Section 2

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  • Section 2
    • Charged particles in magnetic fields
      • Forces act on charged particles
        • F=BIl
        • I=q/t
        • v-l/t
        • I=qv/l
        • F=qvB
      • Centripetal force
        • Newton's 2nd law: F=mv^2/r
        • Charged particle (F=qvB) qvB=mv^2/r
        • r=mv/Bq
      • Circular path
        • Force on moving charge is perpendicular to direction of travel
        • Used in particle accelerators
        • Radius of curvature of the path gives info about charge and mass
    • Electric fields
      • In radial field E is inversely proportional to r^2
        • E=kQ/r^2
        • Positive Q q is repelled
        • Negative  Q q is attracted
        • Inverse square law
      • Charge has electrical potential energy in EF
        • Elec protential energy is work needed to move q from infinity to distance r away from Q
        • At infinite distance from Q q has 0 PE
        • In repulsive force field have to do work against repulsion to reduce r and increase PE
        • In attractive field q gains PE as r increases
      • Calculate F using Coulomb's law
        • F=kQ1Q2/r^2 k=1/4piE
        • Force on Q1 is equal and opposite to Q2
        • Inverse square law: bigger r weaker force
        • F also depends on permmittivity E
      • EP is potential energy per unit charge
        • V=kQ/r
        • V is +ve when the F is repulsive and -ve when F is attractive
      • Around charged object
        • Charge Q measured in coulombs C (+or-)
        • Opposites attract
        • Charged object placed in EF will experience force
      • Field strength is same every where in uniform field
        • Uniform field produced by two parallel plates connected to opposite poles of battery
        • E is the same at all points
        • Measured in V/m
      • Elec field strength is force per unit charge
        • E is a vector pointin in direction that +ve charge would move
        • E=F/q
        • UNits are N/C
        • A point charge has a radial field
    • Millikan's oil-drop Experiment
      • Before field is on
        • Weight of drop acts downwards and viscous force up
        • Drop stops accelerating when 2 forces are equal
      • Stoke's Law
        • An object in fluid experiences a viscous drag force
        • Happens in the opposite direction to velocity
        • F=6(pi)nrv n is viscosity
      • When field is on
        • When drop was still viscous force had dissapeared
        • Adjusted p.d. until drop was still
        • E=F/q E=V/d F=QV/d
        • Third factor - electric force
        • Charge can never be smaller than 1.6x10^-19


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