- Joined
- Jan 18, 2008
- Messages
- 144
- Reaction score
- 0
Can someone summarize and explain how to do a problem with Faraday's Law & Lenz's law? I'm kind of confused.
faraday's law/lenz's law
- Thread starter sweetsaja
- Start date
- Joined
- Jan 10, 2008
- Messages
- 106
- Reaction score
- 7
Let's consider a loop of copper wire, or any other conducting material. Now let's say we put this wire into a pre-existing magnetic field. Imagine the magnetic field lines as we're moving the wire further into the magnetic field... they're going to be getting more and more dense because we're moving closer to the field. We could say the magnetic flux is changing because the strength of the field is changing (it's getting stronger).
In the case of this wire, Faraday's Law is basically saying that a potential difference (also known as an electromotive force, or emf) is created within the wire due to the changing strength of the magnetic field... more change in flux, the greater the emf created. And we know what happens when there's a potential difference... we'll have a flow of electrons. This means we have a current flowing, and because of this, an electric field is produced as well. And if we consider the fact that a moving charge creates a magnetic field, we realize that not only are a current and electric field induced, but a new magnetic field as well.
Lenz's Law takes this principle one step further and considers the induced current and the resultant magnetic field. The basic idea is that the magnetic field that is created by the induced current opposes motion towards or away from the first magnetic field that started the whole thing. If we're moving our wire into the north side of a magnetic field, the induced current will create a magnetic field with a north pole repelling the north pole of the original magnetic field. However, if we're moving the wire out and away from the north pole of the first magnetic field, the induced current will create its own south pole that is attracted to the first field's north pole. So basically the wire doesn't want to be too close or too far away, so it opposes anything that is too extreme, and does so through directing its magnetic field in reference to the first magnetic field.
I haven't done too many problems yet, so other people will be able to help you more and maybe clarify what I've said. But the problems I have come across have asked for the direction of the induced magnetic field or the induced current under different conditions (where a wire is moving in reference to a field, where the field is pointing, etc)... just understand and apply these principles and you should be fine.
In the case of this wire, Faraday's Law is basically saying that a potential difference (also known as an electromotive force, or emf) is created within the wire due to the changing strength of the magnetic field... more change in flux, the greater the emf created. And we know what happens when there's a potential difference... we'll have a flow of electrons. This means we have a current flowing, and because of this, an electric field is produced as well. And if we consider the fact that a moving charge creates a magnetic field, we realize that not only are a current and electric field induced, but a new magnetic field as well.
Lenz's Law takes this principle one step further and considers the induced current and the resultant magnetic field. The basic idea is that the magnetic field that is created by the induced current opposes motion towards or away from the first magnetic field that started the whole thing. If we're moving our wire into the north side of a magnetic field, the induced current will create a magnetic field with a north pole repelling the north pole of the original magnetic field. However, if we're moving the wire out and away from the north pole of the first magnetic field, the induced current will create its own south pole that is attracted to the first field's north pole. So basically the wire doesn't want to be too close or too far away, so it opposes anything that is too extreme, and does so through directing its magnetic field in reference to the first magnetic field.
I haven't done too many problems yet, so other people will be able to help you more and maybe clarify what I've said. But the problems I have come across have asked for the direction of the induced magnetic field or the induced current under different conditions (where a wire is moving in reference to a field, where the field is pointing, etc)... just understand and apply these principles and you should be fine.
- Joined
- Jul 2, 2009
- Messages
- 94
- Reaction score
- 0
I'm pretty sure this is fair game for the MCAT. Raab's answer is very thorough, but if you want a simplification of the two law's combined:
when a conducting loop is exposed to a changing magnetic field, the a current will start in the loop to create a magnetic field opposing the change.
According to right hand rule #something, if you point your thumb in the direction of the magnetic field created by a current, your fingers will wrap around in the direction of flow of the current. Since Lenz's law saws the magnetic field created will oppose the change, you point your thumb in the opposite direction of an increasing magnetic field to see the direction of flow (or in the same direction as a decreasing magnetic field).
Hope that is makes solving problems with these rules simpler 😳.
when a conducting loop is exposed to a changing magnetic field, the a current will start in the loop to create a magnetic field opposing the change.
According to right hand rule #something, if you point your thumb in the direction of the magnetic field created by a current, your fingers will wrap around in the direction of flow of the current. Since Lenz's law saws the magnetic field created will oppose the change, you point your thumb in the opposite direction of an increasing magnetic field to see the direction of flow (or in the same direction as a decreasing magnetic field).
Hope that is makes solving problems with these rules simpler 😳.
Yea, basically - Lenz's law (LL) is a re-statement of the conservation of energy. LL can be used to find the direction of the induced electromotive force (emf) and current resulting from electromagnetic induction.
LL states: "an induced current always in such a direction as to oppose the motion or change causing it".
Basically, you can't get more energy out of nowhere (law of conservation of energy), so the only direction for an induced current is to oppose the change causing it.
Hope this helps.
LL states: "an induced current always in such a direction as to oppose the motion or change causing it".
Basically, you can't get more energy out of nowhere (law of conservation of energy), so the only direction for an induced current is to oppose the change causing it.
Hope this helps.
- Joined
- Sep 17, 2007
- Messages
- 765
- Reaction score
- 3
Here's another way to look at it.
Lets say you have a magnetic field with lines pointing INTO the screen.
You have a coil and you are moving it towards the magnetic field, and eventually into it.
Lenz's law is basically saying that the magnetic field in the coil will be pointing OUT of the screen. Now, with the same magnetic field, if the coil is being moved away from it, the magnetic lines are going to be pointing INTO the screen.
Same thing will happen with a magnetic field pointing OUT, except in opposite for moving the coil into and away from.
Basically, this is kind of like Le Chatlier's principle. The system will do whatever is necessary to reduce the stress on it. In this case, the magnetic field lines. If the coil is moving towards a magnetic field, the coil will respond in a way as to create a magentic field that will cancel off the strengthening, hence, creating a magnetic field in the opposite direction. If the coil is being moved away from the magnetic field, the coil will respond in a way that will amplify the magnetic field, hence, creating one that will be in the same direction as the magnetic field.
Lets say you have a magnetic field with lines pointing INTO the screen.
You have a coil and you are moving it towards the magnetic field, and eventually into it.
Lenz's law is basically saying that the magnetic field in the coil will be pointing OUT of the screen. Now, with the same magnetic field, if the coil is being moved away from it, the magnetic lines are going to be pointing INTO the screen.
Same thing will happen with a magnetic field pointing OUT, except in opposite for moving the coil into and away from.
Basically, this is kind of like Le Chatlier's principle. The system will do whatever is necessary to reduce the stress on it. In this case, the magnetic field lines. If the coil is moving towards a magnetic field, the coil will respond in a way as to create a magentic field that will cancel off the strengthening, hence, creating a magnetic field in the opposite direction. If the coil is being moved away from the magnetic field, the coil will respond in a way that will amplify the magnetic field, hence, creating one that will be in the same direction as the magnetic field.
