Voltage

Assume two containers connected with a channel.  When one is higher than the other one the water in the higher container starts running from the top one to the bottom one creating a current. This happens because the water in the higher level relative to ground has a higher potential energy.

 

Voltage is the potential energy that makes the electrical current flow in a circuit by pushing the electrons around. The unit of voltage is volt shown as ‘v’. To be more accurate, one Volt is equal to one Joule of energy that can move one Coulomb of electrical charge. Formula below shows this relation.

V = W / q

In this formula, V is the voltage in volt, W is the energy in joule and q is the electrical charge in Coulomb. Similar to the example above, in a circuit there is an electrical component that acts like the channel in the figure above. When the voltage on one side of the component is higher than the other side, the electrical current flows from that side to the other side, which is in the reverse direction of the electron flow as described in the Current section. Following equations show the relation between the Electrical Field Strength described in the Electrical Charge section and voltage.

 

In this formula F is the force in Newton, E is the electrical field strength in Newton per Coulomb or volt per meter, r is the distance in meter between two points with potential difference of V. When they want to show the voltage across an electrical component, they use plus and minus signs on its terminals, showing the side with a higher voltage with the plus sign as shown in the figure below.

 

The arrow in the figure above shows the direction of current I in the component. You may think that the current direction is always from the higher voltage to the lower voltage, but that’s not always true. This is only true when a component consumes energy in a circuit (consumer). There are components that give back energy to the circuit such as a battery (generator) and also there are components that are both consumer and generator depending on the way they used in a circuit. In a battery the direction of the current is from the negative side to the positive side inside the battery and vice-versa outside, as shown in the figure below.

 

Voltage is a relative entity similar to energy. To be able to tell the voltage of a point, it must be compared to a reference voltage of another point. As explained in the Circuit section, a circuit can have many different nodes. Each node can have a different voltage. To measure the voltage of each node, usually a reference node is selected as Ground with an assumed zero voltage and the voltage of other nodes are measured relative to this node. The voltage across a component is the difference of the voltages at its ends. It doesn’t matter how you choose the voltage plus and minus locations across a component or to the ground, it only changes the sign of the final result. But whatever you choose, you should stick with it to be consistent in your calculations. Figure below shows voltages in a circuit. To understand the figure completely, you should know an important rule: “Voltages across parallel components in a circuit are equal”. Please refer to Kirchhoff laws section for more information.

 

As you can see in node 2, the negative side of one components can be the positive side of another component. Also as mentioned in the rule of parallel components above, the voltage across C₁ is the same as the voltage across the two ends of the branch with C₂ and C₃ in series, equal to V₁.