Knowledge Organiser: Resistance and Ohm's Law
Key Terms
- Resistance: Opposition to current (Ω)
- Ohm's Law: V = IR (at constant temperature)
- Ohmic conductor: Constant resistance
- Thermistor: R decreases as T increases
- LDR: R decreases as light increases
- Diode: One-way current flow
Key Facts
- Metal: hotter = more resistance
- Longer wire = more resistance
- Thicker wire = less resistance
- Filament lamp: curved V-I graph
- Fixed resistor: straight V-I graph
Key Equations
- V = I × R
- R = V / I
- I = V / R
Exam Tips
- Unit of resistance: Ω (ohms)
- Filament lamp = non-ohmic
- Thermistor: hot → less R → more I
- Required practical: control temperature
Common Mistakes
- Saying filament lamps obey Ohm's law: Filament lamps are non-ohmic — resistance increases as temperature increases, so the I-V graph curves
- Rearranging V = IR incorrectly: R = V ÷ I (not V × I) — always divide voltage by current to find resistance
- Confusing thermistor and LDR behaviour: Thermistor — resistance decreases as temperature increases; LDR — resistance decreases as light intensity increases
- Not controlling variables in the practical: Temperature must be kept constant when investigating resistance — heating changes resistance and ruins the relationship
- Plotting I-V graphs the wrong way: Current (I) goes on the y-axis, potential difference (V) on the x-axis — a straight line through the origin means ohmic
Memory Aid
- VIR Triangle — "Very Important Relationship":
- Write V at the top, I and R at the bottom. Cover the letter you want to find: V on top alone = I × R; I on bottom-left = V ÷ R; R on bottom-right = V ÷ I.
- Non-ohmic components — "FDT Changes":
- Filament lamp, Diode, Thermistor — these three all have changing resistance. Everything else (fixed resistors) is ohmic.
- Thermistor vs LDR — "Hot is Less, Bright is Less":
- Hotter thermistor = less resistance. Brighter LDR = less resistance. Both sensors work the same way — the thing they detect reduces their resistance.
- Wire resistance factors — "LAMT":
- Length (more = more R), Area (more = less R), Material, Temperature.