Electric generator (A.C. & D.C.) | Magnetic effects of current | Khan Academy

Summary

This educational script explains how power stations generate electricity for cities without using giant batteries. It delves into the technology of electromagnetic induction, discovered by Michael Faraday, which involves spinning turbines to produce electric current. The script describes how generators convert mechanical energy into electrical energy and introduces Fleming's Right Hand Generator Rule to determine the direction of induced current. It also explains the differences between AC and DC generators, using slip rings for AC and commutators with split rings for DC, to maintain current direction. The explanation is enriched with visual examples and animations to illustrate the concepts.

Takeaways

• 🌐 Power stations generate electricity by spinning giant turbines using steam, falling water, or wind energy.
• 🔧 The spinning of a turbine is converted into electrical energy through electromagnetic induction, a principle discovered by Michael Faraday.
• 🧵 A wire moving within a magnetic field induces an electric current, which is the basis for generating electricity in power stations.
• 🤚 Fleming's Right Hand Generator Rule helps determine the direction of the induced current in a rotating coil.
• 🔄 The rotation of a coil in a magnetic field produces alternating current (AC), which changes direction periodically.
• 🔌 To avoid tangled wires, power stations use slip rings and brushes to maintain electrical contact as the coil rotates.
• 💡 The alternating current (AC) produced by a generator can cause a light bulb to glow differently when the current direction changes.
• 🏡 AC is preferred for long-distance transmission due to its advantages over direct current (DC).
• 🔵🔴 By using split rings or commutators, the direction of current can be maintained in one direction, creating a DC generator.
• 🔁 The continuous change in electrical contact for every half rotation of the coil is achieved automatically with the use of commutators in a DC generator.
• ⚙️ The technology of generators allows for the conversion of mechanical energy into electrical energy, which is then distributed to power houses and cities.

Q & A

How do power stations generate electricity for cities without using giant batteries?

Power stations generate electricity by spinning giant turbines. This spinning can be achieved through various means such as using hot steam, falling water, or wind energy from windmills.

What is the basic principle behind generating electricity from a spinning turbine?

The basic principle is electromagnetic induction, discovered by Michael Faraday. It involves moving a wire within a magnetic field, which induces an electric current.

How does the rotation of a coil of wire inside a magnetic field produce electricity?

As the turbine rotates, the attached coil of wire also rotates, moving up and down within the magnetic field. This motion induces an electric current that can be used to power devices.

What are electric generators and how do they convert energy?

Electric generators are devices that convert mechanical energy from a spinning turbine into electrical energy through the process of electromagnetic induction.

How can we determine the direction of the induced current in a rotating coil?

The direction of the induced current can be determined using Fleming's Right Hand Generator Rule, which involves aligning the thumb, forefinger, and middle finger of the right hand to represent the direction of wire motion, magnetic field, and current, respectively.

What happens to the direction of the current when the wire moves upwards or downwards in the magnetic field?

When the wire moves upwards, the current is induced inwards, and when the wire moves downwards, the current is induced outwards, as determined by the right hand rule.

Why does the direction of the current change as the coil rotates?

The direction of the current changes because as the coil rotates and passes through positions perpendicular to the magnetic field, the motion of the wires within the magnetic field reverses, causing the current direction to flip.

How are electric generators connected to an external circuit without causing wire tangling?

Generators use an arrangement with brushes and slip rings instead of direct wire connections. The slip rings rotate with the coil, and the brushes maintain electrical contact without rotating, preventing wire tangling.

What type of current is produced by the generator described in the script, and why is it called that?

The generator produces alternating current (AC) because the direction of the current continuously changes as the coil rotates. This is in contrast to direct current (DC), where the current flows in one direction.

How can a DC generator be created to maintain a constant current direction?

A DC generator can be created by using split rings, which act as commutators to automatically change the contact between the brushes and the wires for every half rotation of the coil, ensuring the current flows in one direction.

Why is AC preferred for long-distance electricity transmission, and what are its advantages over DC?

AC is preferred for long-distance transmission due to its ability to be easily transformed to higher or lower voltages using transformers, which reduces energy loss during transmission and makes it more efficient for powering homes and businesses.

Outlines

00:00

🔌 Power Generation and Electromagnetic Induction

The script explains how power stations generate electricity without using giant batteries. Instead, they use turbines powered by steam, falling water, or wind to spin and generate electricity. The underlying principle is electromagnetic induction, discovered by Michael Faraday. This involves moving a wire within a magnetic field to induce an electric current. The script details the construction of electric generators that convert mechanical energy into electrical energy by attaching a coil to a turbine and placing it in a magnetic field. The direction of the induced current is determined using Fleming's Right Hand Generator Rule, which is demonstrated with the pink and blue wires example. The current direction changes as the coil rotates, and this alternating flow is visualized through an animation.

