Voltage and Current
The Voltage(V or sometimes E) between two points is the cost in energy (work done) required to move a unit of positive charge from the more negative point (lower potential) to the more positive point (higher potential).
So what does this mean for you and me?
Voltage is not thought of like this in practice by most engineers, we gain an intuitive understanding of what voltage really is as we learn. However, in the meantime we can think of Voltage being what we apply to cause current to flow. Note: Voltage may also be called ‘Potential Difference’ or Electromotive Force (EMF).
Hold on a minute, slow down, what is current?
Current is the rate of flow of electric charge past a certain point. It can also be thought of as the rate of flow of electrons past a point as flowing electric charge is typically carried by moving electrons. The important and often confusing thing here is that there are two ways of thinking about current.
It’s not that complicated don’t worry..
So ‘conventional current’ is thought to flow from positive to negative, so in a simple LED circuit powered by a battery, we would say that the current is flowing from the positive side of the battery (The + side) to the negative side (The – side). Whereas the actual flow of electrons is from negative to positive, so in this scenario the current would flow form the negative side of the battery to the positive side.
This next point is very important to remember and it will simplify things going forward!
Voltage appears 'across' things
Current flows 'through' things
Try and really think about it and wrap your head around it, it makes sense: trust me. To measure voltage, you measure between or across two points, across the two ends of a battery will give you the voltage of that battery.
To measure current, you measure the current through things and in a simple series circuit the current will be the same through the whole circuit (more on this later). If we put a break in our circuit and attach a multimeter, and let the current flow through the multimeter to the rest of the circuit we can get a reading of the current.
Let’s wrap this up!
You now know what they are separately but let’s visualise it and bring it together. This is an analogy I found really useful when I was beginning to learn about electronics.
The Waterfall Analogy
The height of the waterfall will represent the voltage. The higher the waterfall is, the more potential energy the water has.
The potential difference/voltage here is large as it is far from the bottom of the falls, or ground.
The current is represented by how much water is falling over the edge of the waterfall per second.
The resistance is represented by any obstacles that might slow the flow of the water, I.e. rocks before the water gets to the edge and air resistance.
From this analogy you can see that, the voltage or height of the waterfall dictates the amount of push the electrons are exerting to get to their destination. The current is the rate of flow of electric charge or water over the edge of the waterfall. And resistance is the obstacles that try to slow the flow of electric charge or the flow of the water off the waterfall.
This leads us on to Ohms Law. Check out our article on Ohms law to learn more.
