4.2

• • electrostatic charges arise when electrons are transferred between objects by rubbing
• • attraction occurs when a rubbed strip of cellulose acetate is brought near a strip of polythene
• repulsion occurs when two strips of the same material are rubbed together
• an electric current flows from positive to negative terminals through a complete circuit
• • charge is measured in coulombs and is defined in terms of the ampere
• the region of space where an electric charge experiences a force due to other charges is called an electric field
• the direction of an electric field at a point is the direction of the force on a small positive charge placed in the field
• electric current is the charge passing a point per unit time, measured in amps (A)

  $I =$$Q/t$
• conventional current flows from positive to negative whereas the flow of free electrons is from negative to positive
• in a direct current the electrons flow in one direction. In an alternating current the direction of flow reverses regularly
• the number of complete alternations or cycles in 1 second is the frequency of the alternating current
• electromotive force is the electrical work done by a source in moving unit charge around a complete circuit
• potential difference is the work done by a unit charge passing through a component
• $V =$$W/Q$

  $E =$$W/Q$

  $W =$$ItV$
• the opposition of a conductor to current is called its resistance
• the ohm is the resistance of a conductor in which the current is 1 ampere when a voltage of 1 volt is applied across it

  $V =$$IR$
• the resistance of a metal wire

  • increases as its length increases
  • decreases as its cross-sectional area increases
  • depends on the material
• the current in a metallic conductor is directly proportional to the p.d across its ends if the temperature and other conditions are constant
• $P =$$IV$

  $E =$$IVt$

  $P =$$I^2R$

  $P =$$V^2/R$