# Section 2

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- Created by: Rebecca Pearson
- Created on: 20-05-13 17:54

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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

- Forces act on charged particles
- 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

- In radial field E is inversely proportional to r^2
- 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

- Before field is on

- Charged particles in magnetic fields

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