05:01

🔄 Understanding Alternating Current (AC) and Generators

This paragraph delves into the concept of alternating current (AC), which is produced when a coil rotates within a magnetic field and the direction of the induced current changes. The animation illustrates how the current direction flips each time the coil is perpendicular to the magnetic field. The script discusses the practical issue of connecting the rotating coil to an external circuit without causing wire tangling. To solve this, slip rings and brushes are used to maintain electrical contact without physical attachment. The resulting AC has advantages for long-distance transmission, which is why household electricity is AC. The paragraph also introduces the idea of creating a direct current (DC) generator by changing the connection method.

10:01

🔄 Transitioning from AC to DC Generation

The final paragraph explains the transition from AC to DC generation. It starts by removing the slip rings used in AC generators and introduces the concept of split rings or commutators to maintain a constant current direction. As the coil rotates, the split rings automatically change the contact between the brushes and the wires, ensuring the current flows in one consistent direction. This mechanism is essential for creating a DC generator, where the current does not reverse but remains unidirectional. The script summarizes the process of generating electricity by spinning a coil in a magnetic field and the difference between AC and DC generation, highlighting the role of commutators in DC generators.

Keywords

💡Power stations

Power stations are facilities that generate electricity by converting various forms of energy into electrical energy. In the video's context, power stations utilize methods such as steam, falling water, or wind to spin turbines, which in turn generate electricity. This is central to understanding how electricity is provided to numerous houses in a city.

💡Turbines

A turbine is a rotary mechanical device that converts the kinetic energy of a moving fluid (like steam or water) into mechanical energy. In the script, turbines are described as being spun by hot steam, falling water, or wind to generate electricity, illustrating the crucial role they play in power generation.

💡Electromagnetic induction

Electromagnetic induction is the process by which an electric current is generated in a conductor when it moves through a magnetic field. This principle, discovered by Michael Faraday, is the foundation of electricity generation as described in the video. The script explains how moving a wire within a magnetic field induces an electric current, which is key to understanding how generators work.

💡Electric generators

Electric generators are devices that convert mechanical energy into electrical energy using the principle of electromagnetic induction. The video script describes how attaching a coil of wire to a spinning turbine and placing it within a magnetic field results in the production of electricity, making the concept of electric generators central to the theme of electricity generation.

💡Fleming's Right Hand Generator Rule

Fleming's Right Hand Generator Rule is a method used to determine the direction of the induced current in a wire moving through a magnetic field. The script uses this rule to illustrate how to ascertain the direction of the current in the generator's coil, which is essential for understanding the process of electricity generation.

💡Alternating current (AC)

Alternating current (AC) is an electric current whose direction changes periodically. The video script explains that the current generated by a standard generator changes direction with every half rotation of the coil, thus creating an alternating current. AC is highlighted as the type of current used in household electricity due to its advantages in long-distance transmission.

💡Direct current (DC)

Direct current (DC) is an electric current that flows in a constant direction. The script discusses how to modify a generator to produce DC, where the current direction remains the same throughout, by using split rings or commutators to maintain the current direction in the external circuit.

💡Slip rings

Slip rings are mechanical components used in rotating electrical connections. In the script, they are described as metallic rings that allow a continuous electrical connection to a rotating coil through stationary carbon brushes. This arrangement prevents the wires from twisting and tangling, which is crucial for the practical operation of generators.

💡Split rings

Split rings, also known as commutators, are components used in DC generators to maintain the direction of the current. The script explains that by using split rings, the generator can automatically change the contact between the brushes and the wires with every half rotation of the coil, ensuring a constant current direction.

💡Magnetic field

A magnetic field is a region around a magnetic material or a moving electric charge within which the force of magnetism acts. In the context of the video, the magnetic field is essential for electromagnetic induction, as the movement of the wire coil within this field is what induces the electric current in the generator.

Highlights

Power stations generate electricity by spinning a giant turbine using steam, falling water, or wind energy.

The spinning of turbines is based on the principle of electromagnetic induction discovered by Michael Faraday.

An electric current is induced when a wire moves within a magnetic field.

Electric generators convert mechanical energy into electrical energy by attaching a coil of wire to a rotating turbine within a magnetic field.

Fleming's Right Hand Generator Rule is used to determine the direction of the induced current in a coil.

The direction of the current in the wire changes as the wire moves up or down within the magnetic field.

The animation demonstrates how the current direction changes as the coil rotates within the magnetic field.

Direct connection of the coil to an external circuit would cause wires to tangle; hence, brushes and slip rings are used.

Slip rings allow for continuous rotation of the coil without tangling the wires by maintaining electric contact through brushes.

AC (alternating current) is produced when the current direction changes continuously, as seen in the generator's operation.

AC has advantages over DC (direct current) for long-distance electricity transmission.

