Couple physics theory questions

[ptavasso]

Hi everyone. I have a few quick physics questions that aren't necessarily focused on what is tested on the MCAT, but more so for my understanding and curiosity. I understand the formulas and information required for the MCAT, but I would like to make sure that I have the correct fundamental knowledge behind this information.

1) my textbook says that "a charge moving through a magnetic field experiences a force". I'm wondering is it correct that the moving charge would have a net force of zero if it were moving at constant velocity (because then a=0 and thus, net force=0)? How I see it is that any field (gravitational, magnetic, electric, etc.) will have a net force of 0 if the velocity is constant because force is dependent upon mass and acceleration and constant velocity means a=0. Note that I am saying net force, and not individual force. Please correct me if I'm wrong.

2) since a changing electric field creates a magnetic field, then would all of the cases we're presented with that discuss a current moving in a circuit (as in a DC circuit) be creating a magnetic field? Please provide an example of when charge is not moving in an electric field (and thus B field is not created) since the examples I have encountered have been of a current moving in a circuit.

3) this question may sound silly... if what I have said in question 2 is all correct, would all circuits then be considered magnets since there is a current moving in the circuit, and this creates a B field? Or can you have a magnetic field without being a magnet?

Thank you!

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[m.z]

Force is a vector quantity, and any charged particle moving through a magnetic field will be forced into a curved path. Changing the magnitude or direction of forces exerted on an object changes the net force (sum of all forces) exerted on the object. The net force exerted on an object is defined as mass times acceleration (F = ma), where mass, m, is constant. This means that when the net force exerted on the object changes in magnitude (or direction), its acceleration will also change in magnitude (or direction). In addition, acceleration is defined as the change in velocity, so when the magnitude (or direction) of acceleration changes, the magnitude (or direction) of velocity will also change.


Electric and magnetic fields are really the same entity that we are merely treating as two distinct fields. Changing an electric field induces a magnetic field, and changing a magnetic field induces an electric field. Thus, a stationary charge does not induce a magnetic field, while a moving charge does induce a magnetic field.

Current is moving charge, thus any and all current creates a magnetic field.

 

[ptavasso]

Awesome thank you! I was thinking too much about it and the connection between direction change and velocity totally slipped my mind... oops.

I understand the theory behind it, but I'm just curious then why a circuit isn't also considered a magnet since it has a magnetic field in the same way that a magnet has a magnetic field. Can you have a magnetic field without being a magnet?

Force is a vector quantity, and any charged particle moving through a magnetic field will be forced into a curved path. Changing the magnitude or direction of forces exerted on an object changes the net force (sum of all forces) exerted on the object. The net force exerted on an object is defined as mass times acceleration (F = ma), where mass, m, is constant. This means that when the net force exerted on the object changes in magnitude (or direction), its acceleration will also change in magnitude (or direction). In addition, acceleration is defined as the change in velocity, so when the magnitude (or direction) of acceleration changes, the magnitude (or direction) of velocity will also change.


Electric and magnetic fields are really the same entity that we are merely treating as two distinct fields. Changing an electric field induces a magnetic field, and changing a magnetic field induces an electric field. Thus, a stationary charge does not induce a magnetic field, while a moving charge does induce a magnetic field.

Current is moving charge, thus any and all current creates a magnetic field.

 

[m.z]

You're welcome. Magnetic fields are what define magnets. Current carrying wires are what make up electromagnets, which are like temporary magnets (whenever there is current going through it produces a magnetic effect, and when there is no current there's no magnetic effect). In regards to circuits I guess that their purpose wouldn't be one that involves magnetism, or their magnetic fields are so small that they don't produce that much of an effect; but this is just my speculation, I don't know too much about electricity and magnetism to give a better answer.

 

[ptavasso]

Excellent, I'm happy to read that your answer is identical to what I thought! I wasn't willing to accept it at first since I hadn't read/ heard it anywhere but seeing you say the same thing gives me more assurance of this explanation.

Thanks again. You're awesome

You're welcome. Magnetic fields are what define magnets. Current carrying wires are what make up electromagnets, which are like temporary magnets (whenever there is current going through it produces a magnetic effect, and when there is no current there's no magnetic effect). In regards to circuits I guess that their purpose wouldn't be one that involves magnetism, or their magnetic fields are so small that they don't produce that much of an effect; but this is just my speculation, I don't know too much about electricity and magnetism to give a better answer.