Section 2

• 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