Electric Current: Crash Course Physics #28

Summary

This engaging script delves into the fundamentals of electric current, drawing parallels with the flow of rivers. It explains how electric current is formed through the movement of electrons, driven by a voltage difference, and how Alessandro Volta's invention of the voltaic cell paved the way for continuous electric current generation. The concept of a ground as a common conductor is introduced, followed by an explanation of current measurement in amperes. The script clarifies the historical convention of current direction, set by Benjamin Franklin, which is opposite to the actual electron flow. Ohm's Law and its significance in determining current based on voltage and resistance is discussed. The role of resistance in affecting current flow is highlighted, and superconductors are mentioned as materials with zero resistance at extremely cold temperatures. The script concludes with a discussion on power, its calculation, and its importance in understanding the energy transformation in electrical devices, emphasizing the relevance of Ohm's Law in predicting circuit behavior.

Takeaways

• 🌊 The flow of electricity in a wire is compared to the flow of water in a river, with electrons being the charged particles that move.
• 🔋 Electric current is the total amount of charge passing through a wire over a period of time.
• 💡 Voltage, or electric potential difference, provides the energy for charged particles to move, similar to how water flows from high to low elevation.
• 🔌 The first voltaic cell, invented by Alessandro Volta, used chemical reactions to create an electric potential difference between two different metals.
• 🔋 Multiple voltaic cells connected together form a battery, which can provide a higher voltage and continuous current.
• 🌐 A ground connected to the wire serves as a path for the current to a large reservoir of charge, typically the Earth.
• 📐 The strength of the electric current is measured in amperes, with one ampere being equivalent to one Coulomb of charge passing through a wire's cross section per second.
• 🔄 The conventional direction of electric current is positive to negative, opposite to the actual movement of negatively charged electrons.
• ⚙️ Resistance is a property of materials that impedes the flow of electrons, and it is measured in Ohms.
• 📈 Ohm's Law states that voltage is directly proportional to current when resistance is constant, and can be expressed as voltage equals current times resistance.
• 💡 Power is the amount of energy transformed by a device over time, and is calculated as the product of current and voltage in a circuit.

Q & A

How does the flow of water in a river relate to the flow of electricity through a wire?

The flow of water in a river relates to the flow of electricity through a wire in that both involve the movement from a higher potential to a lower potential. In the case of water, it's from a higher elevation to a lower one, while for electricity, it's the movement of electrons from a region of high voltage to one of low voltage.

What is electric current and what is it a measure of?

Electric current is the total amount of electric charge passing through a wire over a period of time. It is a measure of the rate at which charge moves through a conductor.

How does Alessandro Volta's voltaic cell contribute to the generation of electric current?

Alessandro Volta's voltaic cell contributes to the generation of electric current by using chemical reactions to create an electric potential difference between two different metals, known as electrodes. When these electrodes are connected, current begins to flow.

What is the role of a ground in an electrical circuit?

In an electrical circuit, a ground serves as a common conductor that ensures the current always has a path to a large reservoir of charge, which is usually the Earth itself. It provides a stable reference point for electric potential.

How is the strength of electric current described and what is its unit?

The strength of electric current is described by the amount of charge flowing through a wire's cross section over a period of time, measured in Coulombs per second. The special unit for this measurement is the ampere.

Why is the direction of conventional current flow considered to be from positive to negative?

The direction of conventional current flow is considered to be from positive to negative because of the historical convention established by Benjamin Franklin. It was later discovered that this direction is actually opposite to the movement of electrons, which are negatively charged.

What is resistance and how is it measured?

Resistance is a property of materials that impedes the flow of electrons, preventing a perfect flow of electric current. It is measured in units of Ohms, where one Ohm of resistance allows one Volt of potential to generate one Ampere of current.

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

Ohm's Law is a fundamental principle that states the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. It is expressed as V = I * R, indicating that voltage is directly proportional to current when resistance is constant.

How can superconductors impact the efficiency of electrical transmission?

Superconductors are materials that can have their resistance brought to zero when cooled to very low temperatures. This lack of resistance allows for the transmission of electricity with minimal energy loss, significantly increasing the efficiency of electrical transmission.

What is power in the context of electrical circuits and how is it calculated?

Power in electrical circuits refers to the amount of energy transformed by a device over time. It is calculated as the product of the current (I) and the voltage (V) across the device, with the resulting units in Watts (W).

How can Ohm's Law be used to find the power consumed by a resistor?

Ohm's Law can be used to find the power consumed by a resistor by substituting the law's relationship (V = I * R) into the power equation (P = V * I). This allows for the derivation of power equations in terms of voltage and resistance or current and resistance, which can be helpful when not all circuit information is available.

