[number23]

Can someone explain it to me?

So is the direction the direction a positive charge would travel in?

Also when an electron is fired horizontally into a field directed downwards, why does it curve upwards with increasing speed?

[number23]

Another similar question with magnetic fields: an electron beam is fired horizontally into a magnetic field directed perpendicular to the initial beam ... why does the beam curve round in a vertical circle? is it because of flemings left hand rule... ie the force goes "into the paper" ... hmm not sure

[Stonebridge]

(Original post by number23)
Can someone explain it to me?

A field where the force is everywhere constant and in the same direction.

So is the direction the direction a positive charge would travel in?

Yes, it's the direction of the force on the positive charge. It will move in that direction if it is able to.

Also when an electron is fired horizontally into a field directed downwards, why does it curve upwards with increasing speed?

For the reason in the answer above.
A positive charge would move downwards if the field is pointing downwards. A negative charge will move in the opposite direction.

[Stonebridge]

(Original post by number23)
Another similar question with magnetic fields: an electron beam is fired horizontally into a magnetic field directed perpendicular to the initial beam ... why does the beam curve round in a vertical circle? is it because of flemings left hand rule... ie the force goes "into the paper" ... hmm not sure

Yes. The LHR, when applied to the electron beam, will show a force acting at right angles to it at all times. This creates circular motion as it provides the centripetal force.

[number23]

(Original post by Stonebridge)
Yes. The LHR, when applied to the electron beam, will show a force acting at right angles to it at all times. This creates circular motion as it provides the centripetal force.

thanks, im also struggling on this question (the bit in italics)

1).electrons moving at a constant speed are directly horizontally into a uniform electric field due to two parallel plates of length 85.0mm and spaced 40.0mm apart which have a pd of 4000 volts between them

a).when a uniform magnetic field of flux density 2.95mT is applied at right angles to the beam and to the electric field, the beam is undeflected. Calculate the speed of the electrons.
ive done this bit by equating Bev=(eV)/D and rearranging

b). When the magnetic field is switched off, the beam is deflected vertically by the electric field by 55mm where it leaves the field. show that each electron takes 2.51ns to pass through the field and calculate the specific charge e/m of the electron.
think i can do the time thing, but i dont get the specific charge bit

[Stonebridge]

The key to this is to find the vertical acceleration of the electron. You know how far it has travelled vertically (55mm) and you know the time taken. The initial vertical component was zero on entering the field so you have plenty of values to plug into a SUVAT equation to find "a".

Then use F=ma

You know F=Ee for the force on the electron.

This will give you e/m

[number23]

(Original post by Stonebridge)
The key to this is to find the vertical acceleration of the electron. You know how far it has travelled vertically (55mm) and you know the time taken. The initial vertical component was zero on entering the field so you have plenty of values to plug into a SUVAT equation to find "a".

Then use F=ma

You know F=Ee for the force on the electron.

This will give you e/m

thank you soo much