Voltage, Current & Resistance

OK Let's get started with the very basics!

I appreciate many of you do already know these basics, so please treat these first few sections as a quick reminder.

Aims & Objectives

  1. Understand the basics of Voltage, current & resistance
  2. Understand the mathematical link between Voltage Current & Resistance
  3. Understand the correlation between water flow and electric current
  4. Understand the difference between 'Conventional Current' and 'Electron Flow'


TIP: In this section we're going to have a quick look behind the scenes at how electricity behaves in a circuit, please note that even though this section is very basic, it is probably the most important! It will help you massively when it comes to fault finding to think in terms how electricity behaves in a circuit; or should behave!


So...what is Electricity?

Electricity is a form of energy brought about by displacing electrons within a conductor, such as copper wire, and a simpler way to visualise this is to use the popular water flow analogy below:


Boat Electrics Course Memo

In the diagram above:

  • The water pump acts as a source of energy, it creates a pressure, and this pressure creates a flow.
  • This 'flow' is restricted due to a section of increased 'resistance' in the pipe.
  • The restriction reduces the flow rate around the entire circuit, so more resistance = Less Flow.
  • The restriction in the pipe causes a pressure drop, there is a higher pressure on one side than the other.

In an electrical circuit the:

Pressure is called Voltage (Volt V)

Rate of flow is called the Current (Amp A)

Restriction is called Resistance (Ohm Ω)

Tip: I will be using these terms often throughout the course so it's worth spending a little time at this stage learning these terms thoroughly, if you don't already know them.

  • Pressure = Voltage, Volt V
  • Flow = Current, Amp A
  • Restriction = Resistance, Ohm Ω


Ohms Law

Fully understanding how electricity behaves in a circuit is paramount when it comes to finding faults, cable sizing, circuit protection. The units of Voltage, current, and Resistance are all connected by a simple maths formula and each unit is calibrated nicely so that:

1 Volt (V) of voltage will push

1 Amp (A) of current through

1 Ohm (Ω) of resistance


Please watch the lesson below.

Video Lesson: Voltage, Current & Resistance

We're going to use the terms and concepts from this lesson often throughout the course. Please ensure at this stage you fully understand the following terms.

Resistance. The Ohm (Ω) (R)

Resistance is the amount of restriction, expressed as Ohms Ω (or the letter R in formulas) Less resistance will allow more current to flow.

Voltage. The Volt (V)

Voltage is the driving force, synonymous to the pressure of a plumbing system, expressed as V, it's how much potential force can be applied to the electrons. The more voltage, the more current can flow

Current. The Amp (A) or (I)

Current is the Flow, expressed as A (or the letter I in formulas for 'Intensity') is essentially how many electrons flow at once. Synonymous with the flow of water in a plumbing system. With less resistance or more voltage, a higher current will flow.

Optional section

This whole course is based on the accepted conventional idea that electricity flows from positive to negative. If you're interested in the science behind what's actually happening then section 3 below may be of interest, whilst it is not required in order to complete this course, it will be useful if you are doing the level 2 course in the future.

3.0 Electron flow vs Conventional flow

3.1 Electron flow.

In conductors, such as copper wire, the copper atoms are more than happy to let one of their outer most electrons jump to the other atoms around it. The electrons are already in the cable, surrounding the copper atoms nuclei, the Negative terminal of a battery offers a surplus of electrons, whilst the positive terminal has a deficit. Forming a conductive connection between the two provides an opportunity to redress the balance, but the electrons encounter resistance on the way in the form of work that must be completed in order for them to complete their arduous journey.

Electrons flow from negative to positive, but this was only discovered well after electricity was put into practical use. Prior to this there was an incorrect but well established 'conventional' understanding that electricity flowed from positive to negative, and this convention is still in use.

3.1 Conventional Current.

We still continue to this day with the old convention, we have to imagine there's a 'deficit of electrons' as a form of charge flowing from positive to negative, and we call this the 'Conventional Current' flow.

A good analogy for this is to imagine a hot steel copper rod inserted in a bucket of ice, we can imagine that the cold flows up the rod, even though scientifically it's the heat flowing from the rod to the ice.

Throughout this course we will refer to current flowing in the 'Conventional Current' understanding from Positive to Negative.

Let's get onto the next section and learn about 'Power'.

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