Choosing the x and y axis for a graph
Changing one thing and measuring another
In physics a standard sort of experiment involves changing one thing and measuring another while keeping everything else constant. For example 'I changed the voltage across a bulb and measured the current through it while keeping the rest of the circuit the same.'
Activity Plotting a graph of current vs. voltage for a filament bulb by dragging the crosses to the right position.GCSE Maths exam soon?
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In this case what you 'change' is what you as an experimenter are in charge of. You can choose to take whatever voltage readings you think are necessary at whatever interval and over whatever range. What you're in charge of is sometimes called the independent variable.
What you 'measure' is what you find out about. The idea is that before you actually do the experiment you don't know what you'll find out. What you find out about is sometimes called the dependent variable because it depends on what you changed.
What you measured goes on the y-axis
When you look at a graph you tend to look at whether it goes up or down. Going left to right is just a given. That's why what you measured always goes on the y-axis. It's what you found out: it's what the experiment was actually about.
In our example we changed the voltage and found out about the current so current goes on the y-axis. (You can't change the current and find out about the voltage because power supplies provide known voltages not known currents, unless you have a power supply with some electronics that changes its voltage to keep the current constant.)
Swapping axes to make the gradient meaningful
Ohm's law is sometimes by the equation V = IR. You can rewrite this as V = RI and then compare it with y = mx. In this case the gradient, m, is equal to the resistance, R.
So plotted with the voltage up the y-axis a steeper line means a bigger resistance.
For components that don't obey Ohm's law the general rule is that the steeper the curve the bigger the resistance but it's not numerically equal to the resistance.
Animation explaining why the gradient of a voltage vs. current curve is not equal to the resistance.