P4 - Electric circuits

• Charge


  • When electrons are removed from an atom it becomes positively charged
  • When electrons are added to an atom it becomes negatively charged
• Static charge

  • Insulating materials become charged when they are rubbed with another insulating material
  • Electrons are transferred from one material to the other
  • Materials that gain electrons beome negatively charged
  • Materials that lose electrons become positively charged
  • Positive charge does not usually transfer between materials
• Electric charge is measured in coulombs (C)
• Electric fields


  • A charge object creates an electric field around itself
  • If a charged object is placed in the electric field of another charged object, it experiences electrostatic force
  • The two charged objects exert a non-contact force with each other:
  • like charges repel each other
  • opposing charges attract each other
  • The electric field between two objects gets stronger as the distance between the objects decreases
• Drawing electric fields


  • Electric fields can be represented using a diagram with field lines
  • these show the direction of the force that a small positive charge would experience when placed in the electric field
• Drawing electric fields


  • field lines meet the surface of charged objects at 90°
  • arrows always point away from positive charges and towards negative charges
• Sparks


  • If two objects have a strong electric field between them, electrons in the air molecules will be strongly attracted towards the positively charged object
  • If the electric field is strong enough, electrons will be pulled away from the air molecules and cause a flow of electrons between the two objects
  • These are sparks
• Electric current

  • When charge flows
  • Carried by electrons
  • Unit - amperes (A)
• Conventional current


  • In circuit diagrams, current flows from the positive terminal of a cell or battery to the negative terminal
• In a single closed loop, the current has the same value at any point in the circuit
• Metals are good conductors because they contain delocalised electrons, which are free to flow through the structure
• Potential difference


  • A measure of how much energy is transferred between two points in a circuit
  • Measured in volts (V)
• • The p.d. across a component is the work done on it by each coulomb of charge that passes through it
  • The p.d. across a power supply or battery is the energy transferred to each coulomb of charge that passes through it
• For electrical charge to flow through a circuit there must be a source of potential difference
• Resistance


  • When electrons move through a circuit, they collide with the ions and atoms of the wires and components in the surface
  • This causes resistance to the flow of charge
  • Measured in ohms (Ω)
• A long wire has more resistance than a short wire because electrons collide with more ions as they pass through a longer wire
• Resistance equation


  potential difference = current x resistance
• • Current is directly proportional to p.d.
  • only applicable at a constant temperature
• • Current only flows in one direction through a diode
  • This is called the forward direction
  • There needs to be a minimum voltage before the current will flow
• • As more current flows through a filament lamp, the temperature increases
  • The atoms in the wire vibrate more, and collide more often with electrons flowing through it
  • This increases resistance
• Series circuits


  • The components are connected one after the other in a dingle loop
  • If one component stops working, the whole circuit will stop working
• Voltage in a series circuit


  the sum of all the voltage of the components
• Resistance in a series circuit


  the sum of the resistance of all the components
• Parallel circuits


  • Made up of two or more loops through which current can flow
  • If one branch stops working, the other branches will not be affected
• Resistance in parallel circuits


  • the total resistance is always less than the smallest resistance of any branch
  • adding a loop provides another route for the current to flow
  • more current can flow even if the p.d. has not changed
  • adding more resistance in parallel decreases the total resistance of a circuit
• Open switch

  
• Closed switch

  
• Lamp

  
• Voltmeter

  
• Ammeter

  
• Resistor

  
• LDR - Light Dependent Resistor

  
• Thermistor

  
• Variable resistor

  
• Diode

  
• LED - Light Emitting Diode

  
• Fuse

  
• Cell

  
• Battery