Electric circuits

• what is an electric current

  flow of electrical charge
• what can be said about the value of current at any point in a series circuit

  current is the same at all points in a closed loop
• 2 factors does current in a circuit depend on

  . potential difference (V)

  . resistance (R)
• equation for voltage

  V = current × resistance
• ohmic conductor conditions

  . a conductor for which current and voltage are directly proportional

  . resistance remains constant as current changes

  . temperature must be constant
• 4 components for resistance not constant as current changes

  lamps

  diodes

  thermistors

  ldr
• what happens to resistance of a filament lamp as temperature increases and why

  . resistance increases

  . ions in metal have more energy, so vibrate more, causing more collisions with electrons as they flow through metal, creating greater resistance to current flow
• what is special about current flow through diode

  . the current only flows in one direction

  . resistance is very high in the other direction, preventing current flow
• what happens to resistance of thermistor as temperature increases

  thermistor's resistance decreases
• 2 examples of when thermistor may be used

  . in a thermostat to turn a heater on below a certain temperature

  . in a freezer to turn on a cooler when temperature becomes too high
• what happens to resistance of a LDR as light intensity decreases

  LDR's resistance increases
• application for LDR

  street lamps
• 2 ways a component can be connected in a circuit

  . series

  . parallel
• how does voltage across two components vary when connected series and parallel

  . series - total voltage is shared between each component

  . parallel - voltage across each component is same
• if 2 resistors are connected in parallel, what can be said about their combined total resistance

  total resistance is less then the smallest of 2 individual resistances.
• if 2 resistors are connected in series, what can be said about their total resistance.

  their total combined resistance is equal to the sum of the induvial resistances
• 2 equations for power of circuit

  P = current × voltage

  P = I2 × R
• equation linking energy transferred, power and time

  E = p × t