B.1 VOLTAGE, CURRENT, RESISTANCE AND POWER
Figure B.1 shows a voltage source (V) connected to a resistance (R). When the two terminals of a voltage source (such as a battery) are connected across a resistance, a current (I) flows from the positive terminal of the voltage source, through the resistance and back to the negative terminal of the source. The direction of the current is indicated by the arrows on the wires connecting the voltage source and resistance. Voltage is measured in units of volts (V) named after Alessandro Volta, and current is measured in units of amperes, or amps (A), named after Andre Marie Ampere.
The ability of a material to conduct electricity depends on the freedom of electrons in the material to move. A good conductor, such as copper, has many more electrons able to move freely than a poor conductor, such as glass. The conductance of a material is a measure of its ability to carry current. In practice, the resistance of a material to the flow of electrons, which is the inverse of conductance, is more commonly quoted. Resistance is measured in ohms (Ω), named after Georg Ohm.
The voltage across a circuit, the current flowing through it, and the resistance of the circuit are related by Ohm’s Law:
For example, if a current of 100 mA flows through a resistance of 50 Ω, the voltage across this resistance must be 5 V.
When a current passes through a resistance, heat is generated. This heating is caused by the dissipation of energy in the resistance. Power (P) is the rate at which energy is dissipated, which is measured in watts (W). The power dissipated in a resistance, R, is given by:
Since Ohm’s law tells us that V=IR, the equation can also be written: