Capacitors and Dielectrics And you Recall that In

Capacitors and Dielectrics And you

Recall that: • In an electric field, as a point charge moves, the amount of EPE changes. We define the change in EPE/charge (on the point charge) as VOLTAGE • There are different voltages in different regions of the electric field around a point charge Q

New Stuff: • A voltage difference can also exist between 2 charged conductors that are near to each other but not touching. • This arrangement is called a capacitor and it has the capacity to store electric charges + + + -

If you also: • Place an insulating material (dielectric) between the 2 conductors, you increase the capacity of the charge that can be stored. • The magnitude of the charge on a plate (conductor) is given the symbol q • q α V that is to say: q = CV C = a constant known as capacitance

The energy stored • Estored = ½ q. V = ½ CV 2 = q 2/ 2 C • NO time for derivations on this one – you will spend a good deal of time on this if you take an electrical engineering class in college. . .

The Battery • A number of capacitors in sequence is known as a “battery” • Although the term battery is used in a more general sense now.

The Lyden Jar Demonstrations • 1) Simple circuit: Feel the flow of charges Flow of charges = current (I) = charge/time = Δq/Δt = 1 columb/ 1 sec = Ampere (amps) • DC = direct current (moves in same direction) • AC = alternating current (changes directions)

• 2) Circuit with fiberglass insulator: Resistance (R) = voltage/current = V/I = Ohm’s law • 1 volt/ampere = Ohm (Ω) • That is to say, lg. V with small current = high resistance • In a wire: R = ρ (L/A) • ρ = resistivity of the material (Ω meters) • L = length, A = cross sectional area

Power • • • Power = change in energy / time interval P= (Δq)V / Δt Recall that (Δq) / Δt = current (I) Therefore: P = I V (current Voltage) Units of power are the Watt (W)

Voltage • As stated earlier: V = ΔEPE/qo = Work/qo = FIId/qo = qo. E d/qo = E d • Also keep in mind that ΔEPE = ΔKE • SO: Eqod = ½ mv 2 = q. V • NOTE: I use little v for velocity and big V for voltage here.

Summary • (I) = current (in amperes) = flow of charges • (V) = Volts = energy per charge • (Ω) = Resistance (in Ohms) = resistance to flow of current • (P) = power (in Joules/s = Watt) = current x voltage = (I)(V) • Now lets use this stuff in a lab so you understand it better!!!!