ELECTRIC POTENTIAL ENERGY and the ELECTRIC POTENTIAL

Electric Fields - exist in space surrounding a charged particle or object. Fields are represented by lines of force that start on a positive charge and end on a negative charge. electric field (E) E = F/q (fundamental definition) electric field (E) due to a point charge E = kQ/r 2 If any charge experiences an electric force at a certain point in space, it is in the presence of an electric field. E-field is a vector quantity. When a test charge is in the vicinity of more than one charged object, the E-field at that position is the vector sum of each individual field. ELECTRIC FIELD

ELECTRIC POTENTIAL ENERGY A charge has an electric potential energy associated with the electric force, just as a mass has gravitational potential energy due to the gravitational force For a pair of charges: In a uniform E-field: The negative sign means energy increases for a (-) charge and decreases for a (+) charge.

Electric potential : at a point (in a field) equals the potential energy per unit charge at that point. V = PE / q(fundamental definition) Potential (Voltage) can be determined from a point charge or in a uniform field V = kQ/r(due to a point charge) If more than one point charge is present, the potential at a certain point in space is equal to the sum of each potential. (Electric potential is a scalar quantity.) V = E d(uniform field – produced by 2 plates that are a certain distance apart.) ELECTRIC POTENTIAL

Like mechanical potential energy, the difference in voltagemechanical potential energy (electric potential), is the quantity which is meaningful. The difference in voltage measured when moving from point A to point B is equal to the work which would have to be done,work on the charge, against the electric field to move the chargeelectric field from A to B. V = V f - V i And often the initial point (V i ) is chosen to be zero (as if the charge coming into the field started out “infinitely” far away) V = V f – 0

Electric potential energy: a measure of the work done to move a charge from point A to point B through a certain potential difference. Work = PE = qV Work is positive and the change in energy is positive Work is negative and the change in energy is negative.

A capacitor stores electric charge and consists of two conductors separated by a substance (insulator) known as a dielectric. The ability of a capacitor to store charge is called capacitance (C) and is found by: C = Q / V The SI unit of capacitance is the farad (F). (Typical capacitors have values which range from 1 pF to 1 F.) The capacitance depends on the physical build of the capacitor (and the dielectric material). For parallel plate capacitors, capacitance is: C =  A / d Where  is a constant = 8.85 x C 2 /Nm 2 CAPACITANCE

A charged capacitor stores energy. The electric energy stored when a neutral capacitor is charged with an amount of charge – Q – from a potential (battery) of V is: energy (PE) = ½ QV Combining this expression with the definition of capacitance gives alternate expressions for the energy stored: PE = ½ QV = ½ CV 2 = ½ Q 2 /C CAPACITANCE