This topic summary covers Knowledge Organiser: I-V Characteristics within I-V Characteristics for GCSE Physics. Revise I-V Characteristics in Extra Topics for GCSE Physics with 13 exam-style questions and 11 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 12 of 12 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
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
Practice questions for I-V Characteristics
What does an I-V characteristic graph show for a component?
Explain why the I-V graph for a filament lamp is not a straight line.
Quick recall flashcards
What is an ohmic conductor?
A component where current is directly proportional to voltage at constant temperature. The I-V graph is a straight line through the origin
What is a diode used for?
Converting AC to DC (rectification), because it only conducts in one direction