Chapter 17 Section 17.2 Capacitance PowerPoint Explanation PDF

CAPACITANCE Grade 12 A capacitor is a devise that is used to store electrical potential energy. Uses: in tuning the frequency of radio receivers, sparking in car’s ignition systems. A simple design for a capacitor consists of two parallel metal plates separated by a small distance. This type of ca...

CAPACITANCE Grade 12 A capacitor is a devise that is used to store electrical potential energy. Uses: in tuning the frequency of radio receivers, sparking in car’s ignition systems. A simple design for a capacitor consists of two parallel metal plates separated by a small distance. This type of capacitor is called a parallel-plate capacitor. The other type is a sphere capacitor. The capacitor is energized (charged) by connecting the plates to the two terminals of a battery. Charge transfer between the plates stops when the potential difference between the plates is equal to the potential difference between the terminals of the battery. The ability of a conductor to store energy in the form of electrically separated charges is called the capacitance.  !!! Capacitance C= C-capacitance, Farad (F) q-electric charge, Coulomb (C) ΔV-potential difference, Volt (V)  !!! Capacitance C= C-capacitance, Farad (F) [C]=1=1=1Farad=1F q-electric charge, Coulomb (C) ΔV-potential difference, Volt (V)  !!! The SI unit for capacitance is [C]=1=1=1Farad=1F  !!!Capacitance of the parallel-plated capacitor ε₀=8.85 x 10 ̄¹² , the permittivity of the vacuum A-area of the plates (m²) d-distance between plates (m)  Permittivity reflects the ability of a medium to store a charge in an electric field.  !!!Capacitance of the sphere capacitor Csphere= R R-radius of the sphere (m) The equation indicates that the capacitance of a sphere increases as the size of the sphere increases. Because Earth is so large, it has an extremely large capacitance. Earth can provide or accept a large amount of charge. How to increase capacitance  How to increase capacitance  increase area of plates Or  How to increase capacitance  increase area of plates Or  decrease the distance between plates  How to increase capacitance We have assumed that the space between the plates of a parallel- plate capacitor is a vacuum. However, in many parallel-plate capacitors, the space is filled with a material called a dielectric. A dielectric is an insulating material, such as air, rubber, glass, or waxed paper. A dielectric (air, rubber, glass) inserted between the plates of a capacitor, increases the capacitance. The molecules in a dielectric can align with the applied electric field, causing an excess negative charge near the surface of the dielectric at the positive plate and an excess positive charge near the surface of the dielectric at the negative plate. A dielectric reduces strength of electric field (E=). A dielectric reduces strength of electric field (E=). Use this formula (C=) to prove that capacitance increases. A dielectric reduces strength of electric field (E=), it means increases the charge that can be stored on a capacitor and capacitance (C=) increases.  !!!Capacitance for the parallel-plate capacitor with a dielectric In the vacuum: C= With a dielectric between plate: C= ε0-permittivity of the vacuum k-the dielectric constant (”kappa”) (must be taken from the reference data) A material with high dielectric constant polarizes more in response to an applied electric field than a material with low permittivity, thereby storing more energy in the material. Capacitors in flash attachment of a camera A battery is used to charge the capacitor, and this stored charge is then released when the shutter- release button is pressed to take a picture. One advantage of using a discharging capacitor instead of a battery to power a flash is that with a capacitor, the stored charge can be delivered to a flash tube much faster, illuminating the subject at the instant more light is needed. Capacitors in keyboards Each key is connected to a movable plate, which represents one side of the capacitor. The fixed plate on the bottom of the keyboard represents the other side of the capacitor. When a key is pressed, the capacitor spacing decreases, causing an increase in capacitance. External electronic circuits recognize that a key has been pressed when its capacitance changes.  !!! PE electric=  !!! PE electric= PE electric= In practice, there is a limit to the maximum energy (or charge) that can be stored because electrical breakdown ultimately occurs between the plates of the capacitor for a sufficiently large potential difference. So, capacitors are usually labelled with a maximum operating potential difference. Electrical breakdown in a capacitor is like a lightning discharge in the atmosphere. 1. What happens to the charge on a parallel-plate capacitor if the potential difference doubles? q=C x ΔV 1. The charge on each plate doubles. 2. You want to increase the maximum potential difference of a parallel-plate capacitor. Describe how you can do this for a fixed area of the plates and plate separation. C= 2. Use a dielectric between the capacitor plates. 3. Why is Earth considered a “ground” in electric terms? Can any other object act as a ground? 3. The Earth is large enough that it can accept or supply an unlimited number of charges; Any object with this ability can act as a ground. 4. If the potential difference across a capacitor is doubled, by what factor is the electrical potential energy stored in the capacitor multiplied? PE electric= 4. PEelectric is 4 times as great. PE electric= The capacitance is constant characteristic for the certain capacitor. 5. If you were asked to design a small capacitor with high capacitance, what factors would be important in your design? C= 5. The plates should be close together and have a dielectric between them.