P7.2 Flashcards
How does an electric current transfer energy?
Electrical devices = energy is transferred when CHARGE moves
- from chemical store of battery or
- from chemical/nuclear store or the fuel in power station used to produce mains electricity
How is energy in the wire transferred?
- e- in wire produce light, heating effect or make a motor turn
What is power rating and what is it measured in?
- Watts or kW
- tells you the rate at which the appliance transfers energy between the stores
What does a large power rating mean?
- more power means appliance transfers more energy per second when used
- battery goes flat faster/ fuel in power station is used up more quickly
What unit is used to measure energy transfer by domestic electrical appliances and what does it mean?
- domestic appliances = kilowatt-hour (kWh)
- it’s the energy transferred by a 1kW appliance when switched on for 1 hour
How do you calculate electrical work done?
Electrical work done (kWh) = power (kW) x time (h)
How are you charged for the electricity bill?
Charged for each kWh or unit that you use
How much is 1 MJ (mega joules) in Joules?
1,000,000 J
How much is 1 kWh in Joules?
3,600,000 J
How do electrical appliances transfer energy?
- transfer energy electrically from chemical stores
- changes electrical transfer to other forms of energy
How do microwaves transfer energy?
- changes energy transferred electrically to energy transferred by heating
- current produces microwaves = heat food
How do hair dryers transfer energy?
- energy transferred electrically produces heating by radiation (IR)
- energy transferred mechanically(the fan and air move) to dry hair
What happens if you use a device with a high power rating?
- needs more fuel to run than lower power device
- phone = requires high power so battery drain quicker
Why would you buy an appliance such as a hairdryer with a higher power rating?
Transfers electrical energy to heat/kinetic energy faster so it works quicker. Hair dries quicker.
Is using higher power devices cheaper, more expensive, or does it make no difference?
Higher power devices transport electrical energy faster to other forms. This means that in the same time they will use more energy, therefore costing more to run. However, if the higher power of the device makes it more efficient in doing its job, it won’t need to be on for as long a period of time, so it may actually end up using less energy and so costing less money to use.
How do you know energy is wasted in a wire?
- current flows = wire heats up
- energy is always transferred to thermal store when using electrical appliances
- energy is always wasted
- motors = friction = energy transfer to thermal store
How does a TV transfer energy between the stores?
- runs off mains
- store with less energy when appliance used = chemical (fuel and oxygen)
- store with more energy when appliance used = thermal store (surroundings)
How does a DVD player transfer energy between the stores?
- runs off mains
- store with less energy when appliance used = chemical (fuel and oxygen)
- store with more energy when appliance used = kinetic (turntable when it speeds up)
How does a smartphone transfer energy between the stores?
- runs off battery
- store with less energy when appliance used = chemical (battery)
- store with more energy when appliance used = thermal store (surroundings)
How is energy transferred to a: Motor Kettle’s heating element Speaker Bulb
- motor = kinetic store
- kettle = thermal store
- speaker = carries away from speaker by sound waves
- bulb =light carries away from bulb by light waves
Why is it better to use an electrical appliance to heat water than to burn wood?
- burn wood = bad for environment
- electrical = clean way to transfer energy
What is the physical situation for an electrical kettle when boiling water?
- kettle on = cold water in kettle and more fuel/ oxygen
- transfer = current flows and hears an element in kettle
- water boils = boiling water in kettle and less fuel/ oxygen
What is the energy analysis for an electrical kettle when boiling water?
- kettle on = medium in chemical store and low in thermal store
- transfer = energy transferred electrically, heating by radiation and heating by particles
- water boils = low in chemical store and medium in thermal store
How would you calculate the energy transferred in heating water?
Energy transferred = temp change x SHC (J /kg k) x mass (kg)
Energy = power ( in watts not kW) x time (s)
How can the energy in the thermal store be increased and how do you calculate the energy transferred?
- burn a fuel
- use electricity to transfer energy from fuel - conduction
- convection
- radiation
- energy transferred = m x SHC x temp change
When is energy transferred in the thermal store?
- temp diff between 2 objects = energy transfer
- hot object = source - temp decreases
- colder object = sink - temp increases
- more temperature diff = energy transfers quicker
What happens when a storage heater releases energy when the heating is off
- hot piece of concrete in room at the start of the day = room has lower thermal store
- radiator gets hot and emits radiationwhich heats the air = through conduction, infrared and convection
- cold piece of concrete in room at the end of the day = concrete has lower thermal energy
What happens when you heat up a solid?
- heat up object = energy is transferred to object is shared across the kinetic energy stores of particles in object
- particles in hotter part of object vibrate and collide more, transferring kinetic energy to neighbouring particles
- also vibrate faster = increase temp in that part of an object
- continues until extra energy is spread out evenly across particles and temp of the obj is same everywhere
What is conduction?
