script delves into the fundamental concepts of insulators and conductors in electrical materials, explaining their atomic structures and how electrons behave within them. It highlights the free movement of electrons in conductors versus their restricted movement in insulators, and how these materials respond to applied electric fields or charges. The script also illustrates charging methods, including direct contact and induction, using everyday examples like balloons and metal rods, and emphasizes the polarization ability of insulators to create temporary attractions.

Takeaways

• 🌌 All materials in the universe can be categorized into insulators, conductors, semiconductors, and superconductors, but for basic physics, distinguishing between insulators and conductors is often sufficient.
• 🔬 Both insulators and conductors are made up of atoms and molecules with positively charged nuclei and negatively charged electrons.
• 🏠 In solids, the positively charged nucleus of atoms in both insulators and conductors is essentially immobile, only able to vibrate in place.
• 🔋 Conductors have electrons that can move freely with minimal resistance, which is the key difference from insulators where electrons are not free to move.
• 🚫 Insulators' electrons are confined and cannot jump freely from atom to atom, limiting the flow of electrical charge.
• 🌀 Even though insulators do not allow free electron movement, they can still polarize under an electric field, creating an electrical effect.
• 🔌 When extra charge is added to insulators, it remains static and can be uniformly distributed or concentrated on one side.
• 🌐 In contrast, conductors repel extra charge to the outer edges, and any excess charge can move to the surface, distributing evenly or concentrating depending on the shape.
• 🛠 Common insulators include glass, wood, and most plastics, while metals like copper, gold, and silver are typical conductors.
• 🤝 When two conducting rods touch, charge redistribution occurs, spreading the charge to maximize distance between like charges.
• 🌐 Charge by induction involves bringing a charged object near an uncharged conductor, causing a redistribution of charges within the conductor, and can even lead to charging the conductor by grounding it and cutting the connection to the ground.
• 🎈 The example of a balloon sticking to a ceiling demonstrates how insulators can still interact electrically through polarization, even though electrons do not transfer between the materials.

Q & A

What are the three main categories of materials in terms of electrical properties?

The three main categories of materials in terms of electrical properties are insulators, electrical conductors, and semiconductors. Additionally, superconductors and other exotic forms of electrical materials exist, but they are often not considered in introductory physics classes.

What is a common characteristic of both insulators and conductors?

Both insulators and conductors are composed of a large number of atoms and molecules, which in turn consist of a positively charged nucleus surrounded by negatively charged electrons.

Why can't the positively charged nucleus move freely in a solid insulator or conductor?

The positively charged nucleus cannot move freely in a solid insulator or conductor because it is fixed in place. It can only wiggle or jiggle due to thermal vibrations but cannot travel throughout the material.

How do electrons behave differently in conductors compared to insulators?

In conductors, electrons can move about relatively freely with almost no resistance, allowing for the flow of electricity. In contrast, in insulators, electrons do not have the right energy levels and bands to move around freely and are essentially stuck, unable to jump from atom to atom.

What happens when an insulator is connected to a battery or subjected to an electric field?

When an insulator is connected to a battery or subjected to an electric field, the electrons cannot jump from atom to atom. However, the nucleus and the cloud of electrons can shift, causing one side of the atom to become more negative and the other side more positive, leading to an overall electrical effect where the insulator can interact with nearby charges.

What is the effect of adding extra charge to an insulator?

When extra charge is added to an insulator, the charges cannot flow through the material and become stuck. They can be distributed uniformly throughout the insulator or concentrated on one side, but they do not move freely.

How do charges behave when added to a conductor?

When extra charges are added to a conductor, they repel each other due to their like charge and move to the outside edge of the conductor. Charges can only be added to the outside edge of a conductor, as any internal charges will quickly redistribute to the edge.

What physical materials are considered insulators?

Physical materials that are considered insulators include glass, wood, and most plastics. These materials do not allow charges to flow through them and can hold charge on their surface.

What are some examples of conductors?

Examples of conductors include metals such as gold, copper, and silver. These materials allow charges to flow freely through them.

How can charge be transferred between two conducting rods?

Charge can be transferred between two conducting rods by touching them together. The charges will spread out to maximize the distance between them, resulting in a sharing of the charge between the rods based on their size.

What is charge by induction, and how does it work?

