IB Physics Sub-topic B5 Notes

Voltage

Topic B.5 primarily focuses on electricity, which is the movement of charges. Their movement is brought about by the electric force, and in Topic D.2 you will learn about electric fields that exert this electric force. However, you will learn the basics before this. Remember from Topic A.2 that when an object moves in the direction of an applied force, work is being done on the object. So, if a test charge in an electric field moves in the direction of the exerted electric force, work is done on the charge. 

This is the voltage (V), officially defined as the work done per unit charge and measured in Volts (V). The formula for this is:

$V = \frac{W}{q}$

However, voltage typically applies to large scale movements of electrons. When work is done on a single electron, it is measured in electronvolts (eV). To convert, moving an electron through 1 Volt is 1eV, which is equal to 1.6 x 10^-19 J.

You may also hear voltage referred to as potential difference (pd), and you can use both terms interchangeably. However, let's explain the term nonetheless:

1. If you place test charge right next to a charge distribution and then move it, we know that work is being done on the test charge. 
2. As a result, the test charge gains energy, called its electric potential energy or electric potential.
3. Right next to the charge distribution, it had no electric potential, but further away from it, it now has electric potential. 
4. It is this change in potential that is called the potential difference.

However, this is functionally the same as voltage and thus both terms can be used. It should be noted that potential difference is more often used when discussing individual charge movement, whilst voltage is more often used when discussing circuits.