Circuit Builder
Wire up the fundamentals of electricity: current, potential difference and resistance, tied together by Ohm's law. Read a circuit, take the measurements, and plot how current responds to voltage.
Get the method right under pressure
Free interactive practice on the steps that lose marks under exam pressure.
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Circuit Builder
Electricity comes down to three linked quantities: current (how much charge flows), potential difference (the push driving it) and resistance (what slows it down). One equation ties them together. Build up from the basics and every circuit question becomes readable, including the one about why a filament lamp refuses to obey the equation once it gets hot.
The words for it
Six terms and their units. The last two are instruments, and where each one goes in a circuit turns out to follow from what it measures:
Charge flowed
A current of 2 A flows for 5 seconds. How much charge flows past a point in that time, in coulombs (C)?
Ohm's law
The three quantities are tied together by one equation: V = I R (potential difference = current × resistance) It rearranges two ways, so any one can be found from the other two: • I = V ÷ R to find a current • R = V ÷ I to find a resistance A check worth doing every time: volts divided by amps must give ohms. If your rearrangement would produce amps per volt, you have divided the wrong way round. And note what the law actually claims. It says these three are related, not that resistance never changes. That distinction matters later in this module.
Find the potential difference
A current of 2 A flows through a 3 Ω resistor. What is the potential difference across it, in volts (V)?
Find the resistance
A component has 12 V across it and a current of 3 A through it. What is its resistance, in ohms (Ω)?
Find the resistor
This circuit has a cell, a resistor and a lamp. Tap the RESISTOR: drawn as a plain rectangle.
Where does each meter go?
An ammeter is connected in series and a voltmeter in parallel. Which explanation actually justifies that, rather than just restating it?
- An ammeter has to carry the current it is measuring, so it must be in the path; a voltmeter has to sit across the two points whose difference in potential it is measuring, so it must bridge the component
- It is a convention agreed for drawing circuit diagrams clearly
- It is done for safety, to keep the meters away from the current
- Because ammeters have high resistance and voltmeters have low resistance
Order the method
Put the steps for measuring the resistance of a component in the lab in order.
- Build a circuit with the component, a cell and an ammeter in series
- Connect a voltmeter in parallel across the component
- Read the potential difference V and the current I
- Calculate the resistance: R = V ÷ I
Series and parallel
Two ways to wire components together, and the two rows that matter most for reading any circuit diagram:
When resistance will not stay still
Plot current against potential difference and the shape of the line tells you what kind of component you have. For an ohmic conductor, a fixed resistor at constant temperature, the graph is a straight line through the origin. Double the p.d. and the current doubles, because the resistance has not changed. A filament lamp does something different. As the current rises, the thin filament gets hotter, and a hotter metal has a higher resistance. So each extra volt buys less extra current than the one before, and the line bends over into a curve. Nothing is broken and Ohm's law is not being violated: V = IR still holds at every instant, with a value of R that is climbing as you watch.
Plot the I–V point
An ohmic resistor has a resistance of 3 Ω. Plot the current when the potential difference across it is 6 V. (Axes: V across, I up. The points (0,0) and (3,1) are already on the line.)
What does each reading tell you?
- Two ammeters, one either side of a lamp, read exactly the same
- A voltmeter across one lamp reads less than the cell's p.d.
- Doubling the supply p.d. exactly doubles the ammeter reading
- Doubling the supply p.d. increases the ammeter reading by much less than double
- They are in series: one path, so the same current everywhere. Current is not used up
- The p.d. is being shared with the other components in the loop
- The component is ohmic: its resistance has stayed the same
- The resistance has risen, which is what a filament lamp does as it heats up
Why does the line bend?
The I–V graph for a filament lamp curves, flattening as the p.d. rises. Which statement describes what is happening?
- The filament gets hotter as the current rises, and a hotter metal has a higher resistance, so each extra volt produces less extra current than the one before
- Ohm's law breaks down at high potential differences
- The cell runs out of charge, so it cannot push as hard
- The filament's resistance falls as it heats, so the current rises more slowly
Which are true?
Select ALL THREE statements that are TRUE.
- An ammeter is built with very low resistance and a voltmeter with very high resistance, so that neither meter changes the circuit much by being there
- A straight line through the origin on an I–V graph means the resistance is constant
- Current is not used up going round a circuit: the same current flows at every point in a series loop
- A filament lamp is an ohmic conductor
- A voltmeter is connected in series with the component it measures
- Doubling the potential difference always doubles the current, whatever the component
The circuit rules
Ohm's law states that potential difference = current × _____. Current is measured with an ammeter connected in series, because it must carry the current, while a voltmeter is connected in _____ across the component. In a series circuit the current is the _____ everywhere. An I–V graph that is a straight line through the origin shows a component whose resistance is _____, while a filament lamp gives a curve because it heats up.