Sources in a circuit can be independent or dependent. Independent voltage or current sources supply voltage and current unaffected by other circuit elements. Symbols of independent voltage sources have polarities, while independent current sources use arrows to indicate the current direction. A short circuit is a zero-voltage independent voltage source, while an open circuit is a zero-current independent current source. Dependent sources are voltage or current generators controlled by other circuit elements. They are represented by a diamond shape. Based on their controlling and controlled parts, dependent sources are divided into four categories. Consider a circuit where the dependent source's voltage is controlled by the current through the resistor. If a short circuit is added in series with the resistor, the controlling current is equivalent to the current in the short circuit. The source voltage to controlling current ratio represents the gain of the dependent source. Similarly, the controlling voltage can be considered as open-circuit voltage. So, the dependent sources' voltage or current is proportional to the controlling voltage or current.