Circuit elements are the basic building blocks of an electric circuit. They can be categorized as active and passive elements. Active elements can generate energy, whereas passive elements absorb energy. Examples of active elements include voltage or current sources, whereas resistors, capacitors, and inductors are examples of passive elements. Circuit elements can be further classified as either linear or non-linear. A circuit or an element is considered linear if the excitation and response of the element follow the principles of superposition and homogeneity. Suppose a voltage v1 leads to a response i1, and a voltage v2 leads to a response i2. If the element responds to the sum of the voltages with the sum of the responses, then it satisfies the superposition principle. In addition, the homogeneity property is satisfied when multiplying the input of a linear device by any constant results in an equivalent multiplication of the output. Linear elements like resistors, capacitors, and inductors exhibit linear characteristics. Elements like thermistors are non-linear, as they do not comply with these principles.