# Is there any way possible a magnetic field can cancel out an electric field?

#### darknight08

Is there any way possible a magnetic field can cancel out an electric field?

Thanks !

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#### berkeman

Mentor
Is there any way possible a magnetic field can cancel out an electric field?

Thanks !
Are you familiar with the Lorentz force? What can you tell us about your question?

#### darknight08

The thought was inspired by the passage and question below. Also, I still don't understand the explanation thoroughly. I understand the electric field portion, but not the magnetic field.

http://imageshack.us/m/847/8320/passage1.jpg

Thanks!

#### darknight08

More specifically, I don't understand this statement: "The magnetic force, in order to cancel the electric force, must point upward" ???????

#### berkeman

Mentor
More specifically, I don't understand this statement: "The magnetic force, in order to cancel the electric force, must point upward" ???????
Because the E field points up, the electric force on the electron is down. Therefore, to cancel the electric force with a magnetic force, the magnetic force has to point up.

And to answer your original question in the context that you've shown, yes, for certain situations you can get a manetic field to cancel the force on a charged particle from an electric field. In the general 3-d case you can't get the fields to "cancel", but when you apply some physical constraints on the situation, you can make the forces cancel.

That's why I asked if you are familiar with the Lorentz Force:

http://en.wikipedia.org/wiki/Lorentz_force

.

#### darknight08

Because the E field points up, the electric force on the electron is down. Therefore, to cancel the electric force with a magnetic force, the magnetic force has to point up.
I am still having hard time understanding why if a magnetic force points in the same direction as an Electric field, it will cancel?

#### berkeman

Mentor
I am still having hard time understanding why if a magnetic force points in the same direction as an Electric field, it will cancel?
The electric force on a *negatively* charged particle is in the opposite direction as the Electric field. Look at the Lorentz Force equation:

F = qE + q(v x B)

If q is negative (like is for electrons), the electric force is opposite the E field direction. Just remember that the E field direction is defined as the direction of force on a *positive* test charge.

Does that help?

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