Fluids, Viscosity and Flow Rate

Fluids, Viscosity and Flow Rate

By Bradley Crouse

​Fluids!

Fluids

Fluids are defined as any substance that is able to flow or will change shape continually under a constant force.

Because liquids and gases do NOT have a definite shape they are able to flow making them fluids.

We use fluids every single day!

​

Science 8

Multiple Select

Which of the following states of matter are considered fluids? (Select all that apply)

• Solids

• Liquids

• Gas

Open Ended

Some fluids flow at different speeds. Why do you think this is?

Viscosity

Viscosity describes the thickness of a fluid. It measures a fluid’s resistance to flow.

• A “thick” fluid is said to be highly viscous, or have high viscosity

• A “thin” fluid is said to have low viscosity.

​

​

Science 8

Multiple Choice

Rearrange the following fluids in order from low to high viscosity:

Ketchup, water, air, honey

• Ketchup, Water, Air, Honey

• Honey, Ketchup, Water, Air

• Air, Ketchup, Water, Honey

• Air, Water, Ketchup, Honey

Multiple Choice

Remember, how do particles move in liquids?

• They vibrate, while in constant contact with each other.

• They are able to move and slide past each other

• They move freely in all directions

• They don't move

Why Are Fluids Viscous?​

• ​Remember that fluids have particles that move past one another (liquids and gasses can flow)

• Viscosity describes the collisions between these particles. This rubbing together of the particles creates friction, which in effect slows the fluid down. This is called internal friction.

• ​The higher the internal friction, the higher the viscosity.

• Do you think solids can be ​viscous? For example we can pour cereal into a bowl - this is sort of a flow.

Why is molasses extremely viscous compared to water? We know it’s caused by internal friction.

To answer this, we must look at the molecular level (particles).

​

Why viscosity changes?

Here is a water molecule vs. fructose molecule (type of sugar).

​

The internal friction caused by the large bulky shape of fructose molecules is much higher than the internal friction caused by the small compact shape of the water molecules.

​

Why viscosity changes?

Open Ended

In your own words, why is molasses more viscous than water?

​The Great Molasses Flood of 1919

“At around 12:40 p.m. on January 15th 1919, the mid-afternoon calm was broken by the sound of a metallic roar. Before residents had time to register what was happening, the recently refilled molasses tank ripped wide open and unleashed 2.3 million gallons of dark-brown sludge. “A rumble, a hiss—some say a boom and a swish—and the wave of molasses swept out,” the Boston Post later wrote. A fifteen-foot wall of syrup cascaded over Commercial Street at 35 miles per hour, obliterating all the people, horses, buildings and electrical poles in its path. Even the solid steel supports of the elevated train platform were snapped.”

Why do we care about viscosity?

• When designing machines, engineers need to know the physical properties of the fluid they are using, including the viscosity

• It is particularly important to know the viscosities of oils and greases when designing things like cars, planes and chains. These fluids are key to maintaining the machine to extend its useful life

• Who here has heard of getting the oil changed on a car?

  • The cost of an oil change at Mr. Lube in Halifax costs between $20 - $150 depending on the vehicle.

Flow Rate

The flow rate of a fluid depends on how viscous it is. If a fluid has a high viscosity, it will have a lower flow rate.​

Flow rate is how fast a fluid flows in a given amount of time

If a fluid is warmer, it will flow faster and must then be less viscous.

​

​

Science 8

Replace this with your body text.

​Happy teaching!

Replace this with text

Replace this with your body text.

​Happy teaching!

Replace this with text

Example:

What is the flow rate of honey if it takes 30 seconds to move 15 mL of fluid?​

​

FR = V/t = 15mL/30 s = 0.5 mL/s​

​

​

Subject | Subject

Some text here about the topic of discussion

Multiple Choice

If the flow rate of molasses is 1.5 mL/second, and we have a volume of 67.5 mL, which formula would we use to solve for time? (Hint: Use the triangle)

• $$t\ =\ \frac{FR}{V}$$  

• $$t\ =\ FR\ \cdot V$$  

• $$t\ =\ \frac{V}{FR}$$  

Multiple Choice

A garden hose has a flow rate of 36 L/minute. If you’re using it to rinse a car for 6.5 minutes, what formula will you use for volume? (hint: Use the triangle)

• $$V\ =\ FR\ \cdot t$$  

• $$V\ =\ \frac{FR}{t}$$  

• $$V\ =\ \frac{t}{FR}$$