Electricity and Magnetism

• Like poles repel and unlike poles attract.
• Coulomb's law: The force between two point charges is directly proportional to the product of their charges.

  F= 1/4piE X q1 q2/ r^2 where 1/4piE is k and k=9x10^9Nm^2C^-2
• Principle of superposition: When more than 2 charges are present, the force on any one of them is a vector sum of the electric forces acting, due to the other charges being present.
• A greater number of lines used to represent an electric field indicates a stronger field.
• Magnitude of an electric field: The strength of an electric field at a point is defined as the electric force per unit charge, experienced by a small positive test charge.
• Equation for a field: E = F/q

  Due to F = k q1 q2 / r^2 the magnitude of a field could be represented by E=kq/r^2

  E is measured in NC^-1
• Hollow Conductor: There is no electric field inside a hollow conductor of any shape, provided there is no charge in the cavity.
• Corona Discharge: Electric fields are strongest near sharp points on charged conductors. These fields may be strong enough to ionise molecules in the air near sharp points, resulting in charge movements away from the conductor. The movement of moving away from the conductor by the ionisation is called a 'corona discharge'.
• W=Fs=mgh=gravitational potential energy
• Electric fields store electric potential energy, which is work done when the charge moves.
• Delta V is the electric potential difference, the work done per unit charge.
• Delta V = W/q where V is measured in JC^-1
• Electron-volt (eV) is a unit of measurement describing the energy carried by a charged particle.

  1 eV = 1.60 x 10^-19 J

  eV to J -> multiply by 1.60 x 10^-19

  J to eV -> divide by 1.60 x 10^-19
• Uniform electric fields can be set by applying a constant potential difference between two oppositely charged plates separated by a distance d.
• Near and beyond the edges of the plate, the electric field is non-uniform as the field lines have to end and arrive on the surface of the plates at right angles.
• E = delta V / d where E is in Vm^-1 and equivalent to NC^-1
• The magnitude of acceleration of a charge is given by a=qE/m.
• Delta V = Work/Charge
• 1 eV is 1.6x10^-19 J
• direction of magnetic field is given by direction that north pole of a compass points
• the cyclotron is used to accelerate protons/ions to high speeds so that they have high kinetic energy when they exit
• maximum magnetic flux when perpendicular to area (cos 90)
• Faraday's Law states that the induced emf in a circuit is the rate of change of the magnetic flux over time
• Lenz's Law: The induced emf creates a current in a direction that opposes the magnetic flux producing the emf
• eddy currents cause an increase in internal energy, hence causing an increase in the temperature of the material. To avoid this, transformers are ofter laminated, or their metallic core is divided into thin sheets with insulation.