Voltage, Resistance and Ohm's Law
What pushes the current, what holds it back, and the simple law that ties them together
- say what voltage is and how a voltmeter is connected;
- explain what resistance is and what it depends on (resistivity, length, area);
- use Ohm's law I = U / R and rearrange it;
- measure a resistance with an ammeter and a voltmeter.
1. Voltage: the push per unit of charge
In the previous topic you saw that a current is a flow of charge, driven by a source (a battery or a generator). But what exactly does the source provide? It provides energy to every charge that passes through it. Voltage (also called potential difference) tells you how much energy the circuit gives to each unit of charge as it moves between two points.
In symbols, voltage U equals the work W done on the charge divided by the amount of charge q:
- U — voltage, measured in volts (V);
- W — work (energy) given to the charge, in joules (J);
- q — the charge that moved, in coulombs (C).
So one volt is one joule per coulomb: 1 V = 1 J / 1 C. If a source has a voltage of 1.5 V, it hands 1.5 joules of energy to every coulomb of charge that passes. A fresh flashlight cell is about 1.5 V, a car battery 12 V, a household socket in Kyrgyzstan is 220 V.
Voltage is measured with a voltmeter. A voltmeter is always connected in parallel — across the two ends of the part you are studying — because it compares the energy of the charges on the two sides. A good voltmeter has a very high internal resistance so it takes almost no current for itself.