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In an oscillating \(L C\) circuit, when \(75.0 \%\) of the total energy is stored in the inductor's magnetic field, (a) what multiple of the maximum charge is on the capacitor and (b) what multiple of the maximum current is in the inductor?

Short Answer

Expert verified
(a) Charge on capacitor is 0.5 times maximum charge; (b) current in inductor is 0.866 times maximum current.

Step by step solution

01

Understand Energy in LC Circuit

In an LC circuit, energy oscillates between the electric field in the capacitor and the magnetic field in the inductor. The total energy in the circuit is given by: \[E = \frac{1}{2} C V^{2} = \frac{1}{2} L I^{2}.\]When 75% of the total energy is stored in the inductor, the remaining 25% is stored in the capacitor.
02

Setup for Energy in the Inductor

The energy stored in the inductor is given by \[E_{L} = \frac{1}{2}L I^{2}.\]Given that 75% of the total energy is in the inductor, we can express it as\[E_{L} = 0.75E.\]
03

Setup for Energy in the Capacitor

The energy stored in the capacitor is given by \[E_{C} = \frac{1}{2}C V^{2}.\]Since only 25% of the total energy is stored in the capacitor, we express it as\[E_{C} = 0.25E.\]
04

Express Total Energy

The total energy of the circuit is the sum of energies in the capacitor and inductor:\[E = E_{L} + E_{C}.\]Using the given percentages, we have:\[E = 0.75E + 0.25E = E.\]This confirms our setup aligns with the given of 75% and 25% energy distribution.
05

Relate Charge and Energy in Capacitor

The energy stored in the capacitor is related to the charge by \[E_{C} = \frac{Q^{2}}{2C}.\]Solving for the charge when 25% of the energy is in the capacitor:\[0.25E = \frac{Q^{2}}{2C}.\]
06

Calculate Charge in Terms of Maximum Charge

Using the equation from Step 5 and knowing that the maximum energy is \[E = \frac{Q_{max}^{2}}{2C},\]we get:\[0.25 \times \frac{Q_{max}^{2}}{2C} = \frac{Q^{2}}{2C}.\]This simplifies to\[Q = Q_{max} \times \sqrt{0.25} = Q_{max} \times 0.5.\]
07

Relate Current and Energy in Inductor

The energy stored in the inductor in terms of the current is \[E_{L} = \frac{1}{2}L I^{2}.\]Given that this is 75% of the total energy:\[0.75E = \frac{1}{2}L I^{2}.\]
08

Calculate Current in Terms of Maximum Current

Using maximum energy for equality, where \[E = \frac{L I_{max}^{2}}{2},\]for the inductor's energy we have:\[0.75 \times \frac{L I_{max}^{2}}{2} = \frac{L I^{2}}{2}.\]Simplifying gives:\[I = I_{max} \times \sqrt{0.75} \approx I_{max} \times 0.866.\]

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Inductor Magnetic Field
In an LC circuit, the inductor plays a crucial role by creating a magnetic field as electric current flows through it. This magnetic field stores energy which can later be returned to the circuit. The inductor resists changes in current, converting excess current into a magnetic field and when the circuit demands, releasing this stored energy back as electrical current.
The energy in an inductor's magnetic field is described by the formula:
  • \[ E_{L} = \frac{1}{2} L I^{2} \]
This equation shows that the energy stored is dependent on two factors: the inductance \( L \) of the coil and the square of the current \( I \).
In a perfectly oscillating LC circuit, energy continuously transfers back and forth between the capacitor and inductor, turning electric energy into magnetic energy and vice versa.
When the problem states that 75% of the circuit's total energy is stored in the inductor, it implies that the magnetic field is quite strong at that moment. The current in the inductor is significant, which results in higher stored energy.
Capacitor Electric Field
A capacitor in an LC circuit is responsible for storing electrical energy in the form of an electric field between its plates. This energy charging efficiently facilitates the oscillation of energy with the inductor. As voltage builds across a capacitor, it stores energy according to the formula:
  • \[ E_{C} = \frac{1}{2} C V^{2} \]
  • Alternatively, energy can also be expressed in terms of charge \( Q \):\[ E_{C} = \frac{Q^{2}}{2C} \]
Here, \( V \) stands for voltage, \( C \) for capacitance, and \( Q \) represents the electric charge stored.
In our oscillating LC circuit scenario, if 25% of the circuit's total energy is held by the capacitor, it implies that the capacitor is only fractionally charged relative to its maximum potential. This means the charge on the capacitor would be a fraction of its maximum charge possible when the entire energy is stored in the capacitor's electric field.
Energy Distribution in Circuits
Energy distribution in an LC circuit signifies how the total energy oscillates between the electric fields in the capacitor and the magnetic fields in the inductor. In essence, the total energy in the circuit remains constant, shifting roles between the two components.
To understand energy distribution, consider the whole circuit's total energy as a fixed quantity:
  • \[ E = E_{L} + E_{C} \]
Where \( E_{L} \) is the energy in the inductor, and \( E_{C} \) is the energy in the capacitor.
During oscillation, if the energy is distributed such that 75% of it is stored in the inductor's magnetic field, the remaining 25% has to be stored in the capacitor's electric field. Each of these energies is dependent on their specific element parameters, such as current through an inductor and voltage across a capacitor.
For students, understanding this balancing act is pivotal in analyzing how oscillating circuits behave and in making predictions based on initial conditions and known formulas.

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Most popular questions from this chapter

An ac generator with emf \(\mathscr{E}=\mathscr{E}_{m} \sin \omega_{d} t\), where \(\mathscr{E}_{m}=\) \(25.0 \mathrm{~V}\) and \(\omega_{d}=377 \mathrm{rad} / \mathrm{s}\), is connected to a \(4.15 \mu \mathrm{F}\) capacitor. (a) What is the maximum value of the current? (b) When the current is a maximum, what is the emf of the generator? (c) When the emf of the generator is \(-12.5 \mathrm{~V}\) and increasing in magnitude, what is the current?

When under load and operating at an rms voltage of \(220 \mathrm{~V}, \mathrm{a}\) certain electric motor draws an rms current of \(3.00 \mathrm{~A}\). It has a resistance of \(24.0 \Omega\) and no capacitive reactance. What is its inductive reactance?

A coil of inductance \(88 \mathrm{mH}\) and unknown resistance and a \(0.94 \mu \mathrm{F}\) capacitor are connected in series with an alternating emf of frequency \(930 \mathrm{~Hz}\). If the phase constant between the applied voltage and the current is \(75^{\circ}\), what is the resistance of the coil?

A series \(R L C\) circuit has a resonant frequency of \(6.00 \mathrm{kHz}\). When it is driven at \(8.00 \mathrm{kHz}\), it has an impedance of \(1.00 \mathrm{k} \Omega\) and \(\mathrm{a}\) phase constant of \(45^{\circ}\). What are (a) \(R\), (b) \(L\), and (c) \(C\) for this circuit?

An oscillating \(L C\) circuit consisting of a \(1.0 \mathrm{nF}\) capacitor and a \(3.0 \mathrm{mH}\) coil has a maximum voltage of \(3.0 \mathrm{~V}\). What are (a) the maximum charge on the capacitor, (b) the maximum current through the circuit, and (c) the maximum energy stored in the magnetic field of the coil?

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