Charge by induction is a method of charging an object without direct contact. It involves bringing a charged object near an uncharged conductor and then grounding the conductor, causing electrons to move or leave the conductor, thus inducing a charge. If the grounding is maintained, the conductor remains charged even after the charged object is removed.

How can a balloon stick to a wall or ceiling?

A balloon can stick to a wall or ceiling due to the insulating material's ability to polarize. When the negatively charged balloon is brought near an insulating surface, the atoms in the surface can reorient, creating a net force that attracts the balloon, allowing it to stick.

Outlines

00:00

🔌 Understanding Electrical Materials

The script introduces the concept of classifying materials based on their electrical properties, such as insulators, conductors, semiconductors, and superconductors. It simplifies the idea for introductory physics by focusing on insulators and conductors, both composed of atoms with a nucleus and electrons. The key difference is the mobility of electrons; conductors allow free movement, while insulators restrict it. The script explains how conductors can be compelled to move electrons through an electric field or battery, whereas insulators' electrons can only shift within the atom, creating a polarized effect when under an electric influence. It also touches on the interaction of materials with external charges despite the restriction on electron flow.

05:00

🚀 Charge Distribution in Insulators and Conductors

This paragraph delves into the behavior of charge when introduced to insulators and conductors. In insulators, extra charges remain static and can be uniformly distributed or localized on one side. Conductors, however, repel like charges, causing them to move to the outer edges of the material. The script uses the example of metal rods to illustrate how charge redistribution occurs upon contact. It also introduces the concept of charging by induction, where a conductor can acquire a charge by being near a charged object without direct contact, leading to a redistribution of electrons within the conductor.

10:02

🌐 Induction Charging and Material Polarization

The final paragraph explains the process of charging by induction in detail. It describes how grounding a conductor and bringing a charged rod near it can cause electrons to leave the conductor, resulting in a net positive charge. By cutting the grounding connection before removing the charged rod, the conductor remains charged. The script also provides a real-world example of charging a balloon by rubbing it against hair, which involves the insulating properties of rubber and the polarization of atoms in the ceiling, allowing the balloon to stick due to the attractive forces created by the polarization.

Keywords

💡Insulator

An insulator is a material that resists the flow of electric current. In the context of the video, insulators are composed of atoms and molecules with a fixed positive nucleus and negatively charged electrons that cannot move freely. The script explains that insulators do not allow electrons to jump from atom to atom, which prevents the flow of electrical current. An example given in the script is glass, which is an insulator and does not allow charge to flow through it.

💡Electrical Conductor

An electrical conductor is a material that allows the flow of electric current with minimal resistance. The video script describes conductors as having electrons that can move freely about the material, which is a key difference from insulators. Metals like copper and silver are highlighted as common conductors, where charges can flow freely through them.

💡Atom

An atom is the basic unit of matter, consisting of a positively charged nucleus surrounded by negatively charged electrons. The script explains that both insulators and conductors are made up of atoms and molecules. The behavior of these atoms, particularly the movement of electrons, determines whether a material is an insulator or a conductor.

💡Electron

Electrons are subatomic particles that orbit the nucleus of an atom and carry a negative charge. The video emphasizes the role of electrons in determining the electrical properties of materials. In conductors, electrons can move freely, while in insulators, they are more restricted, unable to move around freely.

💡Electric Field

An electric field is a region around a charged particle where an electric force is exerted on other charged particles. The script mentions that electrons in conductors can be compelled to move by an electric field or force, such as when connected to a battery, facilitating the flow of current.

💡Polarization

Polarization refers to the process where electric charges become distributed unevenly within a material, leading to regions of positive and negative charge. The video script explains that even insulators can become polarized when subjected to an electric field, allowing them to interact with other charges nearby, despite the inability of electrons to flow freely.

💡Charge

In the context of the video, charge refers to the property of matter that causes it to experience a force when placed in an electric field. The script discusses how materials can be charged by direct contact or by induction, and how the distribution of charge affects the behavior of materials.

💡Conduction

Conduction is the process by which heat or electricity is transferred through a material due to the movement of particles. The script uses this term to describe the movement of electrons in conductors, which is essential for the flow of electric current.

💡Induction

Induction, in the context of the video, refers to the process of charging an object without direct contact by bringing a charged object near it, causing a redistribution of charges within the uncharged object. The script provides an example of charging a metal rod by bringing a negatively charged rod close to it and then grounding the rod, which causes electrons to leave and the rod to become positively charged.

