7 min read•january 8, 2023
Krish Gupta
Daniella Garcia-Loos
Krish Gupta
Daniella Garcia-Loos
Electric Charge is a property of matter that causes it to feel a force in an electromagnetic field. Electric charge must be conserved. The entire topic of current and circuits is based upon the principle of conservation of charge. We learned about charge and its conservation in the last unit. In this unit, we will focus more on current and how it relates to theconservation of charge.
In Unit 3, we studied voltage and defined it as work per unit charge. There are 2 other important quantities used along with voltage to describe the features of a circuit: current and resistance.
A common analogy for how voltage, current, and resistance are related to each other is to think of an electric circuit like water flowing through a hose. Voltage is similar to the water pressure, current is similar to the amount of water that gets through the hose, and resistance is similar to mud or dirt that gets stuck in the hose and starts to clog it.
Conservation of electric charge refers to the principle that the total electric charge in a closed system remains constant over time. This means that electric charge can be transferred or distributed within the system, but the total amount of charge remains the same.
Here are some key points about conservation of electric charge:
Current is defined as the rate at which charge flows through a circuit. It's represented by the equation:
where I is the current (measured in Amps or milliAmps), Q is the charge passing a given point, and t is the time for the charge to pass through that point.
On a microscopic level, current is also related to the drift velocity (vd) of the individual charge carriers. Drift velocity can be thought of as the average velocity of each charge carrier as it moves through a wire. In the image below, we're looking at the path of the electron as it moves through a wire.
Drift velocity is the average velocity of a charged particle, such as an electron, in an electric field. It is the velocity at which the charged particle would move if it were not subjected to any other forces or collisions.
Here are some key points about drift velocity:
We can imagine that the current in the wire would depend on the total number of charge carriers moving through the wire as well. A larger diameter wire would allow for more carriers. Combining these ideas together we can derive an equation for current.
Example Problem:
A metal wire has a cross-sectional area of 1 square millimeter and is subjected to an electric field of 1000 volts per meter. The wire is made of a metal with a drift velocity of 1 millimeter per second for electrons. Calculate the electric current flowing through the wire.
Solution:
To solve this question, you would need to use the formula for electric current, which is I = qA vd, where I is the electric current, q is the electric charge of the charged particles (in this case, electrons), A is the cross-sectional area of the wire, and vd is the drift velocity of the charged particles. Using the given values, the electric current flowing through the wire would be:
I = (1.6 x 10^-19 C) * (1 mm^2) * (1 mm/s) = 1.6 x 10^-19 C/s = 1.6 x 10^-15 A
Note that the electric current is very small because the drift velocity of the electrons is very small and the electric charge of an electron is also very small.
To easily draw circuits, we use a variety of symbols to represent common components. Here are a few common ones, and there are many many more that are not used in AP 2 (although if you become an electrical engineer you'll use them!)
There are also two tools listed in the image above: the voltmeter and the ammeter. The voltmeter is designed to accurately measure the potential difference between two points. Because of this, it is built with a very high internal resistance so as not to create a short circuit when it bridges two points in a circuit. A voltmeter is always connected in parallel around the object you are trying to measure.
On the other hand, an ammeter is designed to measure the current flowing through a part of the circuit. Because it's going to be connected in series with the component it's measuring, the internal resistance of the ammeter is designed to be very small. Hooking up an ammeter or voltmeter in the wrong configuration can lead to short circuits or a meter that doesn't function at all. Be careful in your lab experiments, and check first before you connect them.
An ammeter is a device used to measure the electric current flowing in a circuit, while a voltmeter is a device used to measure the electric potential difference, or voltage, across two points in a circuit.
Here are some key points about ammeters and voltmeters:
Ammeters and voltmeters are usually part of a multimeter, which is a device that can measure multiple electrical quantities, including current, voltage, resistance, and continuity.
Ammeter
: An ammeter is a device used to measure the electric current flowing through a particular point in an electrical circuit.Ampere (A)
: An ampere is the unit used to measure electric current. One ampere represents one coulomb of charge passing through a point in one second.Conservation of electric charge
: Conservation of electric charge is a fundamental principle in physics that states that electric charge cannot be created or destroyed; it can only be transferred from one object to another. In other words, the total amount of positive and negative charges remains constant within an isolated system.Conventional Current
: Conventional current refers to the assumed direction of flow of positive charges in an electrical circuit. It is the opposite direction to the actual movement of negatively charged electrons.Coulomb (C)
: The coulomb is the unit of electric charge in the International System of Units (SI). It represents the amount of electric charge transported by a constant current of one ampere in one second.Current
: Current refers to the flow of electric charge in a circuit. It is measured in Amperes (A) and represents the rate at which charges move through a conductor.Drift Velocity
: The drift velocity is the average velocity of charged particles, such as electrons, in a conductor when an electric field is applied. It represents the net motion of charges due to the electric field.Electric Charge
: Electric charge refers to the fundamental property of matter that causes it to experience a force when placed in an electromagnetic field. It can be positive or negative.Resistance
: Resistance is a measure of how much an object or material opposes the flow of electric current. It determines how difficult it is for electrons to move through a circuit.Voltage
: Voltage refers to the electric potential difference between two points in an electrical circuit. It represents the amount of energy that each unit of charge possesses.Voltmeter
: A voltmeter is a device used to measure the voltage or potential difference between two points in an electrical circuit.7 min read•january 8, 2023
Krish Gupta
Daniella Garcia-Loos
Krish Gupta
Daniella Garcia-Loos
Electric Charge is a property of matter that causes it to feel a force in an electromagnetic field. Electric charge must be conserved. The entire topic of current and circuits is based upon the principle of conservation of charge. We learned about charge and its conservation in the last unit. In this unit, we will focus more on current and how it relates to theconservation of charge.
