Chapter 20 Static Electricity - + Charge by Conduction

Chapter 20 Static Electricity Charge by Induction

Chapter 21 Electric Fields Electric Field A property of space around a charged object that causes forces on other charged objects. Vector quantity It has both direction and magnitude The direction of the force is away from the positive and towards the negative. The electric field is the strongest when the lines are close together Field lines do not exist-only a pictorial guide.

Chapter 21 Electric Fields + -

Van de Graaf Generator In the Van de Graaf generator, charge is transferred onto a moving belt A, and then onto the metal dome B, An electric motor does the work needed to increase the electric potential energy. Chapter 21 Electric Fields

Van de Graaf Generator Chapter 21 Electric Fields

Electric Field Intensity E Electric field intensity (N/C) F Force (Newtons) q’ Test Charge (Coulombs) Similar to Gravitational Field Intensity g Gravitational field intensity (N/kg) F Force (Newtons) m mass (kg)

Chapter 21 Electric Fields g Increase in gravitational potential energy Increase in electric potential energy E

Chapter 21 Electric Fields The electric potential difference (  V) is the work done in moving a test charge in an electric field divided by the magnitude of the test charge. Electric potential difference is measured in joules per coulomb. One joule per coulomb is a volt.

_ _ + + HIGH V LOW V Chapter 21 Electric Fields

+ HIGH V LOW V Chapter 21 Electric Fields ++ +

Electric potential difference in a uniform field And E = F/q so

Chapter 21 Electric Fields A force of.032 N is required to move a charge of 4.2 x C in an electric field between two points that are 25 cm apart. What potential difference exists between the points? V = 190 volts

Chapter 21 Electric Fields An electron is accelerated by a machine that subjects it to a potential difference of 50 Megavolts. What energy has the electron acquired? W = (50 x 10 6 V)(1.6 x C) = 8 x J

Chapter 21 Electric Fields Electric current The flow of electrons Electric current can be maintained only if the electrons are returned to areas of high electron concentration ++ _ _ _ _ pump _ _ _ _ _ _ _

Chapter 21 Electric Fields Millikan’s oil drop experiment Early 1900s – Determined electric charge F1F1 F2F2 When the forces are balanced, F 1 = F 2 Eq = mg so q = mg/E Found that the charge is quantified Multiples of 1.6 x C

Chapter 21 Electric Fields F1F1 F2F2 An oil drop has a mass of 1.9 x kg and is suspended in an electric field with intensity of 6000 N/C. Find the charge on the drop and the number of excess electrons. F 1 = F 2 so Eq = mg = 3.1 x C one extra electron

All systems are in equilibrium when the energy of the system is a minimum. The ball comes to rest when the potential energy is the least. It is the greatest at A and the least at B. A B Chapter 21 Electric Fields

- - - The potential of A decreases and the potential of B increases and both are at the same potential A charged sphere B neutral sphere A is the charged sphere with high potential energy. B is neutral with zero potential energy Chapter 21 Electric Fields

What happens with a large sphere and a small sphere? Low V High V same q High q Low q same V Chapter 21 Electric Fields

Capacitor A device that stores a charge As charge is added, the potential of the body increases. For a given charge, the ratio of the charge to the potential q/V is a constant. Capacitance is the ability to store a charge. C Capacitance (farads) q Charge (Coulombs) V Potential (Volts) Chapter 21 Electric Fields

A 3 x 10 2 pF capacitor has a potential difference of 30 volts across it. What is the charge on the capacitor? C = 3 x 10 2 pF = 3 x F q = V = 30 volts q = CV = (3 x F)(30V) = 9 x C Chapter 21 Electric Fields