What is the significance of understanding the concepts of electric current, Ohm's Law, and power in the study of physics?

Understanding the concepts of electric current, Ohm's Law, and power is significant as they form the basis for analyzing and designing electrical circuits. These principles are fundamental to the operation of all electronic devices and are essential for engineers to optimize performance and efficiency in various applications.

Outlines

00:00

🌊 Understanding Electric Current

This paragraph introduces the concept of electric current by comparing it to the flow of water in a river. It explains that electric current is the movement of electrons and is influenced by voltage, which is the driving force that causes electrons to move from a region of high electric potential to one of low potential, similar to water flowing from high to low elevation. The paragraph also discusses the invention of the voltaic cell by Alessandro Volta, which marked a significant step in generating a continuous electric current. The voltaic cell uses chemical reactions to create a potential difference between two different metals, known as electrodes. When these electrodes are connected, an electric current flows. The concept of a battery is introduced as a combination of one or more voltaic cells, whose voltages add up. Additionally, the paragraph touches on the importance of a ground in an electrical system, the measurement of current in amperes, and the historical convention established by Benjamin Franklin regarding the direction of current flow, which is from the positive to the negative terminal.

05:06

🔋 Batteries, Ohm's Law, and Power Calculations

The second paragraph delves into the properties of materials that affect the flow of electric current, such as resistance, which is measured in Ohms. It explains that resistance is a property that impedes the flow of electrons, and that voltage, current, and resistance are interrelated through Ohm's Law. This law states that voltage is directly proportional to current when resistance is constant, and it can be expressed as voltage equals current times resistance. The paragraph also highlights the existence of materials, known as superconductors, which can have their resistance reduced to zero at extremely low temperatures. Furthermore, it discusses the concept of power in electrical circuits, which is the rate at which energy is transferred or converted over time, measured in Watts. Power is calculated as the product of current and voltage. The paragraph concludes by emphasizing the practical applications of these concepts in understanding and calculating the behavior of electrical devices and circuits.

Keywords

💡Electric Current

Electric current is the flow of electric charge, typically of electrons, through a conductor such as a wire. It is a fundamental concept in the video, as it is likened to the flow of water in a river. The current is described in terms of the total amount of charge passing through a wire over time. It is generated by a voltage difference, which provides the energy for the electrons to move, and is measured in amperes.

💡Voltage

Voltage, also known as electric potential difference, is the force that drives the flow of electric charge between two points. It is compared to the elevation difference in a river that causes water to flow. Voltage is essential for creating a continuous flow of electric current and is generated by devices like voltaic cells or batteries.

💡Voltaiac Cell

The voltaic cell, invented by Alessandro Volta, is a historical device that uses chemical reactions to create an electric potential difference between two different metals, known as electrodes. When connected, it allows for the flow of electric current. The concept of the voltaic cell is foundational to understanding how batteries work and is central to the video's discussion on generating electric current.

💡Terminals

Terminals are the connection points of a voltaic cell or a battery. They allow for the attachment of wires to complete an electrical circuit. In the context of the video, terminals are where multiple voltaic cells can be connected to increase the overall voltage of the battery.

💡Ground

A ground in an electrical system is a common conductor that provides a path for the electric current to return to a large reservoir of charge, often the Earth itself. It ensures that the current has a safe and stable path, which is crucial for the operation of electrical devices.

💡Ampere

The ampere is the unit of measurement for electric current, defined as the flow of one coulomb of charge per second. It is used to describe how strongly the charge flows through a wire. In the video, the ampere is used to quantify the amount of charge moving past a certain point in a given time period.

💡Ohm's Law

Ohm's Law is a fundamental principle in electrical engineering that states the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. It is expressed as V = I * R. The law assumes a constant resistance and is used to predict and calculate the behavior of circuits, as discussed in the video.

💡Resistance

Resistance is a property of materials that impedes the flow of electric current, like rocks and branches obstructing water flow in a river. It is measured in Ohms and is a key factor in determining the current flow in a circuit. The video explains that resistance is directly related to the voltage and current according to Ohm's Law.

💡Superconductors

Superconductors are materials that, when cooled to very low temperatures, exhibit zero electrical resistance. This means that an electric current can flow through them without any energy loss, making them highly efficient for power transmission. The video highlights superconductors as an important area of research with significant potential for improving energy efficiency.

💡Power

Power is the rate at which energy is transferred or converted in an electrical circuit, measured in watts. It is calculated as the product of voltage and current (P = V * I). In the context of the video, power is used to describe the amount of energy used by electrical devices, such as a light bulb, which converts electrical energy into light and thermal energy.