💡Polarized Material

A polarized material is one in which the electric charges are separated into distinct regions, creating an internal electric field. The script explains that even insulators, which do not allow the flow of charge, can become polarized and interact with other charges, such as when a charged balloon sticks to a ceiling.

💡Net Charge

Net charge refers to the overall positive or negative charge of an object, resulting from the imbalance of protons and electrons. The script describes how conductors and insulators handle net charge differently, with conductors distributing it to the outside edge and insulators maintaining it uniformly or localized.

Highlights

Materials in the universe can be categorized as insulators, conductors, semiconductors, and superconductors.

Both insulators and conductors are composed of atoms and molecules with a positively charged nucleus and negatively charged electrons.

The nucleus in a solid material cannot move freely, regardless of whether it's an insulator or conductor.

Electrons in conductors can move with almost no resistance, unlike in insulators where they are unable to move freely.

Insulators can still interact electrically by shifting the nucleus and electron cloud, creating a polarizing effect.

Electrons in insulators can jump within their atoms or share with neighboring atoms but cannot travel freely throughout the material.

Conductors require an external force, such as a battery or electric field, to initiate electron movement.

Insulators can be charged uniformly or have charges bunched up on one side, but the charges remain static.

In conductors, extra charges repel each other and move to the outside edge of the material.

Charges on a conductor always reside on the outside edge due to the repulsion between like charges.

Materials like glass, wood, and most plastics are insulators, while metals like gold, copper, and silver are conductors.

Conductors can be charged by touching another charged object, leading to charge redistribution.

Charge by induction involves bringing a charged object near a conductor without touching it, causing a redistribution of charges.

Grounding a conductor to a large electron supply can cause electrons to leave or enter, effectively charging the conductor.

By cutting the grounding wire before removing the inducing charge, a conductor can be permanently charged.

Insulators like rubber can cause a charged balloon to stick to a ceiling through polarization and attraction forces.

Even insulators can interact with electric fields by polarizing, demonstrating the versatility of material responses to electric charges.

Transcripts

- [Voiceover] It's useful to pretend

like all materials in the universe can be broken down

into a category of insulator, electrical insulator,

or electrical conductor.

That's not completely true.

There are semi-conductors and super conductors

and other exotic forms of electrical materials

but for most introductory physics classes

and problems and tests, you can get pretty far

assuming that it's either an insulating material

electrically or a conducting material electrically.

Before I talk about the differences between these,

here I have two solid cylinders

of either an insulating material or a conducting material.

Before I talk about the differences, one similarity

is that both insulators and conductors are composed of

a huge number of atoms and molecules

and these atoms and molecules,

whether it be insulator or conductor,

are composed of a positively charged nucleus

and a negatively charged swarm of electrons

that surround that nucleus.

Another similarity is that for both conductors

and insulators, the positively charge nucleus cannot move.

I mean it can wiggle around and jiggle

just from thermal vibrations, maybe a little bit in place,

but it can't travel freely throughout the material

for either an insulator or a conductor

as long as it's a solid.

If it was a fluid, I suppose these things can move

and migrate around, but for a solid

the positively charge nucleus is fixed.

They're stuck.

The thing that might be able to move

are the negatively charged electrons,

and here's the difference.

There are electrons in a conductor

that can move about relatively freely.

These can move around with almost no resistance,

whereas for insulators a key difference

is that these electrons cannot move around freely.

These don't have the right energy levels and bands

in order to make these electrons move around freely.

They are also stuck.

For insulators, everything is basically stuck,

These electrons might be able

to jump around in their own atoms

or get shared in a neighboring atom,

but it can't jump around freely from atom to atom

and travel throughout the insulator.

For the conductors, the electrons can do this.

that's the key difference.

Now the electrons aren't just going to do this on their own,

they have to be compelled to start moving

by hooking this up to a battery

or setting up some sort of electric field or force.

If that did happen, the electrons in a conductor

start migrating down the line

but in an insulator, the electrons are stuck

which might make you think that

"Well, okay, shoot, for electrical materials

"all we really care about are the conductors.

"The insulators we will just use if we don't want

"electrical interaction."

While that is somewhat true, it is not completely true

because if I set this insulator up to a battery

or set up some sort of electric field or force in here

even though the electrons in an insulator

can't jump from atom to atom,

what it can do is it can shift.

