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Current is rate of charge flow, I = Q/t. Potential difference is energy transferred per unit charge, V = E/Q. Resistance relates potential difference and current through R = V/I for the measured operating point. Charge is not used up as it passes a lamp: energy is transferred while charge continues around the circuit. Connect an ammeter in series and a voltmeter across the component.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Use charge, current and potential difference relationships
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Where is a voltmeter connected?
In parallel across a component. It measures the potential difference between two points.
After watching 2: What stays equal in all parallel branches connected to the same supply points?
Potential difference. Branch current depends on branch resistance.
What you'll be able to do
Current is rate of charge flow, I = Q/t. Potential difference is energy transferred per unit charge, V = E/Q. Resistance relates potential difference and current through R = V/I for the measured operating point. Charge is not used up as it passes a lamp: energy is transferred while charge continues around the circuit. Connect an ammeter in series and a voltmeter across the component.
In a series circuit current is the same through each component and potential differences add to the supply. Resistances add in series. In parallel, each branch has the same potential difference and branch currents add to the total. Adding a parallel branch lowers total resistance. Explain the result using charge flow and the supply potential difference, rather than claiming every component receives the same current in every arrangement.
An ohmic resistor has a straight I–V graph through the origin at constant temperature. A filament lamp’s resistance rises as it gets hotter, while a diode conducts mainly in one direction. A thermistor’s resistance usually falls as temperature rises in the GCSE model; an LDR’s falls with increasing light. To measure a characteristic, change the supply systematically, record current and voltage and control heating where necessary.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: A junction has 1.2 A entering and one outgoing branch carries 0.5 A. Find the other branch current.
0.7 A.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.