In Unit 3, we studied voltage and defined it as work per unit charge. There are 2 other important quantities used along with voltage to describe the features of a circuit: current and resistance.
A common analogy for how voltage, current, and resistance are related to each other is to think of an electric circuit like water flowing through a hose. Voltage is similar to the water pressure, current is similar to the amount of water that gets through the hose, and resistance is similar to mud or dirt that gets stuck in the hose and starts to clog it.
Conservation of electric charge refers to the principle that the total electric charge in a closed system remains constant over time. This means that electric charge can be transferred or distributed within the system, but the total amount of charge remains the same.
Here are some key points about conservation of electric charge:
Current is defined as the rate at which charge flows through a circuit. It's represented by the equation:
where I is the current (measured in Amps or milliAmps), Q is the charge passing a given point, and t is the time for the charge to pass through that point.
On a microscopic level, current is also related to the drift velocity (vd) of the individual charge carriers. Drift velocity can be thought of as the average velocity of each charge carrier as it moves through a wire. In the image below, we're looking at the path of the electron as it moves through a wire.
Drift velocity is the average velocity of a charged particle, such as an electron, in an electric field. It is the velocity at which the charged particle would move if it were not subjected to any other forces or collisions.
Here are some key points about drift velocity:
We can imagine that the current in the wire would depend on the total number of charge carriers moving through the wire as well. A larger diameter wire would allow for more carriers. Combining these ideas together we can derive an equation for current.
Example Problem:
A metal wire has a cross-sectional area of 1 square millimeter and is subjected to an electric field of 1000 volts per meter. The wire is made of a metal with a drift velocity of 1 millimeter per second for electrons. Calculate the electric current flowing through the wire.
Solution:
To solve this question, you would need to use the formula for electric current, which is I = qA vd, where I is the electric current, q is the electric charge of the charged particles (in this case, electrons), A is the cross-sectional area of the wire, and vd is the drift velocity of the charged particles. Using the given values, the electric current flowing through the wire would be:
I = (1.6 x 10^-19 C) * (1 mm^2) * (1 mm/s) = 1.6 x 10^-19 C/s = 1.6 x 10^-15 A
Note that the electric current is very small because the drift velocity of the electrons is very small and the electric charge of an electron is also very small.
To easily draw circuits, we use a variety of symbols to represent common components. Here are a few common ones, and there are many many more that are not used in AP 2 (although if you become an electrical engineer you'll use them!)
There are also two tools listed in the image above: the voltmeter and the ammeter. The voltmeter is designed to accurately measure the potential difference between two points. Because of this, it is built with a very high internal resistance so as not to create a short circuit when it bridges two points in a circuit. A voltmeter is always connected in parallel around the object you are trying to measure.
On the other hand, an ammeter is designed to measure the current flowing through a part of the circuit. Because it's going to be connected in series with the component it's measuring, the internal resistance of the ammeter is designed to be very small. Hooking up an ammeter or voltmeter in the wrong configuration can lead to short circuits or a meter that doesn't function at all. Be careful in your lab experiments, and check first before you connect them.
An ammeter is a device used to measure the electric current flowing in a circuit, while a voltmeter is a device used to measure the electric potential difference, or voltage, across two points in a circuit.
Here are some key points about ammeters and voltmeters:
Ammeters and voltmeters are usually part of a multimeter, which is a device that can measure multiple electrical quantities, including current, voltage, resistance, and continuity.
Ammeter
: An ammeter is a device used to measure the electric current flowing through a particular point in an electrical circuit.Ampere (A)
: An ampere is the unit used to measure electric current. One ampere represents one coulomb of charge passing through a point in one second.Conservation of electric charge
: Conservation of electric charge is a fundamental principle in physics that states that electric charge cannot be created or destroyed; it can only be transferred from one object to another. In other words, the total amount of positive and negative charges remains constant within an isolated system.Conventional Current
: Conventional current refers to the assumed direction of flow of positive charges in an electrical circuit. It is the opposite direction to the actual movement of negatively charged electrons.Coulomb (C)
: The coulomb is the unit of electric charge in the International System of Units (SI). It represents the amount of electric charge transported by a constant current of one ampere in one second.Current
: Current refers to the flow of electric charge in a circuit. It is measured in Amperes (A) and represents the rate at which charges move through a conductor.Drift Velocity
: The drift velocity is the average velocity of charged particles, such as electrons, in a conductor when an electric field is applied. It represents the net motion of charges due to the electric field.Electric Charge
: Electric charge refers to the fundamental property of matter that causes it to experience a force when placed in an electromagnetic field. It can be positive or negative.Resistance
: Resistance is a measure of how much an object or material opposes the flow of electric current. It determines how difficult it is for electrons to move through a circuit.Voltage
: Voltage refers to the electric potential difference between two points in an electrical circuit. It represents the amount of energy that each unit of charge possesses.Voltmeter
: A voltmeter is a device used to measure the voltage or potential difference between two points in an electrical circuit.© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.