This nucleus and the cloud of electrons

can kind of shift a little bit.

Positive may be this way,

and the the negatives over on the other end

so what you get is overall this side of the atom

would be more negative,

and this side of the atom would be more positive.

Even though the electron doesn't move,

and the electrons don't move,

now because this is set up where the positive

is shifted from the negative,

this material, if you get all of them to do this

or a lot of them, this can create

an overall electrical effect where this insulator

can interact with other charges nearby

and exert forces on them.

Even though the charges can't flow through an insulator,

they can still interact electrically.

Now, let's see what happens if we add extra charge

to these insulators or conductors.

I mean, the way they started off right here

we had just as many positives in the nucleus

as there are negatives surrounding them

and that's true for the conductors and insulators.

What happens if we add extra charge?

Maybe we add extra negatives into here.

Then what happens?

Well, it'll get really messy if we try to draw it

with all the atoms, so since these all cancel out

their overall charge, I am not going to draw

every atom and nucleus.

I'm just going to pretend like those are there

and they are all canceling out.

I'm just going to draw the actual extra charge.

Let's say we added extra negative charges

to this insulator.

What would happen?

Let's say I just add a negative charge here

and a negative charge there,

and here and there, I have added a bunch

of negative charges to this insulator.

What would happen?

Well, we know these negatives

can't move throughout and insulator.

Charges can't flow through an insulator so they're stuck

which means for an insulator, I could charge

the whole thing uniformly if I wanted to

where the charge is spread out throughout the whole thing

or I could make them bunch up on one side if I wanted to

and they'd be stuck there.

The point is that they're stuck.

For a conductor, what would happen if I tried

to put a negative here and a negative there,

some extra negative charge on a conductor?

They don't have to stay here if they don't want to.

If you put extra negatives in here,

they are not going to want to

because negatives repel each other

just like opposites attract, like charges repel.

So what are they going to do?

Well, this negative is going to try to get as far away

from this other negative as it can so go over here.

This negative is going to try to get as far away as it can.

It repels it.

Now, it can't jump off the conductor.

That takes a lot more energy,

but it can go to the very edge.

That's what charges do for conductors.

You've got a solid conducting material,

you put extra charge on it, it's all...

All that charge is going to reside on the outside edge

whether you've added extra negative or positive,

always on the outside edge.

You can only add charge to the outside edge

for a conductor, because if it wasn't on the outside edge

it will quickly find its way to the outside edge

because all these negatives repel each other.

I said this is true for positives or negative.

You might wonder, "How do we add a positive?"

Well, the way you add a positive

is by taking away a negative.

If you started off with a material that had

just as many positives as negatives

and you took away a negative,

it's essentially like adding a positive charge in here.

But again, the net positive charge, the net negative charge

always resides on the outside edge of the conductor

because charges try to get

as far away from each other as possible.

So what physical materials actually do this?

What physical materials are insulators?

These are things like glass is an insulator.

Wood is an insulator.

Most plastics are insulators.

All of these display this kind of behavior

where you can distribute charge and the charge

can't flow through it.

You can stick charge on it.

In fact, you can stick charge on the outside edge

and it will stay there.

There's conductors.

These are things like metals, like gold

or copper is typically used because it's kind of cheap.

Cheaper than gold, certainly.

Or any other metal.

Silver works very well.

These are materials where charges

can flow freely through them.

Now that we see how conductors and insulators work,

let's look at an example.

Let's say you have two conducting rods.

Say these are made out of metal.

One of them has a net amount of negative charge on it

which is going to reside on the outside edge

because that's what net charge does on a conductor,

but this other rod, this other metal conducting rod,

does not have any net charge on it.

What would happen if I took this first rod

touched it to the second rod?

You probably guessed, charges want to get

as far away from each other as possible

so these negatives realize "Hey, if we spread out,

"some of us go on to this rod and some of us stay here,

"we can spread out even father away from each other."

That's what they would do.

If these rods were the same size,

you'd have equal amounts on each.

If the second rod was bigger,

more of them would go on to this second one

because that would allow them to spread out even more.

Some would stay on the smaller one.

That's charged by just touching something.

That's easy.

You can charge something also, you can get clever.

You can do something called

charge, you can charge something by induction it's called.

