Simple A.C Circuits


Welcome to the course material on Simple A.C Circuits in Physics, where we delve into the fascinating world of alternating current (a.c.) and explore its behavior in various circuit setups. This topic is crucial for understanding the principles of electricity and how it is utilized in electronic devices and power systems.

One of the fundamental aspects we will cover in this course is the explanation of a.c. current and voltage. Alternating current periodically changes direction, unlike direct current (d.c.) which flows in one direction continuously. Understanding the nature of a.c. is essential as it forms the basis for numerous electrical applications.

As we progress, we will differentiate between the peak and r.m.s. values of a.c. Peak values represent the maximum magnitude reached by the alternating current or voltage, while the root mean square (r.m.s.) values provide an equivalent steady value in direct current that produces the same heating effect in a resistor as the alternating current.

Furthermore, we will explore the behavior of a.c. sources when connected to different circuit components such as resistors, capacitors, and inductors. The interaction between the a.c. source and these elements leads to phenomena like capacitive reactance and inductive reactance, which influence the overall impedance of the circuit.

In series R-L-C circuits, a combination of resistance (R), inductance (L), and capacitance (C) are connected in sequence. Understanding the dynamics of such circuits involves analyzing vector diagrams to determine the phase angle between current and voltage, as well as calculating impedance and reactance.

Moreover, we will delve into important concepts such as effective voltage in R-L-C circuits, resonance, and resonance frequency. Resonance occurs when the inductive and capacitive reactances in a circuit cancel each other out, leading to a maximum current flow. Determining the resonant frequency is crucial for optimizing the performance of such circuits.

Lastly, we will explore the calculation of instantaneous power, average power, and power factor in a.c. circuits. The power factor indicates the efficiency of power transfer in a circuit and plays a significant role in power distribution systems.

In conclusion, this course material provides a comprehensive overview of Simple A.C Circuits, offering insights into the complex interplay of alternating current, resistive, capacitive, and inductive components in electrical systems. By mastering the concepts covered in this topic, you will develop a solid foundation in understanding and analyzing a.c. circuits.


  1. Determine The Instantaneous Power, Average Power And The Power Factor In AC Circuits
  2. Identify AC Current And DC Voltage
  3. Determine The Phase Difference Between Current And Voltage
  4. Determine The Resonant Frequency Of R-L-C Arrangement
  5. Analyse Vector Diagrams
  6. Differentiate Between The Peak And RMS Values Of AC
  7. Interpret Series R-L-C Circuits
  8. Recognize The Condition By Which The Circuit Is At Resonance
  9. Calculate The Effective Voltage, Reactance And Impedance

Lesson Note

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Lesson Evaluation

Congratulations on completing the lesson on Simple A.C Circuits. Now that youve explored the key concepts and ideas, its time to put your knowledge to the test. This section offers a variety of practice questions designed to reinforce your understanding and help you gauge your grasp of the material.

You will encounter a mix of question types, including multiple-choice questions, short answer questions, and essay questions. Each question is thoughtfully crafted to assess different aspects of your knowledge and critical thinking skills.

Use this evaluation section as an opportunity to reinforce your understanding of the topic and to identify any areas where you may need additional study. Don't be discouraged by any challenges you encounter; instead, view them as opportunities for growth and improvement.

  1. What is the peak value of an alternating current or voltage? A. Twice the peak value B. Half the peak value C. Equal to the peak value D. Irrelevant to the peak value Answer: C. Equal to the peak value
  2. What does the r.m.s. value of an alternating current or voltage represent? A. Average value B. Maximum value C. Minimum value D. Instantaneous value Answer: A. Average value
  3. What is the impedance of a purely capacitive circuit in an a.c. circuit? A. Purely resistive B. Purely inductive C. Zero D. Purely capacitive Answer: B. Purely inductive
  4. In an R-L-C series circuit, at resonance, the current ________. A. Is at its minimum value B. Is at its maximum value C. Is in phase with the voltage D. Is out of phase with the voltage Answer: B. Is at its maximum value
  5. What does the power factor of a circuit indicate? A. How much power is wasted B. How efficient the circuit is C. The total power consumed D. The total resistance in the circuit Answer: B. How efficient the circuit is
  6. What is the resonant frequency of an R-L-C circuit? A. The frequency at which impedance is minimum B. The frequency at which impedance is maximum C. The frequency at which power factor is 1 D. The frequency at which current is maximum Answer: A. The frequency at which impedance is minimum
  7. What is the relationship between the phase angle, resistance, and reactance in an a.c. circuit? A. They are all equal B. They are all perpendicular to each other C. The phase angle is the arctan of the ratio of resistance to reactance D. There is no relationship between them Answer: C. The phase angle is the arctan of the ratio of resistance to reactance
  8. How is the effective voltage in an R-L-C circuit calculated? A. As the sum of the peak and r.m.s. values B. As the product of the peak and r.m.s. values C. As the difference between the peak and r.m.s. values D. As the square root of the sum of the squares of the peak and r.m.s. values Answer: D. As the square root of the sum of the squares of the peak and r.m.s. values
  9. What is the instantaneous power in an a.c. circuit at any given time? A. The product of the current and voltage at that time B. The sum of the current and voltage at that time C. The ratio of the current to the voltage at that time D. The difference between the current and voltage at that time Answer: A. The product of the current and voltage at that time

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Past Questions

Wondering what past questions for this topic looks like? Here are a number of questions about Simple A.C Circuits from previous years

Question 1 Report

You are provided with a battery of e.m.f, Estandard resistor, R, of resistance 2 ? ? , a key, K, an ammeter, A, a jockey, J, a potentiometer, UV, and some connecting wires.

(i) Measure and record the emfE, of the battery.

(ii) Set up the circuit as shown in the diagram above with the key open.

(iii) Place the jockey at the point, U, of the potentiometer wire. Close the key and record the reading, i, of the ammeter.

(iv) Place the jockey at a point on the potentiometer wire UV such that d = UT = 30.0 cm.

(v) Close the circuit, read and record the current, I, on the ammeter,

(vi) Evaluate I1 1 .

(vi) Repeat the experiment for four other values of d = 40.0 cm, 50.0 cm, 60.0 cm and 70.0 cm. In each case, record I and evaluate I1 1 .

(vii) Tabulate the results

(ix) Plot a graph with d on the vertical axis and I on the horizontal axis stalling both axes from the origin (0,0).

(x) Determine the slope, s, of the graph.

(xi) From the graph determine the value I1 1 , of when = 0. (ci) Given that=s, calculate 8.

(xii) State two precautions taken to ensure accurate results.

(xii) Given that E?

 = s, calculate ??

(b)(i) Write down the equation that connects the resistance, R, of a wire and the factors on which it depends. State the meaning of each of the symbols.

(ii) An electric fan draws a current of0.75 A in a 240 V circuit. Calculate the cost of using, the fan for 10 hours if the utility rate is $ 0.50 per kWh.

Question 1 Report

From the diagram above, if the potential difference across the resistor, capacitor and inductor are 60V, 120V and 30V respectively, the effective potential difference is

Question 1 Report

A series RLC circuit is said to resonate, when

Practice a number of Simple A.C Circuits past questions