Transcript Electricity
Rate Processes - Part 2 Electricity In a conductor, a voltage difference forces electrons to jump from atom to atom Pushing an electron in, forces another electron out. That electron is forced into the next atom, which then has to kick one out, and so on… The flow rate of charge is called current See next slide. Electricity Vin e- Valence Electrons e- e- e- e- Vout Nucleus Electricity Analogy 1: Peas in a straw The addition of one pea causes a pea to leave at the other end. Although the peas can move through the straw slowly, the effect of adding another pea is felt immediately. Electricity Analogy 2: Gas Flow through Bed of Sand Pin Gas flow heat Pout Work Blower Vin Sand Bed Electron flow heat Generator Resistor Vout Work Electricity x Vin Electron flow A Vout Work Generator voltage charge current Vin Vout q i V J electron At A x x Electrical conductivity Electrical Conductivity The proportionality constant for electron flux is called the electrical conductivity. If material has a high electrical conductivity, then the charge flows ‘easily’ for a given V. This material is called a conductor, and makes good wires. For a material with the same V, if is small, the flow is more difficult. This material is called an insulator, and makes a good shield around the wires. Pair Exercise 1 For the SI system, what are the units of ? Pair Exercise 2 With V = 500 V, how much current (A) flows through a 0.005-m diameter aluminum wire that is 100 km long ? Pairs Exercise 3 A charge of 0.75 coulombs passes through a copper wire every 15 seconds. What is the current? If that wire is 2.00 km long and has a diameter of 1.00 mm, what is the potential difference between the two ends? Diffusion Species A x Salt Water Cin A Cout Fresh Water Concentration Mass (or moles) Cin Cout MA C JA At x x Diffusivity Pairs Exercise 4 In the SI system, what are the units of diffusivity? Pairs Exercise 5 A 1.00-km long, 0.25-m diameter pipe connects a freshwater lake to the ocean. The pipe has been abandoned for many years. At what rate (kg/yr) does salt (NaCl) diffuse from the ocean to the lake? Seawater has a salt concentration of 35 kg/m3. Assume the average temperature has been 291 K. Resistance Separation Distance Driving Force D x Resistance R Flow Rate r AK Cross-sectional Area Heat Fluid Flow Electricity Diffusion Rheat x Ak 8 x R fluid A2 x Relectron A x RA A Proportionality Constant Electrical Resistance Most commonly, “resistance” implies electrical resistance x V Relectron A i Voltage Difference Ohms Current Ohm’s Law: V=iR coulombs/s Pairs Exercise 6 Two batteries are connected in series (V=1.5 V each) and connected to a circuit of resistors. An ammeter measures the current to be 1.5 mA. What is the resistance in the circuit? + V - i Current black box Resistances in Series R1 R2 R n R Ri R1 R2 i 1 Resistances in Parallel R1 R2 R n 1 1 1 1 R i 1 Ri R1 R2 Pairs Exercise 7 Calculate the equivalent resistance when a 5- and a 10- resistor are connected in a. series b. parallel Summary Thermodynamics tells us where a process is going, and rate processes tell us how long it takes to get there. Regardless of whether we are talking about heat, fluids, electrons, or chemical species: flow rate and flux are derived from the same basic equation, everything flows ‘downhill’ from a higher to a lower potential; only terminology changes.