Voltage & Current - juxtapix/IMA_E4I GitHub Wiki

Voltage is the cause, current is the effect

It's possible to have voltage without current, but current cannot flow without voltage.

Voltage

Voltage is the potential energy between two points in a circuit. In the plumbing analogy, the amount of water acts as the charged particles and voltage is the water pressure.

  • Voltage = Potential difference
  • Voltage is used up inside components, but not in wires.
  • Voltage is measured in Volts, or the symbol V.
  • Voltage is measured across a component
  • Voltage is measured by a voltmeter connected in parallel
  • In electrical equations, the symbol for voltage is V.

Voltage at a point & the concept of 0V (zero volts)

  • Voltage is the difference between two points.
  • In electronics, we talk about voltage at a point in ref to voltage difference between that point and a reference point of 0V.
  • Zero volts could be at any point in a circuit. For consistency, it is usually at the negative terminal of a power supply.

Zero volts for circuits with a dual supply

  • Some circuits require a dual supply.
  • For these circuits, zero volts exist as the middle terminal.
  • On complex circuits, the earth/ground symbol is used to mark zero volts.

Current

  • Current is the rate of flow of a charged electrons.
  • Current is not used up, what flows into a component must flow out.
  • Current is measured in amps (amperes), or the symbol A.
  • The unit of current is coulombs per second: 1 A = 1 C/s
  • Current is measured with an ammeter connected in series.
  • In electrical equations, symbol for current is I.

Voltage and current for components in series

  • Voltages add up for components connected in series.
  • Currents are the same through all components connected in series.
  • Example: two components are connected in series, one component uses 2V and the second uses 4V, they add up to 6V.
  • The current through both of those components stay the same all the way through the circuit.

Voltage and current for components in parallel

  • Voltages are the same across all components connected in parallel.
  • Currents add up for components connected in parallel.
  • Example: two components are connected in parallel, one component uses 30mA and the second uses 60mA, they add up to 90mA.
  • The voltage supplied by the power supply to the whole circuit stays the same.

Important notes when talking about electronics:

Current flows through things while voltages are applied across things. Make sure we use the correct vocabulary - say "current through a device" and "voltage between two points" or "across two points" in a circuit. This is because voltage is used up by components at different points in a circuit, so the voltage at any given point could vary and we only get current by placing voltage across things, pushing the charged electrons through the circuit.

To say something like “the voltage through a resistor ... ” is nonsense. However, we do frequently speak of the voltage at a point in a circuit. This is always understood to mean the voltage between that point and “ground,” a common point in the circuit that everyone seems to know about."" - The Art of Electronics