Download Lab Experiment on Electric Charge Storage and Flow in a Capacitor - Prof. Curtis W. Foltz and more Lab Reports Physics in PDF only on Docsity! Phy.204 Lab 2 : Electric Charge Storage & Flow, E & Voltage Fall`09 PURPOSE To show that electric charge can be stored on a conductor’s surface and can move through wires; to verify that the electric potential at a conductor’s surface follows the excess charge stored in it; to notice that electric current only flows thru a wire while an Electric field in that wire pushes it; to measure the current as it flows onto a capacitor plate, and later flows off that plate, so we can a) see whether the quantity of charge which entered it is equal to quantity of charge which left it, b) see if the positive charge accumulates on one plate, at the same rate as the negative does on its, c) compute an experimentally-measured value for the capacitance of the device, and finally d) estimate the effective Area of the capacitor plates, to validate C A do with a real example. BACKGROUND Suppose the Electric potential of a Power Supply “+” jack is +5V, while the potential of a nearby capacitor plate is zero. This occurs if there’s excess positive charge at the power supply (V~kQ/r) but zero excess charge on the metal plate. This potential difference must be accompanied by an Electric Field, since dEV . A wire between the plate and the P.S. jack would be immersed in that Electric field; many free electrons (–) would move toward the P.S.(+) end, since EqF . If the wire actually touched the P.S. (+) jack, all those excess electrons would be sucked into it, flowing inward as electron current; we’ll usually use engineering convention which pretends that mobile positive charges leave the P.S.(+) jack, flowing as conventional current thru the wire onto the capacitor plate. In either description, the Indicated Current ( vqI ) is out from the (+) jack. As (+) charge accumulates on the top plate, the Electric field strengthens ; it is pointing outward from +Q, according to insideout QkAE 4 ; it points inward toward the –Q that accumulates on the bottom plate. Eventually, the potential everywhere on each plate equals the potential at each P.S. jack ; then, the E-field can’t move charge anymore. Current can be measured as it flows onto a capacitor plate ; the charge on the plate changes by ΔQ = Iavg Δt while the current flows. We’ll use big capacitors, and will make the charges go thru a narrow tungsten filament to get to the plate – that way it will take seconds (rather than picoseconds) to “fill” the capacitor. EXPERIMENT A. Inspect your capacitor ; one of its two “leads” must be kept at lower potential than the other (labeled “–“, or the other is labeled “+”)… connect it to the Power Supply ground (black) jack with black wire. Connect the capacitor (+) lead to the (–, black) jack of the ammeter, connect the ammeter’s “1 Amp” red (+) jack to the lamp socket; connect the other side of the lamp socket to the P.S. (+ , red) jack with a red wire IF the PS is OFF. Remove the red-wire plug from the red PS jack and insert into the back of the black plug that is in the P.S. ground jack). Turn Voltage knobs down, switch ON the P.S.; set the Current-limit knobs very high – adjust Voltage to 4 Volt (is that okay for your capacitor? ____ ok for lamp bulb? ____ ). Put Voltmeter across capacitor.
