Electrical Energy and Capacitance Capacitance

Capacitors and Charge Storage Capacitor – acts as a storehouse of charge and energy –Typically consists of two metal plates separated by a small distance Called a parallel plate capacitor –When connected to a battery, charge is transferred from one plate to another until the potential difference of the capacitor is equal to the potential difference of the battery The two plates will have equal and opposite charges

Capacitors and Charge Storage

Capacitance – the ability of a conductor to store energy in the form of electrically separated charges –Ratio of charge to potential difference –Measured in farads (F) Equivalent to a coulomb/volt (C/V) Named in honor of Michael Faraday who contributed greatly to our knowledge of electromagnetism Capacitors typically range from microfarads ( ) to picofarads ( ) in strength

Capacitors and Charge Storage Capacitance = (magnitude of charge) / (potential difference) C=Q/ Δ V Capacitance for a parallel-plate capacitor in a vacuum Capacitance = (permittivity of a vacuum) * (area of one of the plates) / (distance between the plates)

Capacitors and Charge Storage C=ε 0 * A / d ε (lowercase sigma) represents permittivity of a medium –The subscript 0 means that the medium is a vacuum Permittivity has a value of 8.85* C 2 /N*m 2 Capacitance decreases with increasing distance Capacitance increases with increasing size of the capacitor –Earth is so massive it is a excellent capacitor Used for grounding Can take a lot of charge without changing electric potential

Capacitors and Charge Storage The material between the plates can change the capacitance By inserting an insulating material called a dielectric (such as air, rubber, glass, or waxed paper) between the plates, the capacitance increases –Surface charges build up on the dielectric and reduce the charge on the plates More charge can be stored Charge is rapidly released when discharged by a capacitor –Discharged by connecting the plates with a conductor

Capacitors and Charge Storage

Energy and Capacitors Electrical potential energy stored in a charged capacitor Electric potential energy = ½ * (charge on one plate) * (final potential difference) PE electric = ½Q Δ V The greater the charge on the plates, the more work and therefore energy that is needed to move charges between the plates Other forms of the equation PE electric = ½C Δ V 2 PE electric = Q 2 / 2C Capacitors have a maximum energy and charge they can store –Exceeding this maximum causes electrical breakdowns

Energy and Capacitors