General Facts Flashcards
What do you assume when you measure the density of an object by finding volume with a measuring cylinder?
That its density is more than water.
What is the density of water?
1g/cm3 or 1000kg/m3
Give an experiment to find how extension of a spring varies with force applied.
- Measure original position of spring.
- Add a 100g mass to the hanger and measure new position.
- Repeat adding a 100g mass each time.
- Calculate extension by subtracting original position from subsequent position.
Why, with centripetal force, is no work done on the object?
The object does not move in the direction of the force.
What should you remember when you construct wave-fronts for refraction?
Draw a normal
Draw direction of wave travel
How do you increase sensitivity of a thermometer?
- Mass of liquid
- Diameter of thread
- Change liquid so has higher expansivity
Why is acceleration rarely steady?
- Air resistance and terminal velocity
- Gear change
- Mass of car changes as fuel burns
What is responsible for making objects float?
Upthrust
What is tension?
The force in a stretched material
Newton’s first law?
When no external force is acting on an object:
- If moving, keeps moving at steady speed in straight line
- If still, remain still
What is inertia?
A resistance to change in velocity.
What is true of heavier objects?
Takes more force for acceleration.
What is Newton’s Second Law of Motion?
F=ma
How do you reduce friction?
- Mount wheels on roller bearings
2. Add oil/lubricant
As force is increased, what happens to friction on a stationary object?
Friction rises until object slips
How do you increase static friction?
Add greater downward force on block
Why is friction a nuisance?
Has heating effect.
Makes moving parts so hot they seize up.
Which type of friction heats materials up?
Dynamic friction
Three features of gravitational force:
- Closer together, more attraction
- More mass, more attraction
- All masses attract each other
What is a gravitational field?
Region where mass experiences a force due to gravitational attraction.
What are forces?
Pushes and pulls between two objects.
What is Newton’s Third Law of Motion?
To every action there is an equal but opposite reaction.
Formula for resultant force using change in momentum and time.
Resultant force = change in momentum / time
What is an impulse?
(Resultant) Force x time
Object with most mass has ____ velocity.
Lowest
Why is momentum conserved?
Newton’s third law - to every action there is an equal but opposite reaction.
So if change in momentum = force x time, and force and time are equal (in magnitude) for both cases, mass x velocity = mass x - velocity
Law of conservation of momentum?
When two or more objects act on each other, their total momentum remains constant, provided no external forces are acting.
Conditions of equilibrium:
- Sum of anticlockwise moments about point …
2. Sum of forces in one direction = sum of forces in other direction
Why does a beam balance at centre of gravity?
Particles have turning effects which cancel out at centre of gravity.
What is a centre of gravity?
Point where particles have a resultant force = weight of object
What is load (on a spring)?
Force applied to a spring
Characteristics of direct proportion?
- X doubles, Y doubles
- X / Y gives same value
- Increase in X gives constant increase in Y
- Straight line through origin
What happens beyond elastic limit?
- Permanently stretched
2. Breaks
Typical pressure of shoe sole?
20 kPa
Typical pressure of high heel?
500kPa
Typical pressure of pin?
5 million kPa
Conditions of pressure in liquid:
- Depends on depth
- DEPENDS ON DENSITY OF LIQUID because mass of liquid in certain volume depends on density
- Acts in all directions
- Does not depend on shape
Why is the output piston only moved a fraction of the distance?
Because the volume of oil used to push both pistons is the same.
Same volume means that area x height of X = area x height of Y`
So if area of Y is larger, height must be smaller
What reduces output force in a hydraulic jack?
Friction
What is atmospheric pressure?
100kPa
How does a straw work?
Lungs lower air pressure in straw
So atm pushes liquid up straw
How do you find actual pressure of gas supply?
Find excess pressure in mm of mercury
Add atmospheric pressure in mm of mercury
Convert to Pa
Relationship between pressure and volume?
Inverse proportion
Why pressure increases with decreased volume:
Decreased volume = more particles / unit volume
So collisions per second of particles with walls increases
So more force exerted on walls in one second
So more force / same area = higher pressure
Conditions of a real gas:
Minimum attraction between molecules
- Low density
- Not full of water vapour
- Well above liquefying point
How to find experimentally link between pressure and volume
- Trap air in glass tube with oil
- Pump air into reservoir in stages so oil compresses air so volume reduces
- Pause for few moments so air returns to original temperature
- Measure pressure of oil in stages (using pressure gauge in oil reservoir)
What is work?
Work is done whenever a force makes something move
Why does a falling object do work?
Work done = force x distance
Gravitational force makes object move (downwards)
What is electrical energy?
Energy transferred by electrons in a circuit
What holds the nucleus together?
Strong nuclear force
How do engines use thermal energy to do work?
Thermal energy turns liquids to gas, gas pushes on pistons
Waves carry ____
energy
Sources of wasted energy (all of them turn into thermal energy):
- Air resistance
- Inefficiency of a system (e.g. in muscles)
- Sound (so any vibration)
What does sound turn into and why?
Sound is absorbed, so leaves objects warmer than before, so thermal energy
Efficiency =
useful power output / total power input
useful work done / total energy input
Causes of inefficiency in power stations?
