Electrostatics Forces and Fields The study of stationary charges and their interactions

Electric Charge Two types of charge – “positive” and “negative”. Charge is measured in “coulombs” - C “e” – “elementary charge” – the amount of charge on a proton or electron e = 1.602 x 10-19 C (+ for proton;- for electron) “q” or “Q” is the variable used for charge

Law of Charges - Opposite charges attract, like charges repel Law of Conservation of Charge – charge cannot be created or destroyed only transferred. So the net amount of charge produced in any process is zero. Conductors allow electrons to move freely, Insulators do not!

Methods of Charging Charging by friction scrapes electrons off one object and places them on the other Charging by induction requires no contact. This works only with conductors. Charging by contact requires a physical connection (insulators and conductors)

Polarization Polarization: a “separation of charge” is induced in an insulator by the influence of a charged object

Electroscopes – a device to detect charge

“Sharing” charge - charged objects will transfer charge to come to a better equilibrium If two charged conductors are connected (such as by a conducting wire) they will share charge to achieve a more balanced (shared) charge. -8mC 6mC 1mC 1mC

Electric Forces – Coulomb’s Law +q1 F + r Coulomb’s constant k = 9 x 109 Nm2/C2 F + +q2

Force interactions – “superposition” Linear – charges on an axis Find Fnet on q3 q1 q2 q3 30 cm 20 cm - + - -8.0mC +3.0mC -4.0mC F3,1 = 1.2N Fnet = -F32 + F31 = -2.7N + 1.2 N = -1.5N F3,2 = 2.7N The Fnet is 1.5N and points to the left (Note: found magnitude only)

Right angles Find Fnet on q3 at origin F31 = 140N F32 = 330N Use Pythagorean theorem and tan-1 to find net force and direction y F32 F31 + 52cm - q3 = 65mC q3 = -86mC 30cm + q2 = 50mC

Superposition – the net force of a system of charges b

Electric Fields ( N/C) Electric fields - vectors Electric fields - drawn always in a direction that is away from “+” and toward “-”. Definition: force per unit charge

A charge, Q, produces a field in space A charge, Q, produces a field in space. This exerts a force (N) on a small “test” charge “q” placed in the field. Note: Q can be either + or – but the small q is always + q +Q r The E-field can also be calculated using the strength of the charge “Q” and the distance to a particular point in the field.

E-Field lines Field lines drawn away from positive and toward negative Field lines do not cross Number of lines is proportional to charge strength

Interaction of field lines between charges of equal magnitude and opposite sign

Electric Potential (Volts) Energy per unit charge at a given point due to an Electric Field. 1 volt = 1 joule per coulomb (1 V=1 J/C) Potential Difference is required to make current flow.

Work in an Electric Field “W” is the work required to move a charge through a potential difference in joules Remember – work is equal to the change in PE “q” is the magnitude of the charge in Coulombs “V” is the potential difference in volts

Capacitors A device used to store charge Capacitance is measured in “farads” “C” is capacitance in farads “Q” is stored charge in Coulombs “V” is potential difference in volts most common type is parallel plate capacitor.

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: PE = ½ QV = ½ CV2 = ½ Q2/C