How Power Gets to Your Home: Crash Course Physics #35

Summary

This video explains how electricity is produced and transmitted, focusing on electric generators and transformers. It describes how generators convert mechanical energy into electrical energy using induction and alternating current (AC). The importance of AC power is highlighted, as it enables transformers to change voltage levels efficiently, minimizing energy loss during long-distance transmission. The video also explores the principle of mutual inductance, which is key to transformers and wireless chargers. Finally, it touches on real-world applications like Tesla coils and wireless charging technology.

Takeaways

• ⚡ Electric generators convert mechanical energy into electrical energy using induction, with a rotating coil within a magnetic field.
• 🔄 Alternating current (AC) is produced because the coil's rotation causes the direction of the induced current to reverse every half-turn.
• ⚙️ The induced electromotive force (emf) in a generator depends on the coil's number of loops, magnetic field strength, area of the coil, and its angular velocity.
• 💡 AC power is commonly used in households because it enables transformers to work, which are essential for efficient power transmission over long distances.
• 🔌 Transformers change voltage using mutual inductance, where a change in current in one coil induces an emf in another nearby coil.
• 📉 Transmitting electricity at higher voltages minimizes power loss, as higher voltages lead to lower current and less heat loss.
• 🔺 Step-up transformers increase voltage by having more turns in the secondary coil than the primary coil, while step-down transformers decrease voltage by having fewer turns.
• 🌩️ Tesla coils are a type of step-up transformer, designed to produce high-voltage electricity, often displayed as lightning-like arcs.
• 📱 Wireless chargers work through mutual inductance, transferring power between coils in the charging pad and the phone without physical connections.
• 🧠 Understanding electric generators and transformers helps explain how electricity is generated, transmitted, and utilized in various technologies, from power grids to wireless devices.

Q & A

What is the main difference between electric generators and electric motors?

Electric generators convert mechanical energy into electrical energy using induction, while electric motors convert electrical energy into mechanical energy.

How does a generator produce alternating current (AC)?

A generator produces AC by rotating a coil of wire in a magnetic field. As the coil rotates, the magnetic flux changes, inducing a current that reverses direction with each half-turn of the coil, resulting in alternating current.

Why is alternating current (AC) commonly used for power transmission instead of direct current (DC)?

AC is used because transformers, which are essential for adjusting voltage levels during transmission, only work with AC. Additionally, AC allows for higher voltages to be transmitted over long distances, reducing energy loss as heat.

What role do transformers play in the transmission of electricity?

Transformers adjust the voltage of electricity during transmission. They increase voltage for long-distance transmission (step-up transformers) and decrease it for safe use in homes and appliances (step-down transformers).

How does the number of loops in a coil affect the voltage in a transformer?

The voltage in a coil is proportional to the number of loops. If the secondary coil has more loops than the primary, it increases the voltage (step-up transformer). If it has fewer loops, it decreases the voltage (step-down transformer).

Why is high voltage used in long-distance electricity transmission?

High voltage is used to reduce power loss as heat. For the same amount of power, a lower voltage requires a higher current, which leads to greater power loss. By increasing the voltage, power loss is minimized.

What is mutual inductance and how does it work in transformers?

Mutual inductance occurs when a change in current in one coil induces an electromotive force (emf) in a nearby coil. This is used in transformers to transfer electrical energy between coils, adjusting the voltage in the process.

How do wireless chargers use the principle of mutual inductance?

Wireless chargers use mutual inductance by having a coil in the charging pad and another in the device. AC current in the pad’s coil induces an emf in the device’s coil, transferring energy wirelessly to charge the battery.

What factors affect the emf induced in a generator’s coil?

The emf in a generator is affected by the number of loops in the coil, the strength of the magnetic field, the area of each loop, the angular velocity of the coil, and the sine of the angle between the coil and the magnetic field.

Why does AC power reverse direction 50 or 60 times per second in most places?

AC power reverses direction because of the alternating nature of the current in the generator. The frequency of this reversal is measured in hertz (Hz), with 50 Hz or 60 Hz being standard frequencies used in power grids worldwide.

Outlines

00:00

🔌 How Electrical Power Reaches Your Home

This paragraph introduces the concept of electrical power distribution, starting with the familiar convenience of power outlets in homes. It highlights the critical roles of electric generators and transformers in the process of delivering electricity to homes, schools, and offices. The paragraph explains that generators convert mechanical energy into electrical energy through induction, emphasizing the reversal of current direction during every half-turn, which results in alternating current (AC). This is contrasted with direct current (DC), explaining that AC is the type of power that flows through household outlets.

