SP14 & SP15: Particle Model & Forces and Matter

What is density?

The mass per unit volume of a material - the measure of the 'compactness' of a substance, relating the mass of a substance to how space it takes up.

What is the equation for density?

Density (Kg/m3) \= Mass (Kg) / Volume (M3)

What is the density of an object dependent on?

The material of an object - density does not vary with size or shape.

What does the average density of an object determine?

It determines whether the object floats or sinks - a solid object would float on a fluid if it has a lower average density than the fluid

How do you find the density of an object with an irregular shape?

Measure the mass of an object with a mass balance. Fill a displacement can (eureka can) with water and place a measuring beaker underneath the can's spout. Submerge the object. Record the volume of water that has been displaced (this is the volume of the object). Calculate the density using the equation mass of object / volume displaced.

How should you fill the displacement can to get the most accurate results?

Fill the can until water starts to drip out of the spout and into the beaker. Wait until the dripping just stops before submerging the object.

What is meant by a state of matter?

A form in which matter can exist, based on the particle arrangement in a substance.

State the different states of matter in order of least to most dense

Gas, Liquid, Solid

What is Kinetic Theory?

The explanation that all matter consists of tiny particles that are in constant motion

Using kinetic theory, explain the movement of particles in a solid

There are strong forces of attraction that hold the particles close together in a fixed regular arrangement. The particles don't have much energy in their kinetic energy stores, so they only vibrate about their fixed positions.

Using kinetic theory, explain the movement of particles in a liquid.

The forces of attraction between the particles are weaker. The particles are close together, but can move past each other and form irregular arrangements. They have a little more energy in their kinetic energy stores, which means they move in random directions at low speeds.

Using kinetic theory, explain the movement of particles in a solid.

There are almost no forces of attraction between the particles. Particles have a lot more energy in their kinetic energy stores, so they are free to move, travelling in random directions at high speeds.

Why does heating a substance causes it to change state?

When a substance e.g. a solid is heated, the heat energy is being transferred into the particles' kinetic energy stores. This causes the particles to vibrate faster until the forces between them are partly overcome and they start to move around - this is an example of melting.

In a closed system, what is always conserved when a substance undergoes a change of state?

The mass of a substance, as the particles of the substance isn't increasing or decreasing, it is only rearranging itself.

Why does the volume of a substance change when it undergoes a change of state?

If, for example, a solid was melted into a liquid, the particles were more closer together when it was a solid.

Why is ice less dense than water?

When water freezes, the water molecules are held in a rigid lattice which holds the molecules further apart

How does a change of state differ from a chemical change?

In a change of state, the material can return to having its previous properties if the change is reversed.

What is the difference between physical changes and chemical changes in substances?

Physical changes can be reversed, chemical changes cannot be easily reversed.

Aside from changing the state of substance, what else can heating a substance do?

Raising its temperature, or increasing the energy in its thermal energy store.

What is the internal energy of a substance?

The energy stored by the particles. It is the sum of the total kinetic and potential energies that make up the system.

What is the specific heat capacity?

The change in energy in the substance's thermal energy store required to increase the temperature of 1kg of a substance by 1°C

What is the equation for specific heat capacity?

Change in thermal energy (J) \= Mass (kg) x specific heat capacity (J/kg°C) x change in temperature (°C)

Describe the experiment to find the specific heat capacity of water

Measure the mass of an insulating container and then fill it again with water and remeasure its mass. The difference in mass is the mass of water in the container. Measure the temperature of the water using a thermometer. Connect a power supply to a joulemeter and connect the joulemeter to an electric immersion heater. Insert the immersion heater and a thermometer into the water. When the temperature has increased by a set degrees e.g. 10 degrees, stop the experiment and record the energy of on the joulemeter and the increase in temperature. Use the measurements to calculate to specific heat capacity.

What is specific latent heat?

The amount of energy needed to change 1kg of a substance from one state to another without changing its temperature

What is the equation for the energy required to change state?

Thermal energy (J) \= Mass (kg) x Specific latent heat (J/Kg)

What is the specific latent heat of fusion?

The specific latent heat for changing between a solid and a liquid

What is the specific latent heat of vaporisation?

The specific latent heat for changing between a liquid and a gas

What happens to the bonds between particles when heating a substance?

The bonds break, rather than raising the temperature. This causes the substance to change state.

Explain the pressure of a gas in terms of the motion of particles.

As the particles move about in random directions at high speeds, they can collide with the walls of a container. When this happens, they exert a force which acts at a right angle to the container. This causes pressure.

What is the equation for pressure?

Pressure (Pa) \= Force (N) / Area (m^2)

Which factors affect pressure?

Temperature, volume, and the number of particles

How does temperature affect pressure in a fixed volume?

An increase in temperature means an increase in pressure. This is because the gas particles have more kinetic energy, which increases the frequency of collisions. More forces are exerted as a result, which leads to the increase of pressure.

How does volume affect pressure?

An increase in volume means a decrease in pressure. This is because in a larger volume, the particles are more spread out and hit the walls of the container less often. As a result, fewer forces are exerted and the pressure decreases. This is the opposite when the volume decreases - less room means more collisions meaning more forces exerted.

How does the number of particles affect pressure?

An increase in particles means an increase in pressure. This is because there are more particles moving around the container which leads to a higher frequency of collisions. As a result, more forces are exerted which leads to an increase in pressure.

