Let’s make this simple ..

Say we have a 1.5V battery. It has 2 terminals: +ve and –ve. The positive terminal measures 1.5V. This voltage is arbitrary meaning it has to be compared to something. This something is the –ve terminal, which is the reference point – and it is considered as 0V.

So, the +ve terminal of the battery is 1.5V higher compared to the –ve terminal. In short, this is a potential difference.

Putting this in a circuit

Electric circuit highlighting the internal resistance of the voltage source

Okay, this is a little bit confusing.
So, let’s break down the above circuit:

  • Vs is the voltage source – for example a battery
    • The arrow is pointing to the higher potential (the 12V side)
  • RL is the load resistance
  • Rs is the internal resistance of the source (battery).

How to Measure Voltage

We use a volt meter to measure the voltage of a battery (or any voltage source). Therefore, it is placed across the battery’s terminals.

To measure the voltage drop across a component (resistor for example), we place the voltmeter’s terminals across that component. This placement will in parallel to the resistor.

Note: the current cannot be measured in parallel. The current meter would have to be integrated in the circuit in series with the components.

Voltage of a battery can be measured in 2 conditions:

  1. Open Circuit
  2. Closed Circuit
Measuring potential difference. Open vs Closed circuit measurement with volt meter

OPEN CIRCUIT

  • Means no lightbulb, resistor or any other component connected to the battery.
  • Only the 2 voltmeter terminals touch the battery terminals.
  • But, the volt meter becomes a component this way, isn’t it? Yes, it will. It acts as a resistor yet there is no current flow:

The volt meter has a very high internal resistance so won’t pass any current when its terminals touch the battery.

Essentially, we have an open circuit thus, no current flow from the +ve terminal to the -ve terminal of the battery. We will measure 12V – this is the EMF (Electro Motive Force of the battery) i.e. the potential difference between the two terminals.

  •  

CLOSED CIRCUIT

  • If we connect the battery to a circuit – we close the circuit; thus, inducing current flow.
  • There will be current passing through the batterie’s internal resistance (Rs) meaning, if there is I and there is Rs, there will be a voltage drop across Rs.
  • Now, if we tap the battery terminals with the volt meter again, we won’t be measuring 12V anymore. This is because there will be a voltage drop across Rs.
    • A battery is not a perfect voltage source. The voltage that it supplies depends on the load and the current that passes through the circuit.
    • So, if we connect a lamp, it will not get 12V from the battery. The battery will supply slightly less because of the voltage drop (loss) across Rs.

The voltage the battery provides is the EMF minus the voltage drop across the source resistance or:

Formula for voltage related to the EMF and the potential difference across the internal resistance

Where:

  • V – is the voltage the battery will provide to the lamp
  • E – is the EMF the battery has (12V)
  • I – is the current
  • Rs – is the internal resistance of the battery

Note: E is a voltage, I x Rs is also a voltage (because of Ohm’s Law: V = I x R), namely the voltage drop across the battery’s internal resistance. We get V by subtracting 2 voltages.

As batteries age, the internal resistance increases.

Some terms to understand

Good Voltage Sourcesupplies a stable voltage that is minimally dependent on the load resistance and the current passing through it.

Ground , common or earth:

  • They mean the same thing.
  • A standalone circuit is said to be floating -> therefore, the 0V terminal is arbitrary.
  • Within the circuit we reference all voltages to the 0V terminal but what do we reference the 0V to?
  • Connecting 2 circuit might result in issues as 0V for one might not be the same 0V for the other. 0V terminals are arbitrary, remember?
  • Solution: connect both 0V terminals to a common earth or ground point that we define as 0V. The earth is so large that its potential is not dependent on the current that flows into it, so it is a stable reference point.

optional reading: Success in Electronics book by Tom Duncan
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