- Energy lost in cooling water and waste gases (which travel away)
- Turbines heating up when hit by air and also direction of air particles used to turn them
- Heat energy lost in turning generator
- Resistance in wires of generator
How sun’s energy becomes energy in waves:
Sun heats earth Air heats up (gains thermal energy) and RISES Air forms belts around the equator Air belts causes wind systems Wind produces waves`
Why do particles in liquids and gases have energy?
- They have kinetic energy because they are moving
2. They have potential energy because they are moving and this movement opposes intermolecular forces
Do solids, liquids or gases have the highest potential energy?
Gases, because they are most separated
What is internal energy?
Kinetic + potential energy
What is thermal energy?
Internal energy + heat
Properties that vary with temperature?
Expansion of liquid (volume)
Expansion of gas (volume)
Expansion of solid (volume)
Resistance of semi-conductor
Explain how a thermocouple thermometer works.
Two different metals (copper and constantan) joined at two junctions
A temperature difference causes tiny voltage
Tiny voltage induces current
Objects at the same temperature have the same:
average kinetic energy per particle
Why is temperature different to heat?
Heat (energy) takes into account mass
How to make thermometer more responsive?
- Change material to be a better conductor
- Use thin glass in bulb
- Use a smaller bulb
How to increase range?
- Use electronic thermometer
- Use different material with lower expansivity
- Make capillary longer
Why does a gas not necessarily expand when heated?
Volume depends on shape of container
How to find experimentally how pressure changes with temperature?
- Trap air in glass flask (so volume constant) and place in water bath
- Change temperature by heating water in stages
- Measure pressure in stages
How does conduction take place?
- Heated so particles gain kinetic energy and vibrate more
- Bump against neighbouring particles
- So motion is transferred from hot end to cold end
How to increase rate of conduction?
- Bigger temp. difference
- Increase cross-sectional area
- Decrease length
How to use insulating materials in home:
- Foam around boiler
- Mineral wool in wall cavity
- Double glazing
- Mineral wool insulation in loft
How to increase radiation emitted:
- Higher temp
2. Higher surface area
Experiment to show black surfaces better absorbers of heat?
Apparatus: radiant heater, thermometers, black and white cans
Actions: Fill cans with water. Place at equal distances from the radiant heater. Measure temperature rise.
Observation: Water in black can as greater temperature rise.
What happens to water vapour when it evaporates?
Bear in mind it may return to the liquid
How do refrigerators work?
- Coolant evaporates, taking thermal energy away from the food
- Vapour drawn by pump and compressed by liquid, releasing thermal energy, heating liquid up
- Hot liquid cooled down by cool pipes at back of refrigerator
- So thermal energy in total carried away FROM FOOD INSIDE to air
Specific heat capacity of water
4200J/(kg degC)
UNITS OF SPECIFIC HEAT CAPACITY
J / kg degC
What is thermal capacity?
Mass x specific heat capacity
Units of thermal capacity?
J / deg C
Specific latent heat of fusion of water?
330,000 J/kg
Specific latent heat of vaporization of water?
2,300,000 J/kg
What is specific latent heat?
Heat needed to turn 1 kg of solid into liquid etc.
With no rise in temperature
What does specific latent heat calculations not allow for?
Any heat gained by surroundings e.g. funnel
How to measure latent heat?
- Place boiling water in beaker and place electric heater of known power inside, place this on balance
- Measure initial mass of water and beaker
- For set amount of time (on stopwatch), heat boiling water
- Measure final mass of water and beaker
- Calculate l using E = ml
Why does refraction cause change in direction?
Frequency stays same
So slowing down means that wavelength decreases
If wave-fronts close up on each other (due to decreased wavelength) direction of travel must change unless at right angles
Evidence for reflection
Light reflects in mirrors
Echoes from hard surfaces
Evidence for refraction
Light bends from glass to water
Evidence for diffraction
Sound spreads around walls so can hear round corners
Some radio signals bend around hills
Characteristics of sound waves:
- caused by vibrations
- need a medium to travel through, no vacuum
- can travel through liquids, solids and gases
- longitudinal waves
- can be diffracted
- can be reflected and refracted
How microphone works:
Sound enters microphone
Crystal in microphone vibrates
Vibrations changed into electrical oscillations
Graph produced of how air pressure in microphone varies with time
Speed of sound depends on:
Temperature of air
Does not depend on pressure of air
Material
Principle of echoes used in:
- Echo-sounders
- Radar
Induction results in ____ charge compared to inducing object:
Opposite
What an arrow signifies in an electric field?
Force on a positively charged particle.
What happens at a sharp curve on a charged object?
- Field lines closest together
- So charges most concentrated
- May ionise air
Why does charge gather on the outside of an object?
Because they repel each other
Under what conditions does a cell make electrons move?
When there is a conductor between its terminals (i.e. complete circuit)
What inside a cell pushes electrons round a circuit?
Chemical reactions
Current in electric kettle element?
10A
What is an ampere?
flow of 1 coulomb / second
Why do electrons in a circuit have potential energy?
They repel each other because they have the same charge
What is voltage a measure of?
The energy a cell gives to electrons it pushes out.
What is a volt?
1 Joule / Coulomb
What is emf?
Work done / unit of charge by the cell in driving charge round a complete circuit, including the cell itself.
All the energy supplied by a battery is…
…radiated out or used up by its components.
What is potential difference of a component?
Work done / unit of charge in driving charge around component.
