To understand the relationship between alternating current (AC) and alternating electromotive force (emf), particularly why the current lags behind the emf, we need to delve into the concepts of phase difference and the behavior of inductive and resistive components in an AC circuit.

Phase Difference Explained

In an AC circuit, the voltage and current can be out of sync, which is described as a phase difference. When we say that the current lags behind the emf, it means that the peak value of the current occurs after the peak value of the voltage. This lag is typically measured in degrees, with a complete cycle being 360 degrees.

Understanding the Lag

Imagine a simple circuit with an inductor. When the alternating voltage is applied, the inductor resists changes in current due to its property of inductance. This resistance to change causes the current to take some time to reach its maximum value after the voltage does. The physical interpretation is that the inductor stores energy in its magnetic field when the voltage is applied, and it releases that energy when the voltage decreases, causing the current to peak later than the voltage.

Initial Voltage and Maximum Current

Now, regarding your question about how maximum current can flow in the opposite direction when the initial voltage is zero, let's clarify this with an analogy. Think of a swing: when you push it (apply voltage), it doesn't immediately reach its highest point (maximum current). Instead, it swings back and forth. In an AC circuit, when the voltage is zero, it doesn't mean there is no current; rather, the current can still flow in the opposite direction due to the energy stored in the inductor. The current can reach its maximum value when the voltage is at zero because it is transitioning from one peak to another.

Emf and Current Relationship

Regarding the statement that emf varies with respect to current, this typically refers to the dynamic nature of AC circuits. In circuits with inductors and capacitors, the relationship between current and emf is not straightforward. The emf can be seen as the driving force that causes current to flow, but the actual current depends on the circuit's impedance, which includes resistance, inductance, and capacitance. As the current changes, the impedance can also affect how the emf behaves, leading to variations in the overall circuit dynamics.

Summary of Key Points

• The current lags behind the emf due to the inductive properties of circuit components.
• Maximum current can occur when the voltage is zero because of energy storage in inductors.
• The relationship between emf and current is influenced by the circuit's impedance, which can change dynamically in AC circuits.

In essence, the behavior of current and voltage in AC circuits is a fascinating interplay of energy storage and release, influenced by the properties of the components involved. Understanding these concepts helps clarify the seemingly counterintuitive aspects of AC behavior.