- generically in solids as particles are closer but in gas and liquid they are further so low conduction
- process where vibrating particles pass extra energy in their kinetic energy stores to the kinetic energy stores of their neighbouring particles
What is thermal conductivity?
- how well an object transfers energy by conduction
- metal has high thermal conductivity
- liquids and gas have low thermal conductivity
Is air a good conductor?
No, air is a good insulator
What are fluids?
- anything that can flow = liquids and gases
What happens when you heat up a liquid or a gas and how do convection currents form?
- particles move faster and fluid expands becoming less dense
- warmer, less dense fluid rises above its colder, denser surroundings
- as warm fluid rises = cooler fluid takes its place = process so to use until you end up with a circulation of fluid (convection current)
What is convection?
- occurs when the particles with more energy in their kinetic energy store move from the hotter to cooler regions and take their kinetic energy stores with them
- convection can’t happen in solids = particles can’t move
How do radiators rely on convection?
- rely on convection to make the warm air circulate round the room
How can you reduce convection?
- stop fluid moving
- clothes, blankets and foam cavity wall insulation work by trapping pockets of air = air can’t move so the energy gas to conduct very slowly through the pockets of air as well as the material in between
How do radiation travel and how does heat travel through them?
- through a vacuum
- transfer energy by heating by radiation and the energy is carried by infrared waves
How does radiation makes things hot?
- all obj = continually emit and absorb radiation = hotter and object = more radiation it emits
- cooler objects absorb radiation emitted by hotter things so their temperature increases
Give examples of good/ bad absorbers and emitters?
- Matt black = very good absorbers/ emitters of radiation
- light coloured, smooth and shiny = very poor absorbers / emitters
What is dissipation?
-Transfer of energy to stores that aren’t useful and can’t be used for heating/working
What is effect and energy transfer due to friction between parts of a car engine?
- effect = parts of car heat up
- energy to thermal store of the engine parts
What is effect and energy transfer due to conduction between water in a kettle and the plastic of the kettle?
- effect = plastic of kettle heats up
- energy to thermal store of the kettle
What is effect and energy transfer due to radiation from the front of a hot oven?
- effect = room that the oven is in heats up
- energy to thermal store of the surroundings
What happens to the thermal energy from different objects?
Ultimately energy ends up in the thermal store of the surroundings
How can dissipation be reduced?
- lubricating items = less dissipation by friction
- insulating items = less dissipation by heat
- streamlined cars = reduces the friction from air resistance = uses less fuel and more efficient
How does lubrication work?
Place a layer of fluid between 2 solid surfaces = no direct contact = no friction = no/ minimal energy waste
- e.g. using oil on a bike chain
How does insulation work?
- place a poor conductor between a hot object and a cold one = rate of energy transfer decreases as the poor conductor transfers energy slower
What are the negative effects of friction?
- reduces efficiency of energy transfer
- causes objects to heat up
How can energy in a home be transferred usefully and wasted?
- useful = radiators
- wasted = windows
- make things more efficient so they waste less energy = energy saving light bulbs
What are the ways houses can be designed to lose less energy?
- loft insulation
- cavity walls and cavity wall insulation
- hot water tank jacket
- double glazing
- thick curtains
- draught- proofing
How can energy loss be reduced by double glazing windows, thick curtains and draught-proofing?
- double glazing - 2 layers of glass with an air gap between reduce conduction
- thick curtains- reduce heat loss by convection and conduction through the windows
- draught- proofing - strips of foam and plastic around doors and windows stop hot air going out, reducing convection
How can energy loss be reduced by loft insulation and hot water tank jackets?
- loft insulation - fibreglass wool laid on the lost floor and ceiling reduces energy loss from the house by conduction and convection
- hot water tank jacket = reduces conduction and keeps the water hot
How can energy loss be reduced by cavity walls and cavity wall insulation?
- 2 layers of bricks wire a gap in between them reduce conduction but energy is also transferred across the gap by convection
- squiring insulating foam into the gap traps pockets of air to minimise this convection
- energy is still lost from falls by radiation though and if there are any places where air is not trapped, there will be some convection too
How can the energy lost from hot water pipes connecting to boilers and radiators be reduced?
- cover pipes with insulation to reduce conduction and convection
- paint pipes white = less lost by radiation
- make pipes short so water spends less time in pipes
- making pipes wide so small surface area to volume ratio = less friction of the water in pipes with the surface of the pipe = less conduction
What effects the rate at which energy is transferred to the surrounds?
- thickness of wall
- material that wall is made of
- temp diff between inside the wall and outside
Is a thicker or thinner wall better for keeping the warm in for longer?
