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UNC-Chapel Hill PHYS 115 - Final Exam Study Guide

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PHYS 115 1st editionExam # 4 Study GuideLecture 1 - What are the formulas for work, kinetic energy, potential energy, and total energy?o W = Fdcos(theta)o K = ½ mv2o Ugrav = mgyo Uspring = ½ kx2o E = K + U- Describe forces in relation to the vector.o Opposite to vector = negativeo Perpendicular to vector = zeroLecture 2- Are gases fluids?o Yes.- What does incompressible mean?o Molecules can get as close to water as possible.- What is density? Does it change?o Ratio of mass to volume.o The density of a particular substance, ie. Water does not. - Describe pressure.o Force over areao Increases over a smaller area if the same force is applied.- What is the formula for change in pressure?o Density x gravity x depth- What direction does pressure come from?o All directionso Walls of the container, fluid below, above, and beside.- What is the formula for pressure at a given depth?o- How does pressure across a horizontal line compare at different points?o It is the same in one fluid.o It only increases with depth.- What is Pascal’s Principle?o Change the pressure at one point; all points experience a pressure change.- What is Gauge Pressure?o Is __ in excess of atmospheric pressureo 10 psi of gauge pressure is actually 25 psi in a basketball.Lecture 3- What is the Volume flow rate Q?o Aka “flux”o The rate at which a volume of fluid flows through a tubeo- What is the Equation of Continuity?o- What are the properties of an ideal fluid?o incompressible (i.e., density is constant)o flow is steady (i.e., flow is laminar not turbulent)o nonviscous (i.e., the fluid has no resistance to flow)- The aorta has a cross-sectional area of 3.0 cm2. Blood moves through the aorta at approximately30 cm/s. A capillary has a cross-sectional area of 2.8 x 10-7 cm2. Blood moves through a capillary at approximately 0.05 cm/s. Compare the speeds.o The aorta’s cross-sectional area must be greater than the sum of all the cross-sectional areas of all the capillaries in the body.o The speed of the capillaries is slower than the aorta, so the cross sectional area must be greater to keep the two volume flow rates equivalent.o The speed of the capillaries is slower than the aorta, so the cross sectional area must be greater to keep the two volume flow rates equivalent.Lecture 4- Describe viscosity (η).o Resistance to flowo Increase in viscosity, slower the flowo Units of Pa*so Units of centipoise (10-3 Pa*s)o Requires a change in pressure to overcome drag- Does viscosity have a single speed?o No, the speed increases away from the wall.o Friction is created from the wall.- How do you measure speed in a viscous fluid?o Finding the average speed (Q)o L decreases the flow rate because a longer tube has more frctionLecture 5- What is a Reynolds number?o The ratio of inertial to drag forces- Viscosity increases, drag __.o Increases - Size of object increases, drag __.o Increases- Speed increases, drag __.o Increases- Drag force of a fluid of viscosity η on a sphere of diameter d moving at slow speed v:o- What is inertial force?o The force needed to accelerateo What you exerto The harder to change motion, the more inertia- Which is harder to change motion? Fast or slow?o Fast- Increase velocity, __ inertia.o Increase - Increase density, __ inertia.o Increase - Describe Reynolds number.oo Where n =hr- When Re is small, __ forces dominate.o Viscous- When RE is large, __ forces dominate.o InertialEXAM 2 MATERIAL- Coulomb’s Lawoo K = 9 *109 N-m2/C2o r12 = distance between chargeso This is the force exerted by q1 on q1- Conductors and Insulatorso Inside a conductor, the electric field is zero, otherwise the molecules would move around; charges can’t move moleculeso Electric field is the same along plateso- Electric potential and potential energyo A region has a uniform electric field E pointing to the right (produced by two oppositely-charged plates, not shown) o A positive charge +q is placed in the field and is initially at rest. o The magnitude of the force Feq is proportional to the magnitude of the electric field E and to the magnitude of the charge q: FEq = qE o The force causes the charge to undergo a displacement Δx.o It is useful to describe the change in electric potential energy in a way that does not depend on the size or sign of the charge q.o We therefore define the electric potentialo As for potential energy, only changes in the electric potential ΔV are meaningful and the configuration to which we assign V = 0 is arbitrary. o The units of electric potential are energy/charge = J/Co If we move a charge q through a potential difference ΔV the electric potential energy of the system changes by ΔU = q ΔV.o The most common way of producing a potential difference is to use a battery.o A battery produces an electric potential difference ΔV between its two terminals.o An electric potential difference is also called a voltage and is measured in volts (1 volt = 1V = 1 J/C)o The positive terminal is at a value of the electric potential that is higher than the value atthe negative terminalo The potential difference is produced by separation of charge inside the battery.o If the electric field is uniform (same everywhere), the change in electric potential from the initial to the final position of the charge is o The difference in the electric potential between two locations is the work per unit charge needed to move a charge between the two locations.o The value of the electric potential decreases in the direction in which the electric field points. - Direction of the electric fieldoThe value of the electric potential energy decreases as the positive charge moves in the direction in which the electric field pointsoIf change in electric potential energy is negative, the value is negative. The electric field points in the decreasing.- Electrical currentso You can’t see these- Proportionalityo Brightness is an indicator of flow- Resistanceo Adding more bulbs increases resistanceo Resistance increases, current decreaseso Third rule of our modelo When a bulb is added in series to a circuit, the total resistance of the circuit increases, and the current through the battery increases. o If a bulb which is connected in series is removed from a circuit, the total resistance of the circuit decreases, and the current through the battery increases.- Kirchhoff’s Junction Lawo The splitting and combining of current in a parallel circuit is a consequence of conservation of charge.o Kirchhoff’s Junction


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