# More, more physics... T...T?

1. So, if I have a negatively charged partiсle which is placed into a homogeneous magnetic field. What will describe particles motion the best:

- the body moves at a constant speed

- the body moves at a steady acceleration

- the body moves in a circular motion at a steady speed.

- the body moves at an accelerating speed on the circular path.

- the body is resting.

I was thinking about the 3rd option but first one may also suit. Which option is right? I am not quit sure I understand it fully.

2. The charged particle (+q) is in the homogeneous electric field and is affected by the electric force. What happens to the magnitude and direction of the electric force when

- the electric field strength is diminished?

- the charge is given an initial velocity perpendicular to the field?

I have tried to find the answers in the text book but it is so uncertain and literally “like in the fog”, may somebody in brief explain it a little bit?

Relevance

Update 2

You asked: “Because if it is uncharged, there supposed to be no direction any more... or am i wrong? “

I think you misunderstand the terminology and concepts.

Only an *object* can be charged or uncharged. E.g. it is charged if it has extra electrons. If the *object* in the field is uncharged, there is no electric force on the object, so there is no direction for the electric force.

If the field is zero, we say there is no field. We do *not* say the field is uncharged – that makes no sense. The field is a region of space, not an object

The original question is *not* asking about uncharged objects or zero fields.

There are 3 different things to get clear in your mind:

1. The electric field. If it exists it *always* has a direction. The direction is the direction of the force you *would* get on a positive charge *if* you put a positive charge in the field. But the field is still there and has a direction even if there is no charged object in the field. (Just like near the earth’s surface there is always a gravitational field acting down.)

2. The charged object you put in the field. Charge has no direction, just like a mass has no direction.

3. The electric force on the charged object. This force has a direction. If the charge is positive the force’s direction is the same as the field’s direction. Just like weight (gravitational force) acts down due to the earth’s gravitational field. If the charge is negative the electric force’s direction is opposite to the electric field’s direction.

End update 2.

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Update 1..

Your comment saya: “even though the electric force is uncharged is it still right to say that the direction is same?”

I don’t understand. A force can’t be “uncharged”; only objects can be charged or uncharged. Is it a typing mistake? Do you mean “even though the electric force is unchaNged is it still right to say that the direction is same?”?

The direction of the electric force on a positive charge *is always the same as* the direction of the electric field. Reducing the field does not change its direction.

See if working-through this example helps.

Example.

An electric field has magnitude E = 2x10⁶ N/C (or you can use the equivalent unit ‘V/m’ if preferred). The field acts in the +x direction.

An object O with a charge q = +3x10⁻⁶C is in the field.

The electric force on O is F = qE = +3x10⁻⁶ x 2x10⁶ = 6N.

The direction of this force on O is the +x direction (the same direction as the electric field).

The magnitude of the electric field is now reduced to 2x10⁶ N/C. (The direction is unchanged, still the +x direction.)

The new force on O is F = qE = +2x10⁻⁶ x 2x10⁶ = 4N.

The direction of this force is still the +x direction (the same direction as the electric field).

Does that cleatr up the problem? Let me know if you want further explanation!

End update 1.

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Sorry, answer is a bit long as I’ve tried to explain in detail.

1. The magnitude of the magnetic force equals Bqv.

Note that if the particle is stationary, v=0 and the force is zero, because Bqv = Bq*0 = 0.

(For information, if the particle is moving, the magnetic force’s direction is perpendicular to B and to v, in accordance with the right hand rule.)

The question says the particle is ‘placed’ in the magnetic field. That means it is initially stationary (v=0).

Since there is no magnetic force acting on the particle, the particle will remain stationary. Answer 5: ”the body is resting”

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2. A charge (q) in an electric field (E) experiences a force of magnitude qE. Note that ’v’ is not in the equation.

For a positive charge the direction of the force is the same as the direction of the electric field.

For a negative charge the direction of the force is the opposite to the direction of the electric field.

The magnitude and direction of the electric force *are not affected by movement of the charge*.

When the electric field strength is diminished, the force diminishes by the same factor because F = qE. E.g. if you halve E, you halve F.

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When the charge (+q) is given an initial velocity perpendicular to the field [or in any direction], the electric force is unchanged.

The force’s magnitude is still qE, and the direction of the force is still the same as the direction of E as explained above.

The fact that the charge is moving makes no difference to the electric force. (But that’s not the case for the magnetic force.)

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