Resistance

• How does resistance affect current in a circuit?

  As the total resistance of a circuit increases, the current flowing through the circuit decreases.
• How can the current in a circuit be varied?

  Using a variable resistor.
• Voltage =

  Current x resistance
• How is total resistance affected by two resistors in series?

  The total resistance increases; it is equal to the sum of the two resistors
• How is total resistance affected by two resistors in parallel?

  The total resistance decreases; it is less than the resistance of the resistor with the lowest resistance
• What is an Ohmic Conductor?

  A conductor for which current and potential difference are directly proportional, resistance remains constants as current changes, temperature must be constant
• Components for which resistance is not constant current changes.

  Filament lamps, diodes, thermistors, LDRs (light dependent resistors).
• What happens to the resistance of a filament lamp as the temperature increases?

  Resistance increases, metal ions have more kinetic energy, so vibrate more, colliding more frequently with electrons as they flow through the metal, this creates more resistance to current flow
• What do diodes do to the current flow?

  The current can only flow through one way as resistance is very high in the other direction, preventing current flow.
• What happens to the resistance of a thermistor as temperature increases?

  The thermistor‘s resistance decreases
• Uses of thermistors

  In a thermostat to turn a heater on below a certain temperature, in a freezer to turn on a cooler when the temperature becomes too high
• Uses of LDRs

  Street lamps or night lights, when light levels drop resistance increases and the light gains sufficient current to turn on
• How do diodes work?

  A diode only allows current to flow in one direction, if current is flowing the right way, the resistance is large for small voltages (up until about 6.0 V), but then at higher voltages becomes very small.