Two are electrodes. The third is an electrolyte. This is a gooey paste or liquid that fills the gap between the electrodes. The electrolyte can be made from a variety of substances. But whatever its recipe, that substance must be able to conduct ions — charged atoms or molecules — without allowing electrons to pass. That forces electrons to leave the battery via terminals that connect the electrodes to a circuit.
This keeps chemical reactions from taking place on the electrodes. That, in turn, enables energy to be stored until it is needed.
In those reactions, neutral metal atoms give up one or more electrons. That turns them into positively charged atoms, or ions. Electrons flow out of the battery to do their work in the circuit. Meanwhile, the metal ions flow through the electrolyte to the positive electrode, called a cathode KATH-ode. At the cathode, metal ions gain electrons as they flow back into the battery. This allows the metal ions to become electrically neutral uncharged atoms once again.
The anode and cathode are usually made of different materials. Typically, the cathode contains a material that gives up electrons very easily, such as lithium. Graphite, a form of carbon, holds onto electrons very strongly.
This makes it a good material for a cathode. As smaller and smaller products have evolved, engineers have sought to make smaller, yet still powerful batteries. And that has meant packing more energy into smaller spaces. One measure of this trend is energy density. A battery with high energy density helps to make electronic devices lighter and easier to carry. It also helps them last longer on a single charge. In some cases, however, high energy density can also make devices more dangerous.
News reports have highlighted a few examples. Some smartphones, for instance, have caught fire. On occasion, electronic cigarettes have blown up. Figure 2. Automated external defibrillators are found in many public places. These portable units provide verbal instructions for use in the important first few minutes for a person suffering a cardiac attack. A heart defibrillator delivers 4. What is its capacitance?
We are given E cap and V , and we are asked to find the capacitance C. The size of the capacitor would be enormous; c It is unreasonable to assume that a capacitor can store the amount of energy needed. Skip to main content. Electric Potential and Electric Field. Search for:. Energy Stored in Capacitors Learning Objectives By the end of this section, you will be able to: List some uses of capacitors. We just have to divide by the volume of space between its plates and take into account that for a parallel-plate capacitor, we have and.
Therefore, we obtain. We see that this expression for the density of energy stored in a parallel-plate capacitor is in accordance with the general relation expressed in Equation 4. We could repeat this calculation for either a spherical capacitor or a cylindrical capacitor—or other capacitors—and in all cases, we would end up with the general relation given by Equation 4.
Calculate the energy stored in the capacitor network in Figure 4. We use Equation 4. The total energy is the sum of all these energies. We identify and , and , and. The energies stored in these capacitors are. We can verify this result by calculating the energy stored in the single capacitor, which is found to be equivalent to the entire network.
The voltage across the network is. The total energy obtained in this way agrees with our previously obtained result,. The potential difference across a capacitor is. By what factor is the stored energy increased? In a cardiac emergency, a portable electronic device known as an automated external defibrillator AED can be a lifesaver.
A defibrillator Figure 4. Michael G. Mar 21, Explanation: Capacitors consists of two plates. Related questions Why is capacitance important? Is capacitance constant? When the voltage across a capacitor is tripled, what happens to the stored? When the plate area of a capacitor increases, what happens to the capacitance?
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