05:01

⚡ Induction and EMF in Generators

This section dives deeper into the workings of electric generators. It explains how the rotation of a coil in a magnetic field induces a current due to changes in magnetic flux, with a focus on how this process results in alternating current (AC). The mathematical principles behind the induced emf (electromotive force) are detailed, explaining how various factors—such as the number of loops in the coil, the magnetic field strength, coil area, and angular velocity—affect the emf. The description highlights the importance of alternating current for efficient power transmission.

⚙️ The Role of Transformers in Power Transmission

This paragraph explains how transformers work, focusing on their role in changing the voltage of electricity to reduce power loss during long-distance transmission. It introduces the concept of mutual inductance, where a change in current in one coil induces a change in emf in a nearby coil. The explanation covers step-up and step-down transformers, which adjust the voltage to safe levels for household use, while also maintaining high voltage during transmission to minimize energy loss.

🔄 Mutual Inductance and Voltage Transformation

This part of the script details the operation of transformers through mutual inductance, which allows voltage to be increased or decreased depending on the number of loops in the primary and secondary coils. Faraday's law is used to explain how emf in a coil is proportional to the number of loops. Step-up transformers increase voltage, while step-down transformers reduce it. The paragraph also highlights practical applications, like Tesla coils and wireless chargers, which use the same principle to transfer power wirelessly.

🔋 Wireless Charging and Inductive Power Transfer

This paragraph extends the concept of mutual inductance to modern applications, such as wireless charging for phones. It explains how a coil inside a charging pad transfers energy to another coil inside a phone without physical connections, using alternating current to induce emf. The same principle that powers household appliances is now used for wireless devices. The paragraph concludes by summarizing the overall process of electricity generation, transmission, and usage in daily life, ending with credits and a nod to the production team.

Keywords

💡Electric Generators

Electric generators are devices that convert mechanical energy into electrical energy using induction. In the video, it’s explained that the mechanical rotation of a coil within a magnetic field induces an electrical current. This concept is central to how electricity is generated for homes and businesses.

💡Induction

Induction refers to the process where a changing magnetic field induces an electric current in a conductor. This concept is used in both electric generators and transformers. The video emphasizes that induction is key to producing electricity and transmitting it efficiently over long distances.

💡Alternating Current (AC)

AC refers to the type of current where the direction of the flow of electricity reverses periodically. In the video, AC is shown to be the type of electricity used in households, reversing direction 50 or 60 times per second. The alternating nature of AC is crucial for the operation of transformers.

💡Magnetic Flux

Magnetic flux is the measure of the amount of magnetic field passing through a given area, such as a coil of wire. The video explains that changes in magnetic flux are what induce an emf (electromotive force) in a generator, creating the current needed to power electrical devices.

💡Electromotive Force (emf)

Electromotive force, or emf, is the voltage generated by changing magnetic fields. In the video, emf is a crucial part of how electric generators and transformers work, as it is the force that drives the current through a conductor.

💡Transformers

Transformers are devices that change the voltage of an electric current using mutual inductance between two coils. The video explains how transformers are necessary to step up voltage for long-distance transmission and step it down to safe levels for household use.

💡Mutual Inductance

Mutual inductance is the process where a changing current in one coil induces an emf in another nearby coil. In the video, this concept is key to the operation of transformers, which use mutual inductance to change the voltage of AC power.

💡Step-up Transformer

A step-up transformer increases the voltage of an electrical current by having more turns of wire in the secondary coil than in the primary coil. The video explains how step-up transformers are used to boost voltage for efficient long-distance transmission of electricity.

💡Step-down Transformer

A step-down transformer decreases the voltage of an electrical current by having fewer turns of wire in the secondary coil compared to the primary coil. In the video, step-down transformers are essential for reducing high transmission voltages to safe levels for home appliances.

💡Angular Velocity

Angular velocity is the rate at which an object rotates, typically measured in radians per second. The video uses this concept to explain how the rotational speed of a coil in a generator affects the induced emf, and thus the amount of electrical power produced.

Highlights

Electric generators convert mechanical energy into electrical energy using induction.

The rotation of a coil of wire within a magnetic field generates an alternating current (AC) due to changes in magnetic flux.

The angle between the coil and the magnetic field changes during rotation, causing the current to reverse direction with each half-turn.

