An electron travelling at a constant speed enters a uniform electric field at right angles to the field. While the electron is in the field it accelerates in a direction which is

A in the same direction as the electric field.
B in the opposite direction to the electric field.
C in the same direction as the motion of the electron.
D in the opposite direction to the motion of the electron.

How do I tell what the answer is in a way that I can remember

Well I would say it accelerates at a right angle to both the field and the motion of the electron. Which isn't any of your choices. Hmmmm.

The answer is B but I'm looking for a way to explane the answer to me because I don't understand it fully, if you wouldn't mind having a look at my other physics qn

An electric field is defined as a vector pointing from positive to negative. Here's a crude picture:

+|---->|-
^
e

The left side is a positively charged plate, denoted "+" and the right side is a negatively charged plate, denoted "-". The arrow between the plates is the direction of the electric field and the electron is approaching the field from the bottom travelling "up" the page.

Remember that electrons are negatively charged. Electrical charges repel like charges and attract opposite charges, so the electron naturally wants to go towards the "+" sign. It would therefore accelerate AGAINST the field, or in the OPPOSITE direction of the field. Hence the answer is B.

There is no simple "trick" to remembering this - the best way is to understand the principles behind WHY it happens, and then you're set.

Hope this helps.

Apologies, I was thinking of a magnetic field :)

Thanks

The more I think about it, it's actually a bit of a trick question since two answers are technically correct.

The electron will accelerate both against the electric field and opposite to the direction of its initial motion. Since the eletron's path will curve toward the "positive side" of the field, there will be two components of acceleration - tangential and normal.

I think the answer the question was looking for was b, but one could probably legitimately argue for d.

BTA: you are right about answer B, but I don't think your argument works for answer D. For the electron to accelerate toward the opposite direction, there must be a force acting in the opposite direction of its travel, and there isn't. As the electron accelerates toward the + plate it curves towards the + plate, and so one could argue that the acceleration is always toward the + plate (answer B), but as the electron curves there is a component along its velocity vector (answer C).

Option 'b' Is The Correct Answer

Definetely its answer is b