simple circuits

• Current in a series circuit

  the current is the same anywhere in a series circuit
• Current in a parallel cicuit

  the current in each branch adds up to the total current transferred by the cell.
• Circuit Rules: P.D. in Series

  The P.D. of the cell is shared among the components.

  EG. If the P.D. is 6V, and there are 2 identical filament lamps, each lamp will have 3V each,
• What is resistance?

  A measure of how easy it is for current to flow.
• Circuit Rules: P.D. in Parallel

  Each branch after the junction gets the same P.D.
• Circuit Rules: Current in Series


  All components in series have the same current as each other.

  EG. 2 filament lamps will have th same current if they are in SERIES.
• Circuit Rules: Current in Parallel

  Total current into a junction=total current out of a junction.
• What are the conditions needed for charge to flow through a conductor?

  potential difference

  • closed or complete circuit
• for current to flow, it has to be in a closed circuit
• IV Characteristic graph

  A) resistors and wires

  
• IV Characteristic graph

  iv graph

  A) filament lamp

  
• iv characteristic graph

  iv graph

  A) diode

  
• How does resistance change with current?

  as current increases, electrons have more energy. when they flow through a resistor, they collide with the atoms in the resistor. this transfers energy to the atoms, which makes them vibrate more. these increased vibrations make it more difficult for the electrons to flow through. Therefore, as resistance increases, current decreases; they are inversely proportional.
• P.D. in series


  total p.d. is shared between each component
• p.d. in parallel


  p.d. in each component is the same
• thermistor

  thermistor: temperature dependant resistor, in constant conditions. it curves - as the current increases, the thermistor warms up, reducing the resistance.

  
• diode: made from semiconductor material such as silicon. let current flow freely through it in one direction but not in the other.
• thermistor


  • hot: resistance of the thermistor drops
  • cool: resistance goes up
• thermistor



  • hot: resistance of the thermistor drops
  • cool: resistance goes up

  
• series circuits


  the different components are connected in a line, end to end, between the 2 terminals of the power supply. current has to flow through all of the components to get round the circuit. means if you remove one component, it can have a big effect on the others.
• parallel circuits


  each component is separately connected to the terminals of the supply. means that if you remove / disconnect one, it will hardly affect the others.
• series circuit


  connecting several cells in series, all the same way (positive to negative) gives a bigger total p.d. - because each charge in the circuit passes through each cell and gets a 'push' from each one.
• parallel circuits

  potential difference is the same across ALL branches.

  formula:

  V1 = V2 + V3

  A) V1

  B) branch

  C) V3

  
• parallel circuits

  current is shared between branches

  I = I1 + I2

  A) I1

  
• parallel circuits


  the total resistance of a parallel circuit is always LESS than the branch with the SMALLEST resistance.

  • the resistance is lower because the charge has more than one branch to take - only some charge will flow along with each branch.
• explain the shape of the I-V graph of a filament lamp. [3]

  as the current increases, the filament lamp heats up.

  this causes the particles in the bulb to vibrate more.

  this makes it more difficult for charge-carrying electrons to pass through the lamp, and so increasing the resistance.
• why doesn't the current flow in both directions in a diode?

  because there's very high resistance in one direction.
• uses of diodes


  • radio receivers

  turning alternating current into direct current.
• LDR: Explain what happens as the night falls [in a circuit with an LDR]

  • as the light levels falls, the resistance of the LDR increases.
  • this causes the p.d. across he LDR to increase
  • the p.d. across the bulbs increases because components connected in parallel have the same p.d.
  • as the p.d. increases in the bulb, it turns on / gets brighter.
• if the length of the wire increases, the resistance increases too because electrons have to travel further, so more collisions take place