To build a DC generator, split rings or commutators are used to maintain the current direction without change.

Split rings act as commutators, automatically changing the contact for every half rotation of the coil.

The use of split rings ensures that the current flows in one direction through any external circuit in a DC generator.

The process of generating electricity by rotating a coil in a magnetic field is a fundamental principle in power generation.

The practical application of this principle allows for the lighting and powering of thousands of houses from a single power station.

Understanding the direction of current flow is crucial for the design and operation of both AC and DC generators.

Transcripts

- [Instructor] How do power stations provide

electricity to thousands of houses around a city?

They don't use giant batteries.

They do that by spinning a giant turbine like this,

and to spin such a turbine,

some power stations use very hot steam

that blows over the turbines and spins it.

Or maybe we can use the energy of the falling water,

or another example could be we fit these

inside giant windmills and let

the wind do the work for us.

Whichever way you choose, all we do is spin a giant turbine,

but how do does turning something create electricity?

Well, the technology is based

on electromagnetic induction.

Discovered by Micheal Faraday more than 200 years ago.

The basic idea is that if you take a wire

and move it up or down inside a magnetic field,

it induces an electric current.

So, all we have to do is attach

a coil of wire to these giant turbines,

and place them inside a magnetic field.

As the turbine rotates, the coil starts rotating,

and the wires start moving up

and down inside the magnetic field,

that produces the electric current,

and this can now be used to light up things.

These devices are called electric generators,

they convert spinning or mechanical

energy into electrical energy.

So, let's look at them in detail now.

So, let's start by figuring out,

in what direction a current gets induced

or generated in this coil, once it starts rotating.

So, let's say in our example, the coil rotates clockwise,

somewhat like this.

Now, how do we figure out the direction

of the induced current?

Well, we have already learned

something called the Fleming's Right Hand Generator Rule

which says you stretch the fingers

of your right hand like this

as that they are perpendicular to each other,

then the thumb represents the direction

of the motion of the wire,

that is the direction in which you're pushing the wire,

the forefinger gives the direction of the magnetic field,

then our middle finger will

give us the direction of the current.

So, all we have to do is align our right hand,

to make sure the thumb and the forefinger point

in the direction of the motion and the magnetic field,

then the middle finger will give

us a direction of the current.

So, let's use our right hand rule on the pink wire

that is going upwards as you can see,

and on the blue wire that is moving downwards.

So, it'll be great idea if you can first

see whether you can try it yourself.

So, go ahead and use your right hand

and see if you can figure out the direction

of the current in the pink and the blue wire.

Alright, if you've done it,

let's first start with the pink wire,

because the pink wire is going up,

the thumb should be pointing upwards.

The magnetic field is to the right so the forefinger

should be pointing to the right,

and so if we align our fingers that way,

it will look like this.

Since the middle finger is pointing inwards,

this means the current in the pink wire must be inwards.

Similarly, for the blue wire,

the blue wire is moving downwards,

so the thumb will be pointing down,

the forefinger will still be pointing to the right,

and so if we arrange our right

hand over here, it will look like this.

The middle finger is pointing out of the screen,

that means the current in the blue wire

will be coming out of the screen.

So, what we have seen is if a wire

is moving upwards over here,

then the current will be into the screen,

and if the wire is moving downwards,

then the current will be out of the screen.

Remember this, this will be important.

So now, let's get rid of the hands

and put arrow marks to indicate the current,

and now we can guess what direction

the current will be in the rest of the wires.

Since the current has to flow from the pink to the blue,

we can say that the current has to move here like this,

and then the current has to move here this way, it goes out,

it goes to that external circuit

maybe where there is a bulb, we'll look at that later,

then the current comes back like this

and it flows in this way,

and the current continues to flow

like this as the coil keeps rotating,

because you can see the pink wire

is still going up and the blue wire

is still going down until we come to this point,

because now the pink wire starts moving downwards,

and now the blue wire starts

moving upwards, can you see that?

Again, if I go back and come back again,

notice the pink wire is coming down

and the blue wire is going up,

this means now the current in the pink wire

should be out of the screen,

because we already saw using our right hand rule.

When the wire is going down, the current must be out,

and in the blue wire which is going up,

the current must now be inwards.

In other words, the current will now change its direction.

This is the important thing.

So, the current changes it's direction

and it continues to flow this way

until again the pink wire comes

on the left side, because now again,

the pink wire starts going up,

the blue wire starts going down,

and as a result the current

in the pink wire will be inwards,

the current in the blue wire will now be outwards,

the current will again reverse.

So, every time our coil is in this position,

which is perpendicular to the magnetic field,

we will see that the current direction will flip,

and so if you look at the entire animation now,

it looks somewhat like this.