💡Energy Transformation

Energy transformation refers to the conversion of energy from one form to another, such as from electrical energy to light or heat. This concept is central to the video's discussion on how electrical devices operate, as they use electrical energy to perform their functions, like a light bulb producing light.

Highlights

Electric current is analogous to the flow of water in a river, representing the flow of electrons through a wire.

Electric current is the total amount of charge passing through a wire over a period of time.

Voltage provides the energy for charged particles like electrons to move from high to low voltage, similar to water flow from high to low elevation.

Alessandro Volta invented the first voltaic cell, which uses chemical reactions to create an electric potential difference.

Multiple voltaic cells can be connected to form a battery, increasing the voltage and allowing for a continuous flow of electricity.

A ground is a common conductor that ensures the current always has a path to a large reservoir of charge, typically the Earth.

The strength of the electric current is measured in amperes, which is the amount of charge flowing per second.

The direction of electric current is conventionally considered to be the direction positive charge would flow, even though it's actually electrons moving.

Ohm's Law describes the relationship between voltage, current, and resistance, and is fundamental in predicting and calculating circuit behavior.

Resistance in a material impedes the flow of electrons and is measured in Ohms.

Superconductors are materials that can have their resistance brought to zero at extremely low temperatures, significantly increasing electrical transmission efficiency.

Power is the amount of energy transformed by a device over time and is calculated as the product of current and voltage.

In a circuit, devices that consume power can be modeled as resistors, and their power consumption can be determined using Ohm's law.

The power equation can be rearranged using Ohm's law to find relationships between voltage, current, and resistance in a circuit.

Understanding the math behind electric currents reveals their elegance and practical applications in various devices and systems.

The episode was produced in association with PBS Digital Studios and features a variety of educational and entertaining content.

Transcripts

Imagine you’re floating down a river.

Maybe the stream is calm and serene, flowing without obstruction.

Or perhaps it’s a rushing river, churning and splashing over rocks!

Well, the way water moves in a river is a lot like how electricity flows through a wire.

But when we say that electricity is flowing, we’re really talking about the flow of electrons.

Electric current is the total amount of charge passing through a wire over a period of time.

But how does current actually form?

What can it pass through?

And what determines how strong it is?

It’s time to sit back, relax, and go with the flow.

[Theme Music]

Let’s talk about how we get charges to flow from one place to another.

Last time, we learned that when there’s a difference in electric potential between two points, the voltage gives charged particles, like electrons, the energy to move from one place to another.

Just like a river flows from high elevation to low elevation, electric charge flows from high voltage to low voltage.

So, we need a way to generate a voltage that gets charge to flow continuously, creating an electric current.

Before the 19th century, scientists had been able to generate static charge by rubbing different materials together.

But they couldn’t do much more than create small sparks.

They didn’t fully understand what they were doing, so they couldn’t figure out how to create a constant voltage to generate a steady flow of electricity.

To solve this problem, Italian scientist Alessandro Volta invented the first voltaic cell,

which uses chemical reactions to create an electric potential difference between two pieces of different metals, known as electrodes.

When the two electrodes are connected, current begins to flow.

We call these connection points terminals, and we can connect multiple voltaic cells together by placing wires between their opposite electrodes.

When one or more voltaic cells are combined, their voltages add up, and together they form a battery.

Today’s batteries operate under the same principle as the very first voltaic cell.

Now that we have a source for our current, we’ll also need a ground that’s connected to the wire.

This is just a common conductor that ensures the current always has a path to a large reservoir of charge -- usually the Earth itself.

Next, once we can generate an electric current, we’ll need a way to describe how strongly the charge flows.

If you picture the cross section of a wire, you can measure how much charge flows through that cross section over a period of time.

The amount of charge moving past this point, divided by the time period, gives us a value in Coulombs per second, which we call by the special unit, amperes.

So one Coulomb of charge passing through that cross section of wire over one second is equal to one ampere of current.

OK but wait.

Did you notice that we’re talking about positive charge flowing through a wire?

How can that be?

After all, the current is made up of negatively charged electrons moving through the wire.

Well, when American polymath Benjamin Franklin did experiments with electricity in the 1700s,

he established what he thought to be the direction in which electricity flows, and he named it the “positive” direction of current.

It’s a convention we still use today.

It wasn’t until much later that we learned the direction Franklin chose was actually the opposite of how electrons move in a wire.

