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.
• Give the equation linking current with resistance.

  P.D. (V) = Current (A) x Resistance

  V = IR
• 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'? State the condition required.

  A conductor for which current and potential difference are directly proportional. Resistance remains constant as current changes. Temperature must be constant.
• List 4 components for which resistance is not constant as current changes.

  Filament lamps, Diodes, Thermistors and Light dependant resistors (LDRs).
• What happens to the resistance of a filament lamp as the temperature increases? Why?

  Resistance increase. 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 is different about current flow through a diode?

  The current only flows in one direction. Resistance is very high in the other direction, preventing current flow.
• State what happens to the resistance of a thermistor as temperature increase.

  The thermistor's resistance decreases.
• Give 2 examples of when a 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 the temperature becomes too high.
• State what happens to the resistance of an LDR as light intensity decreases.

  The LDR's resistance increases.
• Give an application for a LDR.

  Street lamps or night lights. When light levels drop (at night), 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 0.6V), but then at higher voltages becomes very small.