Electric Circuits

Some basic circuits and components
In physics it is important you know all the basic component symbols in order to construct basic circuit diagrams.

Circuit diagram

All circuit diagrams will include some sort of power source and a component. In this case it is a 5V power source and two resistors. Note how the Voltmeters are placed in parallel to circuit components and the ammeter is placed in series. This is very important! An easy way to remember this order is that voltage is also known as potential difference and therefore needs to measure the difference around a component.



Electric Current

Conventional current is the flow of positive charge, however as we know it is actually the electrons that flow around a circuit. We still, however say that current flows from positive to negative.


Some definitions

The charge passed by a current of one ampere in one second.

The rate of flow of charge due to the passage of charged particles

Charge current and time are related using the following equation:

Q = Charge moved (C)
I = Current (A)
t = Time (s)

Therefore one ampere is equal to one coulomb per second.

Example Question
Calculate the current when 6.77 C of charge is transferred in 3 seconds.


Potential Difference

Potential difference is defined as the work done per coulomb when charge is transferred from one point to another.

Voltage charge and energy are related using the following equation:

Thus the Volt can be defined as:
Two points in a circuit have a potential difference of one volt if one joule of energy is transferred when one coulomb of charge passes from one point to another.