Edexcel IGCSE Physics · Spec 2.4-2.6
Voltage, Resistance & Ohm's Law
Voltage, resistance and Ohm's law.
Physics revision video
Voltage, Resistance & Ohm's Law
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Explained
Voltage, resistance, and the law that connects them
Current is the flow of charge. Voltage is what drives it, and resistance is what opposes it. Those three sentences are the whole topic, and the equation simply states how they trade off.
What voltage actually is
Voltage, properly called potential difference, is the energy transferred per unit of charge. One volt means one joule of energy transferred for every coulomb of charge passing through.
The word difference matters. Potential difference is always measured between two points, which is why a voltmeter has to be connected across a component rather than in line with it. Asking for the voltage at a single point makes no more sense than asking for the height difference of a single place.
Resistance
Resistance is potential difference divided by current, measured in ohms. A component with a high resistance needs a large voltage to push a small current through it.
In a metal, resistance comes from the moving electrons colliding with the vibrating metal ions. Heat the wire and the ions vibrate more, the collisions become more frequent, and the resistance rises. That is why a filament lamp is not ohmic.
Ohm's law
For an ohmic conductor at constant temperature, current is directly proportional to potential difference, and V equals I times R.
The condition at constant temperature is part of the law, not an afterthought. A fixed resistor obeys it, which is why its current voltage graph is a straight line through the origin. A filament lamp does not, because passing current through it changes its temperature.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme divides each calculation here into three separate marks: correct substitution into V equals I times R, the rearrangement, and the evaluation. Crucially, it allows an error carried forward from an incorrect voltage at every stage, so a wrong number early on does not lose the later marks provided the method is visible.
It goes further on one part, allowing a dimensionally correct rearrangement using an incorrect voltage for that mark only. Method is being credited quite separately from arithmetic, which is the strongest possible argument for writing out every step.
Two other details are worth noting. Where a question asked why two components had particular voltages, the mark scheme credits that the components are connected in series and that the voltages must add up to the battery voltage, and it instructs the examiner to ignore the observation that the voltmeter is in parallel. It is a true statement, but it does not answer the question asked.
And on a follow up part it says simply, same answer as (ii), allowing a differently rounded value provided it is consistent. Recognising that a quantity has not changed is sometimes the whole mark.
Using it reliably
Write V equals I times R, put the two numbers you have into it, then solve for the third. Rearranging in your head first is where marks are lost, because a wrong rearrangement takes the method credit with it.
Check the units too. Volts, amperes and ohms go together. A current given in milliamperes must be divided by a thousand first, and that conversion is the most common cause of an answer that is out by a factor of a thousand.
Spec 2.4-2.6
What you need to know
- Describe what voltage, or potential difference, is
- Describe what resistance is
- Use Ohm's law
Active recall
Quick check
Answer each question before opening the answer.
State Ohm's law as an equation.
Voltage = current × resistance (V = I × R).
What is potential difference (voltage)?
The energy transferred per unit charge, measured in volts (V = E ÷ Q).
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