By the end of this section, you will be able to:
- List some uses of capacitors.
- Express in equation form the energy stored in a capacitor.
- Explain the function of a defibrillator.
The information presented in this section supports the following AP® learning objectives and science practices:
- 5.B.2.1 The student is able to calculate the expected behavior of a system using the object model (i.e., by ignoring changes in internal structure) to analyze a situation. Then, when the model fails, the student can justify the use of conservation of energy principles to calculate the change in internal energy due to changes in internal structure because the object is actually a system. (S.P. 1.4, 2.1)
- 5.B.3.1 The student is able to describe and make qualitative and/or quantitative predictions about everyday examples of systems with internal potential energy. (S.P. 2.2, 6.4, 7.2)
- 5.B.3.2 The student is able to make quantitative calculations of the internal potential energy of a system from a description or diagram of that system. (S.P. 1.4, 2.2)
- 5.B.3.3 The student is able to apply mathematical reasoning to create a description of the internal potential energy of a system from a description or diagram of the objects and interactions in that system. (S.P. 1.4, 2.2)
Most of us have seen dramatizations in which medical personnel use a defibrillator to pass an electric current through a patient’s heart to get it to beat normally. (Review Figure 19.29.) Often realistic in detail, the person applying the shock directs another person to “make it 400 joules this time.” The energy delivered by the defibrillator is stored in a capacitor and can be adjusted to fit the situation. SI units of joules are often employed. Less dramatic is the use of capacitors in microelectronics, such as certain handheld calculators, to supply energy when batteries are charged. (See Figure 19.29.) Capacitors are also used to supply energy for flash lamps on cameras.
Energy stored in a capacitor is electrical potential energy, and it is thus related to the charge and voltage on the capacitor. We must be careful when applying the equation for electrical potential energy to a capacitor. Remember that is the potential energy of a charge going through a voltage . But the capacitor starts with zero voltage and gradually comes up to its full voltage as it is charged. The first charge placed on a capacitor experiences a change in voltage , since the capacitor has zero voltage when uncharged. The final charge placed on a capacitor experiences , since the capacitor now has its full voltage on it. The average voltage on the capacitor during the charging process is , and so the average voltage experienced by the full charge is . Thus the energy stored in a capacitor, , is
where is the charge on a capacitor with a voltage applied. (Note that the energy is not , but .) Charge and voltage are related to the capacitance of a capacitor by , and so the expression for can be algebraically manipulated into three equivalent expressions:
where is the charge and the voltage on a capacitor . The energy is in joules for a charge in coulombs, voltage in volts, and capacitance in farads.
The energy stored in a capacitor can be expressed in three ways:
where is the charge, is the voltage, and is the capacitance of the capacitor. The energy is in joules for a charge in coulombs, voltage in volts, and capacitance in farads. Energy stored in the capacitor is internal potential energy.
Recall that we were able to calculate the stored potential energy of a configuration of point charges, and how the energy changed when the configuration changed in Applying the Science Practices: Work and Potential Energy in Point Charges. Since the charges in a capacitor are, ultimately, all point charges, we can do the same with capacitors. However, we write it down in terms of the macroscopic quantities of (total) charge, voltage, and capacitance; hence Equation (19.76).
For example, consider a parallel plate capacitor with a variable distance between the plates connected to a battery (fixed voltage). When you move the plates closer together, the voltage still doesn’t change. However, this increases the capacitance, and hence the internal energy stored in this system (the capacitor) increases. It turns out that the increase in capacitance for a fixed voltage results in an increased charge. The work you did moving the plates closer together ultimately went into moving more electrons from the positive plate to the negative plate.
In a defibrillator, the delivery of a large charge in a short burst to a set of paddles across a person’s chest can be a lifesaver. The person’s heart attack might have arisen from the onset of fast, irregular beating of the heart—cardiac or ventricular fibrillation. The application of a large shock of electrical energy can terminate the arrhythmia and allow the body’s pacemaker to resume normal patterns. Today it is common for ambulances to carry a defibrillator, which also uses an electrocardiogram to analyze the patient’s heartbeat pattern. Automated external defibrillators (AED) are found in many public places (Figure 19.30). These are designed to be used by lay persons. The device automatically diagnoses the patient’s heart condition and then applies the shock with appropriate energy and waveform. CPR is recommended in many cases before use of an AED.
A heart defibrillator delivers of energy by discharging a capacitor initially at . What is its capacitance?
We are given and , and we are asked to find the capacitance . Of the three expressions in the equation for , the most convenient relationship is
Solving this expression for and entering the given values yields
This is a fairly large, but manageable, capacitance at .