But as far as electric current is concerned, the flow of negatively charged electrons in one direction is equivalent to the flow of positively charged particles in the opposite direction.

While this can be confusing, remember that when we talk about the flow of current, the convention is to say the current is in the direction that positive charge flows.

Now, if you’ve ever installed a battery in…anything…you also know that one terminal of a battery is called positive and the other one is negative.

And conventionally speaking, current flows from the positive to the negative terminal.

But this means that electrons must flow from the negative terminal of the battery to the positive.

Picture the positive terminal as the start of our river, up in the mountains, and the negative terminal as the end of the river, draining out into the ocean.

In a river, the strength of the water’s flow depends on how far the river drops from a high point to a low point.

And the same is true for electric current and voltage.

A high voltage typically corresponds to a high current in a circuit.

But voltage alone doesn’t determine how much current flows.

Just like rocks and branches obstruct the passage of water, the materials used to conduct electricity have properties that impede the perfect flow of electrons.

This property is known as resistance.

And resistance is described in Ohms, where one Ohm of resistance would let one Volt of potential generate one Ampere of current.

When resistance is constant, voltage is directly proportional to current.

And this relationship is known as Ohm’s Law.

This law assumes that the resistance of a material is constant.

So we can express voltage simply as current times resistance.

There are substances in which the resistance is not constant, and it changes with current or voltage.

But for many materials -- known as ohmic materials -- Ohm’s law works quite well.

Now, in ohmic materials, we can use our new expression of Ohm’s Law to predict and calculate the behavior of a circuit.

Let’s say we have a 9 Volt battery, and we want to know how much current it supplies to a light bulb when we complete the circuit.

If we know the system has a total resistance of 15 Ohms, we can divide the battery’s voltage by the resistance, to find that it has a current of 0.6 amperes, or 600 milliamperes.

And whether it’s a light bulb, a speaker system, or a supercomputer, most electrical devices and materials contain some level of resistance.

Even wires in a circuit have some resistance, but compared to the resistance of a connected device like a light bulb, it’s so minuscule that we typically neglect it.

But you want to know a neat trick?

Well it doesn't matter because I'm going to tell you, anyway.

If you can make certain conductive materials extremely cold, you can bring their resistance to zero.

We call these materials superconductors, and trust me when I say that research into these materials is a very important -- and lucrative -- field of study.

Because, if you can reduce the resistance of a material, by getting rid of that natural loss of energy, you can significantly increase the amount of electricity you transmit.

And in all aspects of engineering, efficiency is key.

Now, in that spirit, I should probably point out here that the whole point of batteries isn’t to just push electrons around.

We need to put those electrons to work.

For example, when we attach a battery to a light bulb, we take the potential energy in the battery and turn it into what we need, light.

As current flows through the light bulb, the small piece of filament provides resistance, which transfers electric energy into thermal energy and light.

And since we’re dealing with energy transformed over time, what we really want to know is how much power is used by the bulb.

Power is the amount of energy transformed by a device over time.

And by “transformed,” I mean the energy is converted from electric energy into some other useful kind of energy, like heat or light.

We’ve already learned how to calculate how much energy is in a certain amount of charge that’s moving between a difference in potential -- that is, the voltage.

And since charge transferred over time is just current, it follows that the power used by the bulb is equal to the current in the circuit times the voltage across the system.

And just like in our other calculations of power, our resulting units are in Watts, named for Scottish inventor James Watt.

This equation holds true for the power used by any electronic device, or the power supplied by a battery.

Now, anything that consumes power -- from a light bulb to a refrigerator to an entire house -- can be modeled in our equations as a resistor, because they create resistance.

And if you want to find the power that’s consumed by a resistor, you can substitute Ohm’s law into the power equation to discover some helpful relationships that hold true for all ohmic materials.

Let’s start with the fact that power is a function of current through, and voltage across, a resistor.

And Ohm’s law is about relating voltage, current, and resistance together.

So, you can either replace the current in the equation, and get a new power equation in terms of voltage and resistance

…or you can replace the voltage, and get an expression in terms of only current and resistance.

This is especially useful when you don’t have all the possible information about the circuit.

So that’s your introduction to electric currents!

They really are just like flowing rivers!

And, once you understand the elegant math behind them, they’re almost as beautiful.

Today we learned about what defines electric current and how we generate it using voltaic cells that make up batteries.

We also learned about Ohm’s Law and the relationship between voltage, current, and resistance.

Finally, we discussed what power is and how we measure the power produced and consumed in a circuit.

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

You can head over to their channel to check out amazing shows like PBS Game Show, The Good BBQ With Franklin, and Blank on Blank.

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.