PARTICLES1) H 112) H 123) J14) K15) K2

H11 CHARGED PARTICLES IN MOTION Electron beams Read E2 first Beams of electrons and other charges particles are deflected byMagnetic and electric fields. The unit of energy used is theElectron volt (eV) and energies are often given in MeV and GovNote: in all the diagrams in the section, the equipment isEnclosed in a vacuum tube.

Deflection of electrons by an electric field

vacuum

e f d v electron gun

Above electronics pass between two horizontal plates. The electric field strength between the plates is V/d (see141)Force F on electron = electric field strength chargeSo F= Ve dNote : The force on the electron does not depend on its speed The force is always in line with the electric field The path of the electronics is a parabola (just as it is for the thrown ball in B5)

Magnetic force on a moving charge magnetic field B

F

v

vt

Distance movedIn time tAbove a particle of charge Q is moving at a steady speed throughA uniform magnetic field in time t it travels a distance vt. So it is equivalent to a current Q/t a wire of length vt. According to equation (1) in H7, the force on a current carrying wire is bill. Applying this to the above

Force F= B Q/t =vtSo F = BQv

Note: As the speed increase the force increases The direction of the force is given by Flemings left hand Rule (see H6) If the partical is travelling at an angle to the field thenThe above equation becomesF= BQv sin

Deflection of electrons by a magnetic field Vacuum Electron gun B Magnetic (out of paper)Above electrons travel at right angles to a uniform magneticField the force F on an electron is found by putting Q equalTo e in equation (1) on the left so F= BecNote: The force on the electron increases with speed The force is always at right angles to the direction of Motion,as pericted by Flemings left hand rule. But in Applying this rule, remember that the electron has negative Charge,so electron motion to the right represents a Conventional current to the left The path of the beam is circular (see next page)

Mean what you say- say what you meanFor charged particles in magnetic fields if the direction of the Particle is towards or away from a magnetic pole (so that the fieldIs in line with the motion of the particle), the force on the particleIs zero . There is no deflection. This is shown by equation (2)When =0 or 180, sin = 0. It is incorrect to say that theCharged particle is attracted or rappelled by the magnetic fieldThe maximum deflection occurs when the field and the particleMotions are perpendicular to each other. Describing the Directions clearly can be difficult. If you write the force isup or down do you mean towards the top or bottom of the Paper or do you mean into or out of the paper? Make sure your answer are not ambiguous Use diagrams ifThese help,A beam of both positive and negative particles can beSeparated by a magnetic field. The force on the positiveParticles is in the opposite direction to the force on theNegative particle so the deflection will be in oppositeDirection initially as shown below . it is incorrect to say that The particles will move in opposite directions negative charged particles B positive charged particlesMagnetic field

Circular motion in a magnetic field

Vacuummagnetic fieldnote:(out of paper)The radius of the circle is proportional to the speed Increasing B decreeing the radiusSpecific charge of an electron an electronscharge per unit mass elm its value is -1.8 10 11C kg-1Methods of measuring it make use of the above equation. Lon beams if atoms lose elections, they become positive Ions these particles also have circular paths in a magnetic Field if Q is the charge on a particle, and in the mass thenEquation (3) can be written in this more general formr= mv QBThe radius depends on the specific charge (Q/m)Above electrons leave the electron gun at speed vThey move in a circle because the magnatic force BevSupplies the necessary centripetal force (see b14)soBev= m ev2so r = mevteB

How Science WorksMass spectrometerSpeed SelectionFrom equation (4) the radius depends on the specific charge Q/m an ion beam may contain ions with a range of speedsThis idea is used in the mass spectrometer to separate nuclei with ions of only one speed can be selected by the method Different specific chargesshown below This uses the principal that the magnetic force on anion depends on the speed while the electric force does not

The output shows the relative intensity if different isotopes The magnetic field produces a force toward the top of thePresent in a sample (Usually all the irons have charge +1 if, page BQv. The electric field produces a force towards theFor example, some are +2 this must be taken into account) bottom of the page FQ The only ions to pass through the sinThe ratio of different isotopes is used dating techniques are those for which these forces are equal. So if EQ-BQve.g Carbon dating the selected speed v=E/B faster ions are deflected upwards because BQv is more slower ions are deflected downwards