[PHYSICS] Solids, Liquids & Gases - Topic 2

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Density

Mass per unit of volume

Density = mass/volume

How to determine the density of a regular object

1. Measure the mass using a top-pan balance

2. Use vernier calipers to measure the dimensions

3. Repeat measurements & calculate average dimensions

4. Calculate volume = L x W x H

5. Calculate density = Mass/Volume

How to determine the density of a sphere

Measure the diameter and radius. Find volume using —> 4/3 x Pi x R³

How to determine the density of an irregular small object

1. Measure the mass using a top-pan balance

2. Measure the volume using displacement of water

3. Use a measuring cylinder to measure 50cm³ of water

4. Submerge the object and record the new volume

5. Displaced volume = Final volume - Initial volume

How to determine the density of an irregular large object

1. Measure the mass using a top-pan balance

2. Measure the volume using displacement of water

3. Use a displacement can and fill until it overflows

4. Submerge object

5. Measure displaced volume using measuring cylinder

6. Repeat to get an average

How to measure the density of a liquid

1. Place empty measuring cylinder on top-pan balance, hit tare (to avoid zero error)

2. Add 50cm³ of the liquid

3. Record mass

4. Accurately measure the volume

5. Calculate density

How to measure density of a gas

1. Fill 10ml of HCL into a conical flask of limestone chips

2. Hit tare to 0 reading

3. Accurately measure volume of gas by downward displacement once bubbles have stopped being released

4. Remove the delivery tube and record mass loss from reactants

5. Density of gas = mass lost/volume collected

Pressure

Pressure is force per unit area

Pressure = force/area

Measured in Pascals (PA) or N/m²

Why do snow shoes have larger area than feet

To distribute force over a larger area and reduce pressure on the snow, preventing sinking

Why do knives have a small surface area

To concentrate force, increased force/pressure so that knives can cut through things easier with.

The sharper the knife = the less SA = The less force/pressure is required to cut

Why do posh bags have large handles

Less pressure

More comfortable to wear

How to determine the pressure exerted by a regular object?

1. Measure the mass on a balance

2. Calculate the weight = mass x 10

3. Measure the dimensions of the bottom & Calculate the area

4. Calculate the pressure

How to determine the pressure exerted by a irregular object?

(e.g foot)

1. Measure the mass on a balance or bathroom scale

2. Calculate weight = mass x 10

3. Draw around the foot with graph paper

4. Estimate the surface area of the foot by counting squares on the paper

5. Repeat with the other foot to get an average area

6. Calculate pressure

Formula for pressure difference in a liquid

Height of the fluid column above object x density x 10 (grav)

Pressure in a liquid

• The pressure in a liquid is different at different depths

• The deeper the object, the higher the pressure exerted on it

• This is because of the weight of the column of water above

• Pressure exerted on objects in fluids create forces against surfaces which act at 90 degrees

Why are dam walls thicker and stronger at lower depths

Because that is where pressure is greatest

Why do scuba divers equalise their air spaces

Because they experience greater pressure at greater depths

What do pascals vases show us

That the shape/width/angle of the water column does not change height of liquid and pressure. Only depth can change the pressure of a liquid.

Brownian motion

• Random motion of particles due to a large number of collisions with smaller particles that are too small to see

• This is direct evidence of kinetic theory

• All particles are in a state of constant movement

Gas & Pressure (Kinetic theory)

• Gas fills containers

• As gas move equally and in all directions they collide with the walls of their containers

• These collisions provide small net force at right angles to the wall of the gas container

• Therefore, higher pressure gas = more frequent collisions with container walls = greater force

Air pressure in a straw

• When we inhale, we lower the air pressure in the straw

• The air pressure outside pushes the liquid up the straw

Air pressure in a can

• Can is heated with a small amount of water inside

• Water boils, and steam pushes the air out of the can

• Putting the can inverted into the water makes the steam condense

• This leaves a vaccum in the can

• The can is crushed by atmospheric pressure

Boyles law practical - Describe the relationship the pressure and volume of a gas.

Pressure is independent variable, volume is the dependent variable, mass & temp of gas are control variables

• Use a column of air, a bourdon gauge, and oil

• Take repeats to improve accuracy and eradicate anomalies

• Use a scale marked in mm not cm for better precision

• Have a pump that holds the pressure to give more time to take the reading

Relationship between pressure and volume

The greater the pressure, the lower the volume.

Negative correlation because the line goes down.

They are inversely proportional because a graph of 1/V against P shows a straight line that goes through the origin.

Boyles law

• For a fixed mass of gas held at a constant temperature

• Pressure in pascals x volume in m³ = constant

• When the volume decreases, pressure increased

• When the volume increases, pressure decreases

• When the volume decreases, the same number of particles collide with the walls of a container - but more frequently as there is less space. The particles still collide with the same amount of force.

• This can be written as P₁V₁=P₂V₂ to compare pressure & volume before and after a change in gas.

Gay Lussac Law

• Temperature must be in kelvins = celcius + 273.15

• The pressure exerted by a gas varies directly with the absolute temperature of the gas

• The pressure exerted by the gas is proportional to the temperature of the gas

• Increase temp = particles move faster = increased pressure inside the container = more frequent collision with container due to increase in average kinetic energy

• P₁/T₁ = P₂/T₂