Direction of electric field

In summary, according to the statement, the electric field is perpendicular to the magnetic field. The directions of the magnetic force and electric force are the same, but the magnitude of the magnetic force is greater.
  • #1
hmaier
8
3

Homework Statement


beam of protons moving in +x direction at speed v
electric field is perpendicular to magnetic field
magnetic field in +y direction
no deflection
What direction/sign has the electric field?

Homework Equations


not looking for numerical solutions

The Attempt at a Solution



There's no deflection, the magnetic and electric force have same magnitude, but opposing directions, correct?
The magnetic force is perpendicular to the magnetic field, while the electric force is in the direction of the electric field.
How do I then find the direction of the electric field?
+x and -x are perpendicular to +y. Let's say the magnetic force has a magnitude of 5, the electric force would be -5. Is the direction of the magnetic force -x, i.e. magnitude of -5 and thus the electric force a direction of +x, i.e. magnitude of 5?
 
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  • #2
hmaier said:
There's no deflection, the magnetic and electric force have same magnitude, but opposing directions, correct?
Yes.
First you have to find out what can be found out first. In this case, the magnetic force, because you know the directions of the protons' speed and the magnetic field. The magnetic force exerted on a charged particle moving with a velocity ##\mathbf{v}## in a uniform magnetic field ##\mathbf{B}## is given by ##\mathbf{F}_M=q\mathbf{v}\times \mathbf{B}##. What is then the direction of the magnetic force in this problem?
 
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  • #3
Look at your second statement.
What direction is the electric field in according to that statement?
 
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  • #4
my reply got deleted as I wasn't logged in properly...

the electric field, like the magnetic force should be perpendicular to the magnetic field.
I used your formula blue_leaf77 for the calculations. I think I actually need to use the right hand rule to determine the direction?
That gave me a direction pointint out of the paper given the usual x-y-z coordinate system a -z direction for the magnetic force, which whould mean that electric force would be +z.
I hope, I'm not getting totally off track here!
 
  • #5
hmaier said:
a -z direction
Almost correct. The velocity of the particle is in the x direction and the magnetic field is in the y direction. Using the right hand rule, which direction do you get for the magnetic force?
 
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  • #6
ok, so, index finger(particles) on the +x-axis, thumb (magnetic field) on +y-axis, so magnetic force is actually "into the paper", i.e. the positive z-axis?
I will need some sort of hand massage after this ;)
 
  • #7
I am rather confused with the way you define the directions, I think your definition is slightly different than mine. Anyway, you are correct, the magnetic force points to the positive z direction.
(By the way, I am using
images?q=tbn:ANd9GcT-bN45xM88dYJnigK_ZdPs34sSjzillNbfQ4EEHe-T8VfZOWS9Jg.png
)
 
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  • #8
My friend provided me with the teaching materials from a course he took. The assignment of fingers is switched around. B is the middle finger, F thumb and I index finger.
As long as it yields the same result, there's no harm done!
Thanks a million for guiding me along so generously. I'm really enjoying diving into physics and maths again after so many years.
 

1. What is the direction of the electric field?

The direction of the electric field is defined as the direction that a positive test charge would move when placed in the field. It is always directed away from positive charges and towards negative charges.

2. How is the direction of the electric field determined?

The direction of the electric field is determined by the direction of the electric force experienced by a positive test charge in the field. The direction of the electric force is dependent on the relative positions of the charges and their magnitudes.

3. Does the direction of the electric field change?

Yes, the direction of the electric field can change depending on the location and movement of the charges creating the field. It can also change based on the presence of other charges in the vicinity.

4. How does the direction of the electric field relate to the potential difference?

The direction of the electric field is always perpendicular to the equipotential lines, which represent points of equal potential. The electric field points towards regions of lower potential and away from regions of higher potential.

5. Can the direction of the electric field be manipulated?

Yes, the direction of the electric field can be manipulated by changing the position or magnitude of the charges creating the field. It can also be manipulated by using external electric fields or electrically charged objects.

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