What does this mean?

Charge by induction says alright, first

imagine I just take this and I bring it nearby

but don't touch it.

Just bring it near by this other piece of metal

and I don't touch it.

What would happen?

There is negatives in here, I haven't drawn them.

There's positives in here.

The negatives can move if they wanted to.

Do they want to?

Yeah, they want to!

These negatives are coming nearby,

they want to get as far away from them as possible.

Even though there are already some negatives here,

a net amount of negatives

are going to get moved over to this side.

They were located with their atom on this side,

but they want to get away from this big negative charge

so they can move over here, which leaves

a total amount of positive charge over here.

I.E. There is a deficit of electrons over here,

so this side ends up positively charged.

You might think, "Okay, well that's weird.

"They spread out.

"Does anything else happen?"

Yeah because now these positives are closer to the negatives

than the negatives are,

and these positives in this charge rod

are attracting these positives.

These negatives in this conducting rod

are attracting these positive charges

because like charges repel and opposites attract

but they are also repelling.

These negatives in this rod are repelling these negatives.

Do those forces cancel?

They actually don't because the closer you are

to the charge the bigger the force.

This would cause this rod to get attracted

to the other rod.

That's kind of cool.

If you took a charged rod,

brought it to an empty soda can,

let that can sit on the table

in this orientation so it could roll,

if you bring the rod close

the can will start moving towards the rod.

It's kind of cool, you should try it if you can.

But, that's not charge by induction.

Charge by induction is something more.

It says alright, take this piece of metal

and conduct it to ground.

What's ground?

Well, it could be the ground.

If you took a big metal pipe and stuck it in the ground

that would count,

or any other huge supply of electron,

a place where you can gain, steal, basically take

infinitely many electrons or deposit

infinitely many electrons and this ground would not care.

So the frame of your car, the actual metal,

is a good ground because it can

supply a ton of electrons or take them.

Or a metal pipe in the earth.

Some place you can deposit electrons or take them

and that thing won't really notice or care.

Now what would happen?

If I bring this negative rod close to this rod

that was originally had no net charge?

Now instead of going to the other side of this,

they say "Hey, I can just leave.

"Let me get the heck out of here."

These negatives can leave.

A whole bunch of negatives can start leaving

and what happens when that happens is that

your rod is no longer uncharged.

It has a net amount of charge now.

They won't all leave.

You're not going to get left with no electrons in here.

There's going to be some electrons in there,

but some of the electrons will leave

which means that this rod, which used to be uncharged

now has a net amount of positive charge in it.

I've charged this rod without even touching it

because I let the negative electrons leave.

If I'm clever, what I can do is I can just cut this wire

before I take away the thing that induced the charge.

If I remove this now and move it far away,

what these negatives would have done

is they would have said "Shoot, okay,

"I am glad that that's over.

"Now I can rejoin.

"I'm attracted to this positive again.

"I'm going to rejoin my positives."

and this thing will become uncharged again

but now they can't get back.

They're stuck.

There's no way for these to get back

because you've cut the cord here

and you've permanently charged this piece of metal

without even touching it.

It's called charge by induction.

It's a quick way we charge something up.

Let me show you one more example.

Everyone's tried this.

You take a balloon.

What happens? How do you charge it up?

You rub it against your hair.

It steals electrons from your hair

and the balloon becomes negatively charged.

What do you do with it?

You know what you do with it.

You take this thing and you put it near a wall or a ceiling

and if you're lucky, it sticks there,

which is cool!

How does it work?

Well, remember, this is an insulating material rubber.

The ceiling is an insulating material.

Electrons aren't getting transferred

but even in an insulating material, the atom can reorient

or polarize by shifting.

The negatives in that atom can shift to one side

and the other side becomes a little more positive

and what that does, it causes a net force

between the ceiling and the balloon

because these positives are a little closer.

These positives are attracting negatives

and the negatives are attracting the positives

with a little bit greater a force

than these negatives are repelling

the other negatives in the ceiling.

Because of that, because the ceiling

is also attracting the balloon

and the balloon is attracting the ceiling

with greater force than the negatives

are repelling the balloon, the balloon can stick

because of the insulating material's ability

to polarize and cause and electric attraction.

This is what I said earlier.

Even if it's an insulator,

sometimes it can interact with something electric

because the atom can shift and polarize.