Ohm’s Law is one of the most important relationships in IB Physics, forming the foundation of how students understand electric circuits. Whether you are analysing series and parallel circuits, interpreting current–voltage graphs, or designing an Internal Assessment, this law explains how electrical quantities interact.
Because it appears across all exam papers, students aiming for top grades benefit greatly from mastering Ohm’s Law early. A strong grasp of this principle makes later electricity topics far more manageable.
What Ohm’s Law States
Ohm’s Law states that:
The current through a conductor is directly proportional to the potential difference across it, provided the temperature remains constant.
In equation form:
V = IR
Where:
- V is the potential difference (volts, V)
- I is the current (amperes, A)
- R is the resistance (ohms, Ω)
This simple relationship allows you to calculate any one of the three variables as long as the other two are known. It also underpins almost every calculation involving circuits in the IB Physics syllabus.
Conditions for Ohm’s Law
Ohm’s Law only applies when temperature is constant. If temperature changes, resistance may change, and the relationship between voltage and current will no longer be proportional.
This explains why:
- Filament lamps do not obey Ohm’s Law
- Diodes and thermistors are non-ohmic components
- Metals increase in resistance as temperature rises
Understanding these limitations is essential for correctly interpreting real circuits and for answering qualitative questions, especially in longer-response exam problems.
Interpreting I–V Graphs
A key skill in IB Physics is analysing current–voltage (I–V) graphs.
- Ohmic conductors produce a straight line through the origin, showing constant resistance
- Non-ohmic conductors produce curved graphs, indicating changing resistance
- Diodes show little or no current until a threshold voltage is reached
Being able to interpret these graphs quickly and accurately is vital for exam success, particularly in data analysis and explanation questions.
Applying Ohm’s Law in IB Physics
Ohm’s Law is used to:
- Calculate current, voltage, or resistance
- Design circuits with specific current values
- Choose resistors for circuit protection
- Calculate electrical power using
- P = IV
- P = I²R
- P = V² / R
- Analyse series and parallel circuits
- Understand internal resistance in power supplies
Because these applications appear frequently in both Paper 1 and Paper 2, Ohm’s Law is considered a high-value exam topic.
Ohm’s Law in the Internal Assessment
Many strong IB Physics Internal Assessments are based on Ohm’s Law or resistive behaviour. Common investigation themes include:
- I–V characteristics of different materials
- The effect of wire length on resistance
- How temperature affects resistance
- The relationship between cross-sectional area and current
- Measuring internal resistance of a power source
These investigations allow students to combine theory, data collection, and evaluation in a clear and controlled way.
Common Misconceptions
Students often misunderstand Ohm’s Law because they believe:
- All electrical components obey Ohm’s Law
- Resistance always stays constant
- Any straight-line graph represents Ohm’s Law
- Voltage simply “causes” current in a direct way
Clearing up these misconceptions early makes circuit analysis much easier throughout the course.
Frequently Asked Questions
Does Ohm’s Law apply to all conductors?
No. Only materials with constant resistance obey Ohm’s Law.
Why does temperature matter?
Increasing temperature increases particle collisions, which changes resistance.
Is resistance always constant in a resistor?
Yes for ohmic resistors, but not for non-ohmic components.
RevisionDojo Call to Action
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