Many people confuse these terms. Read the following guide, and use simple electrical terms with confidence! Understand why you can touch a huge car battery without harm, and why a small cattle-prod will do you in!

VOLTAGE

CURRENT

  • Current is the number of electrons flowing per unit time.
  • Think of it as the equivalent of water flow rate.
  • You get current through something, not across it.
  • Current is measured in amps, short for ampères.
  • Despite the above, it is not correct to refer to current as amperage.
  • Current causes fire and death.

RELATIONSHIPS

In Ohmic conductors, current and voltage are related by Ohm's law. The ratio between them is fixed by the resistance of the material across which the voltage is applied. So you cannot "turn up the current" without affecting the voltage.

Power is the current times the voltage; it is also the current squared multiplied by the resistance.

PRACTICAL EFFECTS

As mentioned in the death by multimeter node, the heart stops with a few tens of milliamps flowing through it. Knowing the resistance of the heart allows us to calculate safe voltages - note that the skin makes a huge difference here!

Static electricity is very high voltage - tens of kilovolts. However, in the water analogy, this is like a very high pressure thin film of water: there isn't enough of it to harm you, usually. If you are a microchip, however, you're in trouble.

A car battery is capable of delivering enormous current (but does not have to!), but only at 12V. Think of it as a low pressure garden hose. You can safely touch both terminals because your high resistance prevents large current flowing. Think of yourself as a very narrow pipe - running a low pressure garden hose into a tiny pipe won't get much flow. Note: avoid doing this in a running car as the ignition system sparks at tens of kilovolts!

A cattle-prod uses clever tricks to generate a high voltage stored as a large charge. When "fired", the unlucky recipient of the high voltage has a low enough resistance for a moderate current to flow, causing the well-known muscle effects.

Power grid lines run at very high voltages - in the UK, at up to 400kV. This is to keep the current requirement low: power is current times voltage, and wire heating is proportional to the square of the current flowing through it. These are like thin tubes at unbelievably high pressure. If you touched these, you would die: your resistance at that voltage would allow huge currents to flow.

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