The average power delivered to a series AC circuit is given by (symbols have their usual meaning):

Options

A. `E_("rms") I_("rms")`
B. `E_("rms") I_("rms")cos theta`
C. `E_("rms") I_("rms")sin theta`
D. zero

Answer

To find the average power delivered to a series AC circuit, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Power in AC Circuits**:
- In an AC circuit, power is defined as the rate at which electrical energy is transferred by an electric circuit. The average power delivered to the circuit can be expressed in terms of the root mean square (RMS) values of voltage and current.

2. **Identify the Formula for Average Power**:
- The average power \( P \) delivered to a series AC circuit can be expressed using the formula:
\[ P = V_{rms} \cdot I_{rms} \cdot \cos \phi \]
- Here, \( V_{rms} \) is the root mean square voltage, \( I_{rms} \) is the root mean square current, and \( \cos \phi \) is the power factor, where \( \phi \) is the phase difference between the voltage and the current.

3. **Relate EMF to Voltage**:
- In the context of this question, the electromotive force (EMF) \( E \) is equivalent to the RMS voltage \( V_{rms} \). Therefore, we can substitute \( E \) for \( V_{rms} \) in our power formula:
\[ P = E_{rms} \cdot I_{rms} \cdot \cos \phi \]

4. **Final Expression**:
- Thus, the average power delivered to the series AC circuit can be expressed as:
\[ P = E_{rms} \cdot I_{rms} \cdot \cos \phi \]

5. **Conclusion**:
- The correct answer to the question is that the average power delivered to a series AC circuit is given by:
\[ P = E_{rms} \cdot I_{rms} \cdot \cos \phi \]