Physics circuits

• I–V Graphs
• Ohm's Law


  The current through a conductor is directly proportional to the potential difference across it
• Ohmic conductors


  • Fixed resistors
  • Wires
  • Heating elements
• Ohm's Law equation


  V = IR
• Ohm's Law is relevant only at constant temperatures
• 1. IV graph for an ohmic conductor

  • Straight line through the origin
  • Reversing potential difference and current direction makes no difference to the shape
  • Swapping axis labels (current on x-axis, p.d on y-axis) still gives a straight line through the origin
• Filament lamp


  Example of a non-ohmic conductor
• As current increases in a filament lamp

  Temperature of filament increases
• Higher temperature in filament lamp

  Resistance increases
• As resistance increases


  Current increases at a slower rate
• Where the I-V graph is a straight line the resistance is constant</b>
• The resistance increases as the I-V graph curves
• Reversing the potential difference reverses the current and makes no difference to the shape of the I-V curve for a filament lamp
• Resistance and temperature


  Higher temperature causes atoms to vibrate faster, impeding the flow of electrons and increasing resistance
• effect of current on a filament bulb

  The increased current means higher temperature causing the atoms in the metal lattice of the filament to vibrate more
• Increased vibration of atoms in the filament

  Increases resistance as it becomes more difficult for free electrons (the current) to pass through
• Resistance opposes the current, causing the current to increase at a slower rate
• Where the graph is a straight line, the resistance is constant
• The resistance increases as the graph curves
• Reversing the potential difference reverses the current and makes no difference to the shape of the curve
• Resistance & Temperature


  • All solids are made up of vibrating atoms
  • The higher the temperature, the faster these atoms vibrate
  • Electric current is the flow of free electrons in a material
  • The electrons collide with the vibrating atoms which impedes their flow, hence the current decreases
  • If the current decreases, then the resistance will increase (from V = IR)
  • Therefore, an increase in temperature causes an increase in resistance
• Diodes


  • A diode is a non-ohmic conductor that allows current to flow in one direction only
  • This is called forward bias
  • In the reverse direction, the diode has very high resistance, and therefore no current flows
  • This is called reverse bias
• The I–V graph for a diode is slightly different:
• diode: Forward bias

  When the current is in the direction of the arrowhead symbol, this is shown by the sharp increase in potential difference and current on the right side of the graph
• diode: Reverse bias

  When the diode is switched around, this is shown by a zero reading of current or potential difference on the left side of the graph
• An LED is a specific type of diode that emits light and works the same way as a normal diode
• Linear graphs

  the graph is a straight line
• Non-linear


  The I-V graph is not a straight line
• Linear components have an I-V graph that is a straight line through the origin
• Non-linear components have an I-V graph that is not a straight line
• Linear components are said to obey Ohm's Law and have a constant resistance, whilst non-linear do not
• Some components may be linear at low currents, then become non-linear as the current increases (and therefore a change in temperature)
• Linear elements


  • Fixed resistors
  • Wires
  • Heating elements
• Non-linear elements


  • Filament lamps
  • Diodes & LEDs
  • LDRs
  • Thermistors
• Current


  The flow of electric charge through a conductor, measured in amperes (A).
• Potential difference


  The force that drives the current through a conductor, measured in volts (V).
• I = V / R

  The formula for Ohm's Law, where I is the current, V is the potential difference, and R is the resistance of the conductor.
• Wires

  Conductive materials used to connect electronic components in a circuit. Wires have very low resistance and are designed to carry current efficiently. They are often made of copper or aluminum and are coated with insulation to prevent accidental contact with other conductors.
• Fixed resistors

  Electronic components designed to provide a specific amount of resistance in a circuit. They are often used to limit current flow or divide voltage. Fixed resistors have a fixed resistance value that cannot be changed.
• Heating elements

  Resistive devices that convert electrical energy into heat. They are often used in appliances such as toasters, hair dryers, and space heaters. When a voltage is applied across a heating element, it resists the flow of current, generating heat in the process.