- thick wall = warm for longer as it has a lower rate of energy transfer so a lower rate of cooling which is more beneficial
What does the thermal conductivity of a material tell you?
The rate at which it transfers energy through a wall with an area of 1m^2, thickness of 1m and a temp diff between the inside and outside of 1 degrees Celsius.
Is a higher or lower thermal conductivity of a wall better for keeping the warm in for longer?
- lower thermal conductivity is better = retains the higher temperature for longer as it has a lower rate of energy transfer = more beneficial
- high thermal conductivity = high rate of energy transfer
What is used for insulation on a space shuttle and how can igloos be warm?
- Aerogel as it’s very thin = big difference to the others
- ice has low thermal conductivity
What are the ways friction can be reduced?
- lubrication
- change shape of obj = reduce friction from air resistance = make something more streamlines
What is an efficient device?
- an efficient device is better at transferring energy between stores that do the job we want
- LED light bulbs are efficient
- more efficient device = operates at a lower power level = uses fuels more slowly
When is energy useful
- energy is only useful when it’s transferred from one store to a useful store
How is input energy lost?
- often to thermal energy stores by heating
- motor = kinetic store( useful) but also to thermal energy store of surroundings (wasted =can’t easily be used or collected back)
What is the total energy input?
- useful energy output + wasted energy = total energy input
- less energy that’s wasted = more efficient the device
How can we calculate efficiency of a device?
- Efficiency = useful output energy transfer (J)/ input energy transfer (J)
- could times by 100 for a percentage
- can never be higher than 1 (100%)
When does an energy dissipate?
- energy is transferred away from the device to its surroundings, the energy spreads out and becomes less concentrated = dissipates
What does the width of an arrow on a sanely diagram represent?
- width = amount of energy transferred
How can we increase efficiency?
- reduce wasted energy by:
- insulating
- using materials that reduce unwanted transfers
- use improved technology (LED)
How do you convert from Kelvin to degrees Celsius?
Add 273 to the degrees C to get the temperature in K
What are the units of thermal conductivity and how can you prove it?
- (Energy transferred / time) is directly proportional to (area(temp1 - temp2)/ length)
- (Energy transferred / time) = k times (area(temp1 - temp2)/ length)
- k = power x length / (area(temp1 - temp2))
- watts x metres / metres squared x Kelvin
- cross multiply watts x metres x Kelvin = W/mk
- l is thickness and k is constant
Why might LEDs not reduce your electricity bill?
- LEDs = more efficient than incandescent light bulbs = waste less energy by transferring less heat energy to the surroundings
- room lit by LEDs will feel so colder = so you may turn on the heating
- any electricity you have have saved in powering LEDs (instead of less efficient incandescent bulbs) will be used instead in heating the house
Why does metal feel colder than wood at the same temperature?
- metal is a good conductor of heat energy
- when you touch the metal blade, it conducts heat energy away from your skin at a faster rate than wood does
- so you feel it as being colder as your skin has less heat energy
How is energy transferred through water by conduction causing its temp to rise?
- water particles on surface gain ke as they heat up
- start vibrating and bump into neighbouring particles, passing the ke along
- causes all particles in the bucket to gain energy and therefore heat up
- this continues until extra energy is spread evenly across all particles and temperature
Why would a pan cool down at a slower rate if it was shiny and silver in colour?
- energy transferred would be more slow from the thermal energy of the pan
- shiny surface is a poor emitter of radiation compared to black Matt surfaces
What’s the benefit of adding oil on your bike chain?
- oil = lubricant so reduces friction and makes it more efficient
- reduces friction and therefore unwanted energy transfer through heating
- less work needs to be done to move bike
How could you make a house more energy-efficient?
- loft insulation = reduce energy transfer by conduction and convection
- hot water tank fitted with insulating jacket to reduce conduction and keep water hot
- all windows double glazed = reduce energy lost through conduction by windows
- cavity walls = reduce conduction and convection
- central heating pipes converted in insulating material to stop energy lost by conduction and convection and painted white to reduce energy lost by radiation
What happens to a ball’s speed when the height that which it’s dropped from increases?
- if the height it’s dropped from increases then the speed at which it hits the ground also increases
- this is as the higher the ball is raised, the more energy is transferred to its gravitational potential energy store
- and when the ball os dropped, the energy is transferred to the ball’s ke store which is proportional to the speed squared
- however it might be lower as we calculated it without air resistance so all the energy from gpe=ke but resistance reduces its speed
How much is one 1N/cm in N/m?
1N/cm is 100n/m
What is the typical acceleration of a cyclist and car?
- cyclist = take around 7 seconds to acc to 6m/s
- car = around 4 seconds to acc to 25m/s and 2 secs if accelerates to around 10 m/s
