P4 | Electricity and Magnetism

• There are two types of charges, positive and negative charges
• Unlike charges attract and like charges repel
• Adding a charge to something involves the addition or removal of electrons
• Current is related to the flow of charge
• Current in metals is due to a flow of electrons
• The potential difference across a circuit component is measured in volts
• The electromotive force of an electrical source of energy is measured in volts
• V=IR

  Potential Difference = Current multiplied by Resistance
• For a parallel circuit, the current from the source is larger than the current in each branch
• The combined resistance of two resistors in parallel is less than that of either resistor by itself
• The current from the source is the sum of the currents in the separate branches of a parallel circuit
• The sum of the potential differences across the components in a series circuit is equal to the total potential difference across the supply
• Electric charge is measured in units called coulombs
• This is a Cell

  
• This is a battery of cells

  
• This is a lamp

  
• This is a fixed resistor

  
• This is a variable resistor

  
• This is a switch

  
• This is an ammeter

  
• This is a voltmeter

  
• An ammeter measures the current in a circuit and is connected in series
• Conventional current flows in the direction opposite to that which electrons flow in.
• Red = Conventional Current

  Green = flow of electrons

  
• The energy supplied by the source in driving a unit charge around a circuit.
• The maximum voltage a cell can produce is called the electromotive force (EMF), measured in volts.
• The current at any point in a series circuit is the same
• The current splits at each branch in a parallel circuit so the total current is always greater than the current in one branch
• In parallel: Current across the source = sum of currents in the separate branches
• Electrostatic repulsion is caused by the force between charges. When the charges are the same, they repel each other.
• In experiments showing the production of electrostatic charges by friction, insulating solids like plastics are given a charge. This is done using friction to transfer electrons from the surface. By removing negative electrons, the insulator is left with a positive charge.
• A charged object creates an electric field around itself. This can be shown by electric field lines.
• Electric field lines always point away from positive charges and towards negative charges

  A) +

  B) -

  
• • The strength of an electric field depends on the distance from the object creating the field:
  • The field is strongest close to the charged object - this is shown by the field lines being closer together
  • The field becomes weaker further away from the charged object - this is shown by the field lines becoming further apart
• Objects in an electric field will experience an electrostatic force
• The force of an electrostatic force is either attractive or repulsive depending on whether the charges are opposite (positive and negative) or the same (positive and positive)
• Fields lines always go away from positive charges and towards negative charges – they have the same direction as the direction of the force on a positively charged particle at a point in that field

  
• The electric field between two parallel plates is a uniform electric field.

  The field lines are:

  • Directed from the positive to the negative plate
  • Parallel
  • Straight lines

  
• Conductors allow charge carriers to freely move
• Insulators do not allow charge carriers to move