## Motion of electron in Electric and Magnetic Fields

“Both magnetic and electric feilds are perpendicular in a region.If we place an electron at rest in that region.what will be the path followed by that electron ?”

– asked Vimal

When the electron is at rest, it is not affected by magnetic field, but due to electric field, it will begin to move in a direction opposite to the field. This initial direction determines the plane of motion. Then it is acted upon by magnetic field as the motion starts

(The answer is incomplete. I am leaving it for the visitors to think and respond)

**Charge is placed at rest in crossed electric and magnetic fields**

## A Question from Errors in measurement

An experiment measures quantities a,b and c, and X is

calculated from

if the percentage error in a,b,c are ±1%, ±5%, ±2% respectively, the percentage error in X will be

a) 7%

b) 5%

c) 1%

d) 9%

Answer:

Percentage error = 1x percentage error in a + 2 x percentage error in b + 3 x percentage error in c

= 1×1 +2×5+3×2

=1+10+6

= 17%

SO there could be a misprint in the question posted by you.

## A question from Scientific Method

Somanna posted the question:

Explain the discovery of comet halley based on-

- systemic observation
- logical reasoning
- model making
- theoretical prediction

## Examples of scalar product and vector product

Kashan asks:

“will you give me min 10 examples 0f scalar product and vector product known at xii level”

I am just strating the list; members please contribute more as comments

**Scalar Product**

- work
**F.s** - Power
**F.v** - Potential energy of a dipole
**P.E**

**Vector Product**

- Torque on a dipole =
**PxE** - Lorentz magnetic force F=q
**vxB** - Torque =
**rxF**