Download 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.