This topic summary covers Knowledge Organiser: Resistance and Ohm's Law within Resistance & Ohm's Law for GCSE Physics. Revise Resistance & Ohm's Law in Electricity for GCSE Physics with 16 exam-style questions and 30 flashcards. This topic shows up very often in GCSE exams, so students should be able to explain it clearly, not just recognise the term. It is section 16 of 16 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
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
Practice questions for Resistance & Ohm's Law
Which of the following best describes electrical resistance?
Explain what is meant by an ohmic conductor.
Quick recall flashcards
Unit of resistance?
Ohm (Ω)
Ohm's Law equation?
V = IR where V = voltage (V), I = current (A), R = resistance (Ω)