Knowledge Organiser: I-V Characteristics
Key Terms
- I-V characteristic — graph of current vs voltage for a component
- Ohmic conductor — straight line through origin, constant R
- Threshold voltage — ~0.6 V for diode to start conducting
- Forward bias — direction diode conducts freely
- Reverse bias — direction diode blocks current
- Rectification — converting AC to DC using diode
Graph Shapes
- Resistor: straight line through origin
- Filament lamp: curve through origin, flattening at high V
- Diode (forward): flat then sharp upward curve at ~0.6 V
- Diode (reverse): flat (virtually no current)
Calculating Resistance
- R = V/I (from coordinates of a point)
- NOT from the gradient
- Gradient = I/V = 1/R
- Steeper line = lower resistance
Practical Setup
- Ammeter: in series with component
- Voltmeter: in parallel with component
- Variable resistor: adjusts current
- Reverse supply to get negative values
- Plot V on x-axis, I on y-axis
Key Equations
- V = I × R (Ohm's law — applies to ohmic conductors)
- R = V ÷ I (resistance from any point on an I-V graph)
- Ohmic conductor: straight line through origin on I-V graph
- Resistance = 1 ÷ gradient of I-V graph (for ohmic conductors)
Common Mistakes
- Plotting axes the wrong way: The convention for I-V graphs is voltage (V) on the x-axis and current (I) on the y-axis — reversing them makes the gradient meaningless
- Saying all components obey Ohm's law: Only ohmic conductors (e.g. resistors at constant temperature) obey Ohm's law — filament lamps, diodes, and thermistors are non-ohmic
- Reading resistance from an I-V graph incorrectly: Resistance at any point = V ÷ I (not the gradient) — for a straight line through the origin, resistance = 1 ÷ gradient, but for curved graphs use the point values
- Saying a filament lamp has constant resistance: As the filament heats up, its resistance increases — the I-V graph curves because resistance is not constant
- Forgetting diodes only conduct in one direction: A diode only allows current flow above a threshold voltage in the forward direction — in reverse bias, it has extremely high resistance and negligible current flows
Memory Aid
- Resistor = Straight (behaves predictably, like a well-behaved student — straight line)
- Filament lamp = S-curve (gets lazy as it heats up — curve flattens off)
- Diode = Hockey stick (flat then suddenly kicks up at 0.6 V)