Specific charge

Question 1

Electron in electric field - explain what happens to its motion

Answer 1

• Pd between two plates generates electric field deflecting electron beam down • Uniform field so electric force constant within field • So electrons experience vertical electrostatic force which doesn’t change as the electrons move across the field therefore downward vertical acceleration • Constant horizontal velocity so • So parabolic path via projectile motion • Fluorescent screen enables path to be seen and measured

Question 2

Explain the effect on the speed of an electron in a magnetic field

Answer 2

constant speed: path of electron beam in magnetic field, magnetic field perpendicular to direction of motion, force does not do work as W=fd and there is no movement in the direction of force, force causes direction of motion to change, force is centripetal – acts towards centre of curvature, velocity is tangential

Question 3

Thomson’s value for e/m (1.7 ×10^11 C kg−1.) was significant because

Answer 3

it was about 1800-2000 times greater than the specific charge value for the H+ ion/proton that had been obtained from electrolysis experiments which had the largest specific charge known prior to this meaning that the electron either had a much greater charge or a much smaller mass.

Question 4

Direction of electrons entering a magnetic field horizonatlly

Answer 4

• Plan view either circular arc up or down depending on Fleming’s LHR - Side view always horizontal because the horizontal component of velocity (parallel) is unchanged

Question 5

Electrons in a magnetic field

Answer 5

In a magnetic field, electrons move in a circular path with a constant speed because: - magnetic force on each electron is perpendicular to the velocity - no work is done on each electron by the force so there is a constant kinetic energy - a constant kinetic energy means a constant speed and so the magnitude of the magnetic force is constant as no acceleration - magnetic force always perpendicular to the velocity so centripetal so circular motion

Question 6

Flemings LHR for

Answer 6

determining motion caused by something - motor effect. electric motion perpendicular to magnetic field

Question 7

RHR for

Answer 7

generator: motion generating something

Question 8

Crossed/balanced fields method for determining specific charge

Answer 8

• Pd between two plates generates electric field deflecting electron beam down • Uniform field so electric force constant within field • So downward vertical acceleration + constant horizontal velocity • So parabolic path via projectile motion • Fluorescent screen enables path to be seen and measured • Magnetic coils placed so planes parallel to screen • Current passed through to generate magnetic force perpendicular to electric field and beam direction + velocity so no work done so constant speed • Magnetic force on electron upwards to oppose downward electric field • Vary current to vary flux density i.e. field strength to prevent deflection • F=Bev=eE so v=E/B and 0.5mv^2 = eV so e/m = v^2/2V = E^2/2VB^2 (as don’t know v)

Question 9

Fine beam tube for determining specific charge

Answer 9

• Electrons charged so can be deflected by magnetic field • Electrons pass through bulb, collide with gas atoms making them absorb some of their energy and become excited • Almost instantaneous de-excitation and photon emission so the path of the electron is visible • Two circular magnetic field coils generate field inside tube with force perpendicular to electron’s velocity and direction of field • This means force on electron is always perpendicular to velocity so centripetal so deflects electrons into a circular path but doing no work so not changing the electrons speed/energy • So F = Bev = mv^2/r and as 0.5mv^2 = eV (we don’t know v) we get e/m = 2V/B^2r^2