Circuits

• Switch open

  
• Switch closed

  
• Cell

  
• Battery

  
• Diode

  
• Resistor

  
• Variable resistor

  
• LED ( light emitting diode )

  
• Lamp

  
• Fuse

  
• Voltmeter

  
• Ammeter

  
• Thermistor

  
• LDR ( light dependent resistor )

  
• The greater the resistance across a component, the smaller the current flows through it
• Electric current is the flow of electric charge around a circuit
• $Charge=$$Current \times Time$ $Q=$$It$

  • Charge (Q) in coulombs (C)
  • Current (I) in amperes (A)
  • Time (t) in seconds (s)
• $Potential \; difference =$$Current \times Resistance$ $V=$$IR$

  • Potential difference (V) in volts (V)
  • Current (I) in amperes (A)
  • Resistance (R) in ohm (Ω)
• At a constant temperature the current flowing through the resistor is directly proportional to the potential difference across it.
• Resistance practical

  • Attach a crocodile clip to the wire at 0cm and the other at 10 cm
  • Close the switch of the circuit and measure the current and potential difference
  • Open the switch and then move the crocodile a further 10 cm
  • Repeat these steps with a number of different lengths
  • Use the measurements to calculate the resistance and plot it against the wire length
  • The graph should have a straight line
• The current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor. This means that the resistance remains constant as the current changes.

  
• The resistance of a filament lamp increases as the temperature of the filament increases.

  If the temperature of a metal conductor increases, the ions of the metal vibrate more vigorously.

  This increases the number of collisions between the free electrons and the ions.

  
• The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction.

  
• Light dependent resistors depend on the resistance of light ; when it is bright the resistance falls, when it is dark the resistance rises
• Light dependent resistors are used for automatic night lights, outdoor lighting and burglar detectors
• Thermistors are dependant on the temperature ; when it is hot resistance falls, when it is cold resistance rises
• Thermistors are used for car engine temperature sensors and electronic thermostats