Alternating current (AC) is the type of electricity that flows through wall outlets, reversing 50 or 60 times per second depending on the region.

Direct current (DC) generators use special components to ensure current flows in the same direction, unlike AC.

The strength of the emf in a generator depends on factors like the number of loops in the coil, magnetic field strength, coil area, and angular velocity.

Transformers use mutual inductance to change the voltage of electricity between power plants and homes, working only with AC power.

High voltage reduces power loss during long-distance electricity transmission, lowering heat waste significantly.

Step-up transformers increase voltage for efficient transmission, while step-down transformers reduce voltage for safe household use.

In the US, electricity in homes is 110 volts, while in many other countries, it's 220 volts.

Mutual inductance allows for wireless charging by using coils in both the charging pad and the device, transferring energy without physical connections.

Faraday's law helps calculate the voltage in transformers by comparing the number of loops in the primary and secondary coils.

Tesla coils are advanced step-up transformers that can create visible electric sparks.

Wireless chargers are practical applications of mutual inductance, using electromagnetic fields to charge devices.

Generators and transformers are essential for delivering electricity efficiently from power plants to end users.

Transcripts

Wherever you are right now, there’s probably an electrical outlet within easy reach.

Whether you want to vacuum the living room or recharge your phone, all you have to do is plug into the wall, and bam – instant access to electricity.

But have you ever wondered how all of that electrical power gets to your house, school, or office in the first place?

There are a lot of steps, but two of the most important ones involve electric generators and transformers.

And both of these devices work because of a concept we introduced last time: induction.

[Theme Music]

Electric generators are like the opposite of electric motors.

Motors take electrical energy – so, the current running through a coil of wire – and convert it into mechanical energy as the motor turns.

Generators, on the other hand, take mechanical energy – the rotation of a coil of wire – and use induction to convert it into electrical energy, in the form of a current running through a wire.

Generators use a wire wound around something called an armature – basically, a cylinder that rotates within a uniform magnetic field.

As the loop rotates, the changing magnetic flux induces a current in the loops of the coil.

But the angle that the coil makes with respect to the magnetic field keeps changing, which causes the direction of the induced current to reverse itself at every half-turn.

So the magnetic field in the generator stays constant.

But because the coil is rotating, the angle between the coil and the magnetic field changes.

This means that the magnetic flux through the loops in the coil changes over time, which is what induces an emf.

And as we’ll soon see, the emf depends on the sine of the angle between the coil and the magnetic field.

So for half of the rotation, the sine of the angle is positive, and for the other half, the sine of the angle is negative.

When it’s positive, the current flows in one direction.

When it’s negative, it flows in the other.

This means that the direction of the current flips with every half-rotation.

This produces a type of flow of electricity known as alternating current, or AC.

Until now, we’ve mainly been talking about DC, or direct current.

There are DC generators, which include special parts to make the current keep flowing in the same direction the whole time, instead of reversing itself as the coil turns.

But AC power is the kind that flows from the outlets in your walls.

And in most places in the world, it reverses itself either 50 or 60 times per second.

This is called the frequency of the current, and you’ll normally see it written in hertz.

Now, we can calculate the strength of the emf in a generator using some of the principles we talked about last time.

We’ve already described what happens when you move a loop of wire in or out of a magnetic field:

The emf induced in the wire is equal to the strength of the magnetic field, times the length of the loop, times its perpendicular velocity.

In other words, it’s the velocity times the sine of the angle between the magnetic field and the loop.

And the same idea applies to a coil of wire rotating in a magnetic field, too.

We just have to replace some of the variables with ones that apply to a rotating coil.

First, instead of the length of the loop, we’ll use the area of one loop of the coil, A.

And instead of translational velocity, we’ll use angular velocity, ω.

Next, the angle, theta, is just equal to the angular velocity multiplied by time.

And finally, instead of finding the emf in just one loop of wire, we’re finding the emf in a whole coil of wire.

Which means that we need to multiply this equation by N, the number of loops in the coil.

So, the emf induced in a coil rotating in a magnetic field is equal to the number of loops in the coil, times the strength of the magnetic field,

times the area of a loop of the coil, times the angular velocity, times the sine of the angular velocity multiplied by time.

It’s a bit of a mouthful, but that’s because there are so many factors that affect the emf induced in the coil.

Basically, the equation is saying that you’ll have a greater induced current in the generator if there are more coils in the wire, or if there’s a stronger magnetic field,

or if each loop of the coil is bigger, or if it rotates faster.

