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Learning objectives

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 [link] .) 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 [link] .) Capacitors are also used to supply energy for flash lamps on cameras.

In an electronic calculator circuit the memory is preserved using large capacitors which store energy when the batteries are charged.
Energy stored in the large capacitor is used to preserve the memory of an electronic calculator when its batteries are charged. (credit: Kucharek, Wikimedia Commons)

Energy stored in a capacitor is electrical potential energy, and it is thus related to the charge Q size 12{Q} {} and voltage V size 12{V} {} on the capacitor. We must be careful when applying the equation for electrical potential energy Δ PE = q Δ V size 12{?"PE"=q?V} {} to a capacitor. Remember that Δ PE size 12{?"PE"} {} is the potential energy of a charge q size 12{q} {} going through a voltage Δ V size 12{?V} {} . 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 Δ V = 0 size 12{?V=0} {} , since the capacitor has zero voltage when uncharged. The final charge placed on a capacitor experiences Δ V = V size 12{?V=V} {} , since the capacitor now has its full voltage V size 12{V} {} on it. The average voltage on the capacitor during the charging process is V / 2 size 12{V/2} {} , and so the average voltage experienced by the full charge q size 12{q} {} is V / 2 size 12{V/2} {} . Thus the energy stored in a capacitor, E cap size 12{E rSub { size 8{"cap"} } } {} , is

E cap = Q V 2 , size 12{E rSub { size 8{"cap"} } =Q { {V} over {2} } } {}

where Q size 12{Q} {} is the charge on a capacitor with a voltage V size 12{V} {} applied. (Note that the energy is not QV size 12{ ital "QV"} {} , but QV / 2 size 12{ ital "QV"/2} {} .) Charge and voltage are related to the capacitance C of a capacitor by Q = CV size 12{Q= ital "CV"} {} , and so the expression for E cap size 12{E rSub { size 8{"cap"} } } {} can be algebraically manipulated into three equivalent expressions:

Questions & Answers

what is angular velocity
Obaapa Reply
Why does earth exert only a tiny downward pull?
Mya Reply
hello
Islam
Why is light bright?
Abraham Reply
what is radioactive element
Attah Reply
an 8.0 capacitor is connected by to the terminals of 60Hz whoes rms voltage is 150v. a.find the capacity reactance and rms to the circuit
Aisha Reply
thanks so much. i undersooth well
Valdes Reply
what is physics
Nwafor Reply
is the study of matter in relation to energy
Kintu
a submersible pump is dropped a borehole and hits the level of water at the bottom of the borehole 5 seconds later.determine the level of water in the borehole
Obrian Reply
what is power?
aron Reply
power P = Work done per second W/ t. It means the more power, the stronger machine
Sphere
e.g. heart Uses 2 W per beat.
Rohit
A spherica, concave shaving mirror has a radius of curvature of 32 cm .what is the magnification of a persons face. when it is 12cm to the left of the vertex of the mirror
Alona Reply
did you solve?
Shii
1.75cm
Ridwan
my name is Abu m.konnek I am a student of a electrical engineer and I want you to help me
Abu
the magnification k = f/(f-d) with focus f = R/2 =16 cm; d =12 cm k = 16/4 =4
Sphere
what do we call velocity
Kings
A weather vane is some sort of directional arrow parallel to the ground that may rotate freely in a horizontal plane. A typical weather vane has a large cross-sectional area perpendicular to the direction the arrow is pointing, like a “One Way” street sign. The purpose of the weather vane is to indicate the direction of the wind. As wind blows pa
Kavita Reply
hi
Godfred
what about the wind vane
Godfred
If a prism is fully imersed in water then the ray of light will normally dispersed or their is any difference?
Anurag Reply
the same behavior thru the prism out or in water bud abbot
Ju
If this will experimented with a hollow(vaccum) prism in water then what will be result ?
Anurag
What was the previous far point of a patient who had laser correction that reduced the power of her eye by 7.00 D, producing a normal distant vision power of 50.0 D for her?
Jaydie Reply
What is the far point of a person whose eyes have a relaxed power of 50.5 D?
Jaydie
What is the far point of a person whose eyes have a relaxed power of 50.5 D?
Jaydie
A young woman with normal distant vision has a 10.0% ability to accommodate (that is, increase) the power of her eyes. What is the closest object she can see clearly?
Jaydie
29/20 ? maybes
Ju
In what ways does physics affect the society both positively or negatively
Princewill Reply
how can I read physics...am finding it difficult to understand...pls help
rerry Reply
try to read several books on phy don't just rely one. some authors explain better than other.
Ju
And don't forget to check out YouTube videos on the subject. Videos offer a different visual way to learn easier.
Ju
hope that helps
Ju
Practice Key Terms 1

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Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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