Chapter 16- Charge and Field

A summary of A2 advancing physics chapter 16

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Electric Fields

  • There is an electric field around a charged particle. Electric charge is measured in coulombs (Q) and can be +ve or -ve. Opposite charges attract, and if a charge particle is placed in a field, it will feel a force
  • Coulomb's law is an inverse square law- where the force on Q1 is always equal and opposite to Q2
  • Electric field strength is force per unit positive charge. E (NC-1) is a vector pointing in the direction that a positive charge would move. E depends on where you are in the field
  • A point charge has a radial field. In a radial field, E depends on distance r from point charge Q. E is inversely proportional to r2. Field strength decreases as you move away from Q- field lines get further apart
  • The electric potential energy is the work that would need to be done to move a small charge q, from infinity to a distance r away from a point charge Q. At an infinite distance, q would have 0 potential energy
  • In a repulsive force field (q and Q +ve), you have to do work to move q to Q. q gains PE as r decreases. In an attractive force field (Q -ve and g +ve), q gains PE as r increases
  • Electric potential, V (in volts), is electric potential energy per unit positive charge
  • A uniform field can be produced by connecting two parallel plates to the opposite poles of a battery. Field strength E is the same everywhere in a uniform field
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Millikan's Oil Drop Experiment

  • When you drop an object into air, it experiences a vicious drag force in the opposite direction to velocity
  • Millikan sprayed oil droplets between two plates- applying a p.d. produced a field that exerted a force on the charged droplets
  • With the field off, the forces on the drop are weight (down) and vicious force (up). The drop will reach terminal velocity when these are =
  • The field introduced an electric force on the drop. Millikan adjusted the p.d. until the drop was stationary. Since vicious force is proportional to v, once the drop stopped moving the vicious force disappeared
  • Since the drop is stationary, electric force must be equal to weight. Millikan was then able to find the charge on the drop, repeated hundreds of times. The charge was always a multiple of -1.6 x 10-19 C
  • Millikan concluded that charge can never exist in smaller quantities than -1.6 x 10-19 C. He assumed this was the charge carried by an electron. The results suggested that charge is quantised
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Charged Particles in Magnetic Fields

  • Forces act on charged particles in a magnetic field
  • By Fleming's left hand rule (or Coulthwaite's right hand rule!) the force on a moving charge in a magnetic field is always perpendicular to its direction of travel
  • This effect is used in particle accelerators- which used electric and magnetic fields to accelerate particles to very high energies along circular paths
  • The radius of curvature of this path gives you info on charge and mass of the particle
  • The centripetal force and the electromagnetic force are equivalent for a charged particle travelling along a circular path
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