4.2 - Energy, Power, and Resistance

Potential Difference and EMF

• Potential difference is the energy transferred per unit charge from electrical energy to other forms, such as light and sound.
• EMF is the energy transferred from an energy source, such as chemical energy in cells, to electrical energy.

The change in kinetic energy of one particle accelerated through a potential difference is:

... where q is its charge and v is the velocity gained in this acceleration.

Resistance and Ohm's Law

Ohm's law states that the current through a conductor is directly proportional to the potential difference across it provided that physical conditions, such as temperature, remain constant.

... where V is potential difference across a component, I is current through it, and R is its resistance.

Wires can also have resistance. At a constant temperature, this is based on their resistivity rho (which is a property of the material), its length L, and its cross sectional area A.

Resistivity

However, resistivity can also change with temperature. At higher temperatures, metal ions have more kinetic energy, so vibrate more vigorously. This increases the rate of collisions between electrons and metal ions, reducing the flow of electrical current. As such, we can describe components as being an ohmic or non-ohmic conductor based on whether or not current and p.d. are directly proportional. E.g.:

• A normal resistor is an ohmic conductor.
• A thermistor is non-ohmic.
• A lamp is non-ohmic, as heat is generated in the filament in the process of creating light.