Transcript Ch. 4.2
Ch. 4.2 Resistance in Fluid Systems Fluid Resistance When a solid object moves through a fluid, there is a force that opposes the motion of the solid. This force is called drag. There are two ways in which drag can affect the flow of the fluid. These are called laminar or streamlined flow and turbulent flow. Drag is caused by friction between successive layers of fluid. This is called frictional drag. Laminar Flow Laminar, or streamlined flow is a slow, smooth flow over a surface. River water flowing over smooth rocks would be an example. Turbulent Flow Turbulent flow is irregular flow with eddies and whorls causing the fluid to move in different directions. Rapids on a river would be an example. Viscosity Fluids flow at different rates due to their density. Denser fluid tend to resist flowing. The resistance is caused by internal friction between the atoms and molecules in the fluid. This resistance to flow is called viscosity. Fluids that flow slowly, like syrup, have a high viscosity while fluids that flow easily, like water, have a low viscosity. Terminal Velocity The drag on an object caused by air resistance increases with speed. As an object falls through the air, it accelerates until air resistance balances the downward pull of gravity. At that point, the object stops accelerating and falls at a constant speed referred to as the terminal velocity of the object. The terminal velocity is the highest speed a falling object will reach. Ch. 4.3 Resistance in Electrical Systems Conductors & Insulators Electrons can flow through certain materials. These materials, which allow electrons to flow through them, are called conductors. Most metals are good conductors and allow electrons to pass through them easily. Insulators are materials that do not allow electrons to pass through them easily. Plastic, wood, rubber and glass make good insulators. Electrical Resistance is a conductor’s resistance to the flow of electrons. Semiconductors Semiconductors are those materials that are usually insulators, but under certain conditions, can be made to conduct an electric current. Silicon is a common semiconductor used in computers. Ohm’s Law Ohm’s Law states that the current in a circuit equals the voltage difference divided by the resistance. I = V/R I (current) = V(voltage)/R(resistance) Units: Current – amperes (A) Voltage – volts (V) Resistance – ohms () Ohm’s Law ( Sample) An air conditioner uses 18 A of current when plugged into a wall socket that operates at a voltage of 210 V. What is the resistance in the air conditioner? Step 1 – Write the Formula R = V/I Step 2 – Plug in numbers and units (No Naked Numbers!) = 210 V/18 A Step 3 – Do correct mathematical operation. = 11.67 Ohm’s Law ( Sample) A miniature light bulb with a resistance of 3 ohms is connected to a 6-volt source. How much current will flow through the bulb? Step 1 – Write the Formula I = V/R Step 2 – Plug in numbers and units (No Naked Numbers!) = 6 V/3 Step 3 – Do correct mathematical operation. =2A Series Circuits A series circuit is a circuit that only has one path for the electrons to flow through. In a series circuit, the current must flow through all of the devices in the circuit, so if one goes out, the current can’t flow. Some Christmas lights are wired in series. Parallel Circuits A parallel circuit provides two or more branches for the current to flow through. Individual devices can be turned off and the current will still flow. Homes are wired in parallel circuits. Open and Closed Circuits Open circuits are circuits that have a break or gap in them. Current cannot flow through an open circuit. When a switch is turned off, the circuit is open. Closed circuits are complete paths through which current can flow. When a switch is turned on, the circuit is closed. Resistivity and Resistors Resistivity is a measure of the capacity of a material to resist the flow of electric charges. A resistor is an electrical device that has a specific resistance. The value of the resistance is printed on the outside of the resistor.