Every time the coils comes in this position,

perpendicular to the magnetic field,

the current direction keeps changing now of course,

in the animation, I'm stopping,

I'm pausing the animation when my coil

is perpendicular to the magnetic field.

So, that we can see the current flipping it's direction,

but of course in reality,

the coil will be pushed continuously,

there'll be no stopping,

there'll be no jerking motion

like we're seeing over here.

Now, the next question we might have

is how do we connect this coil

to an external circuit like say to a bulb?

Well, we could connect it directly right.

Well, let's see what happens

if we connect the circuit directly.

Current will flow, no problem,

but as the coil starts rotating,

notice the wires start twisting and tangling

and turning and what not.

So, that's going to be a problem.

So, to avoid that, we will not connect the wires directly,

instead we will use an arrangement

involving brushes and slip rings.

It looks somewhat like this.

So, basically we have two metallic rings,

the pink one and a blue one,

and what you may not make out

from my diagram over here,

is that each ring is connected to one wire only.

So, the pink ring is only connected to the left wire,

and the blue ring is only connected to the right wire,

and these wires are connected to carbon brushes,

which is also conducting,

and they're just touching these rings

so that there is a metallic contact,

but they're not stuck to it.

So, right now, there is a contact,

the circuit is complete, and as a result,

the current comes out of the blue ring as you can see,

moves this way, and current flows into the pink ring,

and goes like this, and when the coil starts rotating,

as you can see, the rings rotate along with the coil,

but since the brushes are not stuck to the rings,

the rings just slip through the brushes,

that's why they're called as slip rings.

As a result, the brushes will not rotate,

that solves the problem of wires twisting and tangling,

and all the while, an electric contact is maintained.

Now, once our coil comes in this position,

we have seen that the current reverses.

So now, the current will flow out

of the pink ring, goes like this,

through the bulb and now enters into the blue ring.

So, for every half a rotation,

the current through the bulb also reverses.

Let's say when the current is flowing this way,

the bulb glows blue,

and let's say when the current reverses,

and flows like this, the bulb will glow yellow,

and so now if you look at the entire animation,

it looks somewhat like this,

and so we have successfully built our generator,

and what's interesting to see is that

the current from that generator

is continuously changing it's direction.

Such a current is called alternating current or AC,

and this might sound a little weird,

but it turns out that when you want

to transmit electricity over a long distance,

like from the power station to your houses,

then alternating currents or AC

has some great advantages over unidirectional currents

or DC, and it's for that reason,

the current that we get at our houses,

the electricity what we get at our houses are all AC,

and these generators are called AC generators.

Finally, what if we want to build a DC generator?

Where we don't want the current direction to change,

we want it to remain the same throughout.

Let's say in this direction, how do we do that?

Well, to build that, first let's get rid of these rings.

All right, now to make sure that

the current direction remains the same,

what we will need is that these brushes

to continuously keep changing contacts

between these wires for every half a rotation.

Let's see why.

So, if I want the current to flow this way,

right now in this position,

I can connect this brush to this wire, so the pink side,

so that the current flows like this and comes out,

but as the coil rotates,

notice once it comes to this position,

you've seen that the current starts flipping,

current reverses and so now,

to maintain the current in the same direction,

we would now require this brush

to come in contact with this wire,

that is the blue side, right, and then again,

once we come to this position, again it flips,

the current flips and again we would want now

this brush to come in contact with this side.

And so, as a result can you see that

for every half a rotation,

we would want the contacts to keep changing.

But how do we make sure that happens automatically?

We can do that by attaching split rings.

Split rings as the name suggests,

is a ring that is split in between,

giving two half rings with some gap in between.

Now, let's see how this arrangement automatically

changes contact for every half a rotation.

So, right now, this brush is in contact

with the pink side but as the coil rotates

and comes to this position,

the current reverses and now notice

the brush is in contact with the blue side,

making sure the current still flows

in the same direction through the build.

Again, as the coil comes to now this position,

finishing another half a rotation,

again notice it just changed contact,

it is now in contact with the pink ring

connected to the pink side,

and this way we have now built our DC generator

where the current only flows in one direction

through any external circuit,

and this arrangement which helps us

automatically change contacts, we call them as commutators.

So, split rings act like commutators.

So, to summarize what we learned,

if you take a coil and spin in a magnetic field,

then due to electromagnetic induction,

a current gets generated in that coil.

Now, the direction of the current depends

on whether the wire is going up or where it's going down,

and as a result for every half a rotation,

we see that the direction of the current keeps changing,

and so this generator is called an AC generator,

because it generates an alternating current,

a current whose direction keeps continuously changing.

On the hand, if we use split rings,

then it acts like a commutator,

it keeps changing the contacts

for every half a rotation and make sure that

the current does not change the direction

in the external circuit.

We call this a DC generator,

and so this is how we can generate electricity

just by rotating a coil in between a couple of magnets.