G484 written sections/ definitions

JR

6/26/2011

G484 written sections/definitions

Newtons laws and momentum

Newtons first law: An object will remain at rest or keep travelling at a constant velocity unless acted on by an external force

Newtons second law: The net force of an object is equal to the rate of change of its momentum

Newtons third law: When two bodies interact, the forces they exert on each other are equal and opposite

Linear momentum: The product of an objects mass and velocity. It is a vector quantity as velocity is a vector, and as it is the velocity multiplied by the mass, it makes momentum a vector quantity.

Impulse: The product of the force and the time for which it acts

Principle of conservation of momentum: In a closed system, assuming there are no external forces, the momentum before a collision is equal to the momentum after

Perfectly elastic collision: A collision where kinetic energy is conserved (as well as momentum and total energy)

Inelastic collision: A collision where kinetic energy is lost (although momentum and total energy are conserved)

Circular motion

Radian: The angle subtended at the centre of a circle by an arc length equal to the circles radius

Centripetal acceleration: The change in velocity of an object travelling in a circle. It is always directed towards the centre of the circle

Centripetal force: The net force acting on an object moving in a circle, and is always directed towards the centre of the circle

Gravitational field strength: The gravitational force experienced by an object per unit mass

Newtons law of gravitation: Any two point masses attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of their separation

Period (related to a circle): The time taken for one complete orbit

Keplers third law: The square of the period of a planet, is directly proportional to the cube of its distance from its star

Geostationary orbit: The orbit of an artificial satellite which has a period equal to one day so that it stays in the same point above the Earths equator (from Earth the satellite appears to be stationary). They are used for telecommunications (TV and telephone signals)

Simple harmonic motion

Examples of free oscillations (an oscillation which has no driving mechanism and zero friction): - Banging a drum - Hitting a nail with a hammer - Pendulum (clock) - Electromagnetic waves

Oscillation definitions:

Displacement: the distance an object has moved from its equilibrium position

Amplitude: the maximum displacement

Frequency: the number of oscillations per unit time at any point

Period: the time taken for one complete oscillation

Angular frequency: the rate of change of angle

Phase difference: the fraction of an oscillation between the vibrations of two oscillating particles (expressed in degrees or radians)

Simple harmonic motion: When the acceleration of an object in an oscillating system is directly proportional to the displacement from its equilibrium position, and is directed towards the equilibrium position

Resonance: When the driving frequency approaches the natural frequency of an object, the object gains more kinetic energy from the driving force and so vibrates with a rapidly increasing amplitude.

Uses of resonance: Nuclei of atoms resonate when they are in suitable magnetic fields and a radiofrequency pulse is applied that matches the natural frequency of the atoms, this can be used for medical imaging Microwave cooking- the microwaves cook the food as their frequency matches the natural frequency of water molecules, causing them to resonate and heat up the food When a radio is tuned you are adjusting the resonant frequency of the radio to the frequency of the transmitted signal In an earthquake the frequency of the quake can match the natural frequency of the buildings, causing them to resonate and potentially cause a lot of damage

Damping: Deliberately reducing the amplitude of an oscillation - If only small damping forces exist, the period of the oscillation is almost unchanged, but

the amplitude gradually decreases. This is known as light damping. - If the damping forces are larger there is a more noticeable reduction in amplitude, and

the period increases slightly. Eventually no oscillation occurs and the body will slowly move back to its equilibrium position.

- The period between oscillation and no oscillation is called critical damping Investigating damping:

- A springy metal strip is clamped to a bench which will oscillate freely when displaced from side to side

- A mass is attached to the free end and a card is attached to the mass so that there is significant air resistance as the mass oscillates

- The amplitude of the oscillations decreases and can be measured every five oscillations by judging the position of the strip against a ruler to its side

- A graph of amplitude against time will show the exponential decrease - By changing the size of the card the amount of damping can be changed

Thermal physics

Solid: closely packed, evenly spaced, vibrations about a fixed point

Liquid: closely packed (less packed than solids, as there are more gaps between), less regular order than solids, particles can move past each other slowly (in comparison to gases)

Gas: particles far apart, not packed at all. Randomly ordered, fast moving in random directions

Brownian motion: the random movement of microscopic particles surrounded by a liquid or gas, caused by collisions of molecules in the surrounding medium

A simple experiment to demonstrate Brownian motion:

- Some smoke is introduced into a small glass container, and the container is well illuminated and viewed through a microscope.

- The microscope needs to be focused on the smoke, which is seen as tiny dots of light - The smoke particles move haphazardly

Pressure: the total force exerted over the area of a wall/surface- when applied to a gas a force is provided by the change in momentum when particles collide with the walls of the container

Assumptions of the kinetic theory of gases: - Gas consists of a large number of particles in rapid, random motion -Collisions between particles and the walls of container (and other particles) is elastic -Gravitational force on particles is negligible -No intermolecular forces exist except during collisions -Volume of particles is negligible compared to volume of the container

Internal energy: The sum of the kinetic and potential energies associated with the particles of a system.

- When you heat a substance, you increase its kinetic energy, thereby increasing its internal energy. When a substance changes state its internal energy changes, but its temperature doesnt. This is because the change of state alters the potential energy of the particles, not their potential energy.

Absolute zero: the temperature at which a substance has no internal energy

Melting: Particles become more disordered, increase in separation between particles, therefore electrical potential energy increases

Boiling: Particles become completely separate from each other. Large increase in separation between particles therefore electrical potential increases greatly and movement becomes more disorderly as a result

Specific heat capacity: The energy required to raise the temperate of a substance of mass 1kg by 1K.

Latent heat of fusion: The energy required to cause a substance to melt at a constant temperature

Latent heat of vaporisation: The energy required to cause a substance to boil at a constant temperature

An experiment to measure the specific heat capacity of a material:

Apparatus is set up with an immersion heater and thermometer. The immersion heater is connected to a circuit, and a voltmeter is placed across it. An ammeter is also placed in series. The immersion heater is switched on and you need to time how long it takes for the material to heat up by 10K,

while checking the voltage and current at regular intervals. Once this has been done the energy supplied can be calculated using

Once this is known the specific heat capacity equation can be rearranged to make C the subject

Boyles law: The pressure of an ideal gas is inversely proportional to its volume at a constant temperature

Charles law: The volume of an ideal gas is directly proportional to its temperature at a constant pressure

Pressure is proportional to the number of particles, their mass and their speed: 1. The volume of a container- increasing the volume of the container decreases the frequency

of collisions between particles because they have further to travel between collisions. This decreases the pressure

2. The number of particles- increasing the number of particles increases the frequency of collisions between the particles and container, so increases the total force exerted by all the collisions

3. The mass of particles- according to Newtons second law, force is proportional to mass, so heavier particles exert a greater force

4. The speed of the particles- the faster the particles are going when they collide, the greater the change in momentum and force exerted