Current, Potential Difference and Resistance

• Open switch


  • Makes the circuit NOT complete and current cannot flow
• Closed switch


  • COMPLETES the circuit and allows current to pass
• Cell


  Indicates a single cell battery that provides the potential difference (voltage) needed for the current to flow
• Battery


  A combination of cells, supplying electricity to the circuit
• Lamp


  Illuminates when current passes through it
• Ammeter


  Used to MEASURE the CURRENT flowing through a circuit; it is always connected in SERIES
• Voltmeter


  MEASURES the POTENTIAL DIFFERENCE (voltage) across a component and is connected in PARALLEL
• Resistor


  Indicates a component that opposes the flow of current, introducing RESISTANCE to the circuit
• Variable resistor


  A resistor whose RESISTANCE can be ADJUSTED. It is used to CONTROL CURRENT flow
• Thermistor


  A temperature-dependent resistor, meaning its RESISTANCE CHANGES with TEMPERATURE
• Light dependent resistor (LDR)


  A resistor that CHANGES its RESISTANCE based on the LIGHT INTENSITY it's exposed to
• Diode


  Allows CURRENT to flow in only ONE DIRECTION, ensuring components are protected from reverse current
• Light emitting diode (LED)


  Emits LIGHT when current flows through it and also allows current to flow in only ONE DIRECTION
• Fuse


  Acts as a SAFETY device that breaks the circuit if the CURRENT becomes too HIGH
• Electrical Charge


  A circuit works when charge is allowed to flow through it, usually these charges are free moving electrons or ions.

  Units: Coulombs (C)
• Electric current


  Electric current is the flow of electric charge, quantified by the amount of charge passing a point in the circuit over time.

  Units: Amperes (A)
• charge = current x time (Q = IT)
• Potential Difference


  The measure of energy per unit of charge, transferred between two points in a circuit. ; the driving force that pushes the current around the circuit.

  Units: Volts (V)
• Resistance


  Slows down the flow of current. The higher the resistance in a circuit, the lower the current, if the potential difference stays the same.

  Units: Ohms (Ω)
• Potential difference = current x resistance (V = IR)
• Resistors


  Components in an electrical circuit that change the current (I) flowing through due to their resistance (R)
• Resistors


  Resistance can be constant or change as the current changes
• Ohmic Conductors


  The current (at a constant temperature) is directly proportional to the potential difference across the resistor.

  Steeper the line, the lower the resistance.

  
• As the potential difference INCREASES


  The gradient (steepness) of line DECREASES, which shows that RESISTANCE is INCREASING
• Reason for increasing resistance

  The FILAMENT in the bulb gets HOTTER, which happens as more current passes through it
• Filament


  • It is a METAL which means it contains METAL IONS and DELOCALISED ELECTRONS in its structure
• What happens as temperature of filament INCREASES

  The metal ions VIBRATE leading to MORE COLLISIONS between the IONS and ELECTRONS which INCREASES RESISTANCE
• Diode


  Allows current to flow in only ONE DIRECTION, and have a HIGH RESISTANCE in the opposite direction
• Light-dependent resistor (LDR)


  A resistor whose resistance varies according to the LIGHT INTENSITY of the surroundings
• In BRIGHT LIGHT


  The resistance of an LDR decreases, allowing more CURRENT (I) to pass through
• Thermistor


  A resistor that changes its resistance according to the TEMPERATURE of the surroundings
• When it's HOT

  The resistance of a thermistor decreases, letting more current through
• In COOL environments


  The resistance increases, decreasing the current
• On an I-V graph, current flowing in the CORRECT direction is shown by a sharp increase in current on the POSITIVE AXIS 

  When the diode is connected in the OPPOSITE DIRECTION, the graph is FLAT at 0, indicating NO CURRENT flowing.

  
• At low potential differences, there is a straight portion on the graph which means the resistance is CONSTANT; the graph curves at higher potential differences.

  As the potential difference INCREASES, the gradient of line DECREASES, which shows that RESISTANCE is INCREASING, because the FILAMENT in the bulb gets HOTTER, which happens as more current passes through it.

  
• The filament is a METAL which means it contains METAL IONS and DELOCALISED ELECTRONS in its structure.

  When the temperature of the filament INCREASES, the metal ions VIBRATE leading to MORE COLLISIONS between the IONS and ELECTRONS which INCREASES RESISTANCE.

  
• Circuit Symbols

  A) open switch

  B) closed switch

  C) cell

  D) battery

  E) lamp

  F) ammeter

  G) voltmeter

  H) resistor

  I) variable resistor

  J) thermistor

  K) light-dependent resistor

  L) diode

  M) light emitting diode

  N) fuse

  
• Formulas


  Q=IT

  V=IR

  P=IV

  P=I^2 x R

  E=PT

  E=QV

  VpIp = VsIs
• Convectional current goes from positive to negative
• electrons goes from negative to positive