physics//electromagnetism//electric circuit//voltage

Voltage is the energy that each unit of electric charge gains or loses between two points, the push that drives an electric current, and it is the first number on any electrical label: what a device needs, what a battery gives, what an insulator must withstand. One volt means one joule for every coulomb of charge that makes the trip.

Voltage is the energy that each unit of electric charge gains or loses between two points, the push that drives an electric current, and it is the first number on any electrical label: what a device needs, what a battery gives, what an insulator must withstand. One volt means one joule for every coulomb of charge that makes the trip.

Voltage is always between two points, like a height difference. A useful ladder: an AA cell gives 1.5 V, a phone battery about 3.7 V, a USB port 5 V, a car battery 12 V, a European socket 230 V, a high-voltage line hundreds of thousands of volts, a body charged on a carpet several thousand volts, a storm cloud hundreds of millions. The resting voltage across a neuron's membrane is about 70 millivolts, small in volts and enormous as a field, because the membrane is only a few nanometres thick.

Voltage says how hard each charge is pushed, never how much charge there is.

A carpet spark at 3,000 V moves almost no charge and stings; a car battery at 12 V can deliver hundreds of amperes; what a voltage can do depends on how much charge stands behind it and how fast it can flow. Through a given resistance voltage and current rise together (Ohm's law), but either can be large while the other is small.

On an object, voltage depends on charge and size together. The same charge on a smaller object sits at a higher voltage, because its capacitance is smaller (V=Q/CV = Q/CV=Q/C); for an isolated sphere the voltage grows with the charge density times the radius.

Across a gap, voltage divided by distance gives the electric field, which is what decides whether insulation holds or sparks.

A voltage source holds a voltage steady while current flows; a charged object without one loses voltage as its charge drains.