How do you convert from Degrees Celsius, °C, to Kelvin, K?

From Degrees Celsius to Kelvin is +273, from Kelvin to Degrees Celsius is -273. For example, 0°C is worth 273 kelvin. 0K is worth -273°C

What is absolute zero?

0K, or -273°C. Particles have no kinetic energy and do not move.

What equation can be used to calculate the pressure or volume for a gas for which the mass is fixed and temperature is constant?

P1 x V1 \= P2 x V2

What happens when you do work on a gas?

Its internal energy increases, which can increase its temperature

Why does the temperature of air inside a bike pump increase when it is pumped?

Work is done on a gas when it is compressed - the gas exerts pressure on the plunger of the pump, and so exerts a force on it. Work has to be done against this force to push down the plunger i.e. mechanically. This transfers energy to the kinetic energy stores of the gas particles, as doing work on a gas increases its internal energy and therefore the temperature.

What happens when multiple forces are applied on an object in different directions?

The object can stretch, compress or bend.

What are elastically distorted objects?

Objects that can go back to its original shape and length after a force has been removed. They are called elastic objects e.g. a spring.

What is the elastic limit?

The maximum point to which an object stops distorting elastically and begins to distort inelastically.

What are inelastically distorted objects?

Objects that cannot go back to its original shape and length after the force has been removed. This leads to a permanent change in shape.

What is being done when a force stretches or compresses an object?

Work is being done; it causes energy to be transferred to the elastic potential energy store of the object. If it is elastically distorted, all this energy is transferred to the object's elastic potential energy stores.

What is the equation for force, linking spring constant and extension?

Force (N) \= Spring Constant (N/m) x Extension (M)

What is the extension of a stretched spring?

A weight at the bottom of a spring that causes the spring to stretch downwards.

What is the spring constant?

A measure of the stiffness of a spring. The higher the spring constant, the stiffer the spring.

What can be said about the relationship between the extension of the stretched spring and the force applied?

They are directly proportional - a linear relationship.

What happens when too much force is applied to the spring when it is being stretched?

There is a limit for how much force can be applied onto a stretched spring (the limit of proportionality). When this limit has been exceeded, the spring undergoes deformation, and past the elastic limit, the spring is permanently stretched.

Describe the experiment to investigate the link between force and extension.

Measure the natural length of the spring when no load or force is applied - do this by hanging a spring from a clamp stand alongside a millimetre ruler. Add a fixed mass to the spring (e.g. a 10N weight) and allow the spring to come to rest. Record the mass and measure the new length of the spring. The extension is the change in length. Repeat by adding more weights until you have enough measurements. Plot a graph of extension against force.

What does it mean if the elastic limit is always at or beyond the limit of proportionality?

This means that for a linear relationship, the distortion is always elastic - all the energy being transferred is stored in the spring's elastic potential energy store.

What is the equation to find the work done/the energy stored in the elastic potential energy store?

Energy (J) \= 1/2 x Spring Constant (N/m) x Extension^2 (M2)

What is a fluid?

A substance that flows freely, such as a liquid or a gas

What causes fluid pressure?

All of the forces exerted by the individual particles when they collide with a container at right angles in a fluid combine to make up the pressure exerted by the fluid.

How do you calculate pressure?

Pressure (Pa) \= Force (N) / Area of the surface (m^2)

What does the force on a surface due to fluid pressure depend on?

The area of the object the fluid is in contact with.

How does fluid pressure depend on depth and density?

For a given liquid, the density is uniform and doesn't vary with shape or size. The density of a gas can vary though. Assuming the particles have the same mass, a denser fluid has more particles in a certain space than a less dense one. This means there are more particles that are able to collide so the pressure is higher at a given depth in the denser fluid.

What happens to the fluid pressure as the depth increases?

The number of particles above that point increases - the mass of the liquid. This means that the force produced by that mass also increases. Since the force has increased whilst the area has remained constant, the pressure will increase.

Why does atmospheric pressure vary with the heigh above the earth's surface?

The number of air molecules decreases the higher you go. This means that the weight of air above a point decreases, meaning the pressure also decreases.

How do you calculate the magnitude of pressure in liquids at different depths?

Pressure (Pa) \= Column height (M) X Density (Kg/m3) x Gravitational Field Strength (N/Kg)

What is upthrust?

The upwards force exerted by a fluid on a submerged object

What happens when an object is submerged in a fluid?

The pressure of the fluid exerts a force on it from every direction. As pressure increases with depth, the force exerted on the bottom of the object is larger than the force acting on the top of the object (upthrust).

What is upthrust equal to?

The weight of fluid that has been displaced by the object.

Why does an object float?

If the upthrust on an object is equal to its weight, then the forces balance and the object floats.

Why does an object sink?

If an object's weight is more than the upthrust, it sinks.

Why does an object with a density more than that of water sink?

Upthrust is equal to the weight of the displaced fluid. If the density of the object is high, there would not be enough volume displaced to produce an upthrust larger than the object's weight, which is why it sinks.

What creates atmospheric pressure?

Air molecules colliding with a surface.

Why does atmospheric pressure decrease with altitude (height above Earth)?

As the altitude increases, the atmosphere gets less dense, so there are few air molecules that are able to collide with the surface. There are also fewer air molecules above a surface as the height increases. This means that the weight of the air above it, which contributes to atmospheric pressure, decreases with altitude.