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Lecture_19

# Lecture_19 - Determining e/m of an Electron p = q Br mv 2 =...

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Br q p = Br v m q = V q mv Δ = 2 2 m q V v Δ = 2 2 2 2 2 2 r B v m q = 2 2 2 2 r B V m q m q Δ = 2 2 2 r B V m e Δ = Determining e/m of an Electron

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1897: m/e >1000 times smaller than H atom Joseph John Thomson (1856-1940)
What if v is not perpendicular to B ? B v q F magnetic × = Direction? Magnitude? F magnetic = q v + v || ( ) × B v || v B qv F magnetic = Trajectory: helix Exercise F magnetic = qvB sin θ

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How big is the magnetic field in a TV created by the electron beam deflection coil? Acceleration voltage: 15000V Solution: 1. Electron speed: B v q F magnetic × = V e mv Δ = 2 2 v =7.3 . 10 7 m/s 2. Br q p = T 3 10 2 × = = = er mv er p B B Exercise: B in TV set Are we justified in using the nonrelativistic KE?
Can combine electric and magnetic forces: B v q F m × = E q F e = B v q E q F × + = Coulomb law and Biot-Savart law have coefficients 1/(4 πε 0 ) and μ 0 /(4 π ) to make the field and force equations consistent with each other The Lorentz Force

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