Now, it might seem kind of strange to have a generator produce a current that reverses itself dozens of times a second.

But actually, this is incredibly useful.

That’s because another important device that gets electricity from the power plant to your house is a transformer, which is made up of two coils of wire.

And transformers only work with AC power.

They’re necessary because one of the problems with transmitting electricity over long distances is that, if the voltage is low, a lot of power gets wasted as heat.

We’re talking like 80% in some cases.

Which is a huge waste of energy!

When electricity is transmitted at higher voltages, though, much less power gets wasted as heat.

That’s because, for the same power, a lower voltage translates to a higher current, and power loss increases proportionally to the square of the current.

In other words, if you double the voltage, you end up with only a quarter the power loss you had before.

And if you triple the voltage, you get a ninth of the power loss.

So it’s worth transmitting electricity at very high voltages.

But then you need a way to change the voltage of the electricity running through the lines

– from its original voltage from the generator, which could be around 12,000 volts;

then up to a very high voltage as it travels long distances, which might be as high as 240,000 volts.

That’s definitely not safe to use in your household appliances.

So once the electricity gets to where it needs to go, you need to lower its voltage again.

In the US, the power coming out of your wall is 110 volts, and in most other places it’s 220 volts.

All those voltage changes are made using transformers, which take advantage of something called mutual inductance, where a change in the current in one coil leads to a change in emf in another, nearby coil.

And emf is the same thing as voltage.

This change happens because the changing current in the first coil produces a changing magnetic field.

So the magnetic flux through the second coil changes, which induces an emf.

And the emf induced in the second coil will be equal to the change in current in the first coil, divided by the change in time, and multiplied by a constant, M.

M depends on things like the size and shape of the coils, and how they’re positioned relative to each other.

This works in the opposite direction, too:

a change in current in the second coil will induce a corresponding emf in the first coil.

In transformers, the power running through the first coil is AC, which means the current and the magnetic field it produces are constantly changing.

So an emf is induced in the second coil.

But if the second coil has more turns than the first, it’ll have a higher voltage, and vice versa.

Here’s why:

Faraday’s law, which we talked about last time, says that the emf – or voltage – in each coil is equal to the number of loops in the coil, times the change in magnetic flux over time.

We can write this out as an equation for each coil separately, using a subscript P for the primary coil and a subscript S for the secondary coil.

Now, we want to know how the voltage in the secondary coil compares to the voltage of the primary one.

To find out, we divide the voltage in the secondary coil by the voltage in the primary coil.

The change in magnetic flux over time cancels out, which leaves us with a simple but useful equation:

The voltage in the secondary coil divided by the voltage in the primary coil is equal to the number of loops in the secondary coil divided by the number of loops in the primary coil.

So if the secondary coil has twice as many loops as the primary coil, it’ll have twice as much voltage.

And if it has triple the loops, it’ll have triple the voltage, and so on.

If the secondary coil has more loops than the primary coil, so that it increases the voltage, that’s called a step-up transformer.

And if it has fewer loops than the primary coil, so it decreases the voltage, that’s a step-down transformer.

As the electricity in power lines travels from the power plant to your house, it goes through lots of step-up and step-down transformers.

If you’ve ever seen a Tesla coil in action – that’s just a fancy version of a step-up transformer.

The secondary coil is designed so that it shoots out bursts of electricity that look like lightning bolts.

Mutual inductance is also used in lots of other things, too – like wireless chargers, for example.

Lots of cell phones have a wireless charging feature, where you just put the phone on top of a charging pad and it charges.

No need to plug anything in.

It works because there’s a coil inside the charging pad, and another one inside your phone.

The AC power running through the charging pad induces an emf in your phone’s coil, which can use the energy to charge its battery.

So the same technology that helps get electricity to the outlets in your wall can move power from one device to another – even if there are no physical connections between them.

Today, you learned about how electricity is produced and transmitted.

We explained how electric generators work, and derived the generator equation.

We also talked about transformers, and how they use mutual inductance to change voltage.

Finally, we described how mutual inductance is used in wireless chargers.

Crash Course Physics is produced in association with PBS Digital Studios.

You can head over to their channel and check out a playlist of the latest episodes from shows like:

The Art Assignment, Blank on Blank, and Braincraft.

This episode of Crash Course was filmed in the Doctor Cheryl C. Kinney Crash Course Studio

with the help of these amazing people and our equally amazing graphics team, is Thought Cafe.