Physics Flashcards
What’s convection?
It is a method of heat transfer which occurs in fluids (liquids and gasses)
When a fluid is heated, e.g. By a fireplace, the particles gain kinetic energy, vibrate more and collide with each other, causing them to space out and expand.
The fluid, due to expanding, becomes less dense and rises, moving to spaces with less kinetic energy. To fill it’s place by the cooler, denser fluid moves towards the fireplace.
The less dense fluid then loses energy, after moving away from the heat source, and becomes dense, losing kinetic energy, and falling. It then moves in to fill in the space.
This is a convection current.
They can occur small such as heating water in a beaker or large like causing on shore off shore drifts.
What is conduction?
A method of heat transfer in solids; faster in metals.
If one end of a solid is heated, the particles on that end gain kinetic energy and vibrate more. The energy and vibrations are passed on to neighbouring particles and this continues until all the particles are vibrating at the same speed and causing the object to heat up.
This occurs in metals faster as metals have free electrons which, as they move around through the metals, they carry kinetic energy by colliding with other particles, hence why metals are good conductors.
What’s radiation?
A method of heat transfer which can travel through a vacuum e.g. Heat from the sun reaching earth.
It’s energy transfer by electromagnetic waves.
All objects emit infrared radiation, the hotter the object, the more radiation given.
What’s evaporation?
When a liquid turns into a gas because, when the liquid heats up, the most energetic liquid molecules on the surface of the liquid become gas and enter the air due to gaining so much energy and moving away.
This means the kinetic energy if the molecules remaining is less so evaporation causes cooling.
What increases the rate of evaporation?
Increasing the surface area of the liquid
Increasing the temperature of the liquid
Creating a draught of air across the liquid’s surface
What’s condensation?
When a gas turns to a liquid
This usually takes place on cold surfaces such as windows as the gas particles lose kinetic energy and become denser, forming a liquid.
What increases the rate if condensation?
Increasing the surface area
Reducing the surface temperature
What does the rate of heat energy transfer depend on?
The material the object is in contact with
The object’s shape
The objects surface area
The temperature difference between the object and surroundings.
How can we maximise and minimise heat energy transfer?
Maximise energy transfer to keep things cool; minimise to keep things warm
Maximise:
Use good conductors
Painted dull black (better emitters)
Have air flow around them maximised
Minimise:
Use insulation
White and shiny materials
Prevent convection currents by trapping air in small pockets
What is specific heat capacity?
When a substance is heated, the energy transferred to it will increase the temperature.
The specific heat capacity is the amount if energy required to raise the temperature of 1kg of the substance by 1°C
Different substances have different s.h.c. The greater the specific heat capacity, the more energy required for for each degree celsius to change.
The greater the mass of substance being heated, the more energy required.
What is the equation for specific heat capacity?
E = m * c * ø
E is energy transferred (joules)
m is mass (Kg)
c is specific heat capacity ( J/Kg°C)
ø is temperature change (°C)
Many people want to save money by minimising energy transfer out of their homes. How can thi be done?
Fibreglass loft insulation - reduces conduction
Cavity wall insulation - reduces convection
Double glazing windows - reduces conduction
Draught proofing - reduces convection
Aluminium foil behind radiators - reflects infrared radiation back into the room
What is a U-value?
The U-value tells us how much energy per second passes through different materials.
The lower the U-value, the better the material is as an insulator.
(U-value = energy/s passing per m2 for 1°C temperature difference)
What’s the bed colour for a radiator?
Dull black
Why do solids have fixed volumes?
All of the particles are held together in fixed positions.
What are the main types of energy?
Magnetic Kinetic (movement) Heat Light Gravitational potential Chemical Sound Electrical Elastic potential Nuclear
(Remember by:
Most kids hate learning g.c.s.e. energy names)
What’s the rule of energy?
Energy can’t be created or destroyed, just transformed.
What’s kinetic energy?
Energy of a moving object due to it’s motion ( in joules).
What’s electrical energy?
Energy transferred by the by the movement of electrical charge.
What’s elastic potential energy?
Energy stored in an elastic object when work is done to change it’s shape.
What’s gravitational potential energy?
Energy of an object due to it’s position in a gravitational field e.g. A waterfall or energy collected from a dam.
What’s chemical energy?
Energy of an object due to chemical reactions such as eating food.
What is conservation of energy?
It isn’t possible to create or destroy energy, only to transfer it from one form or place to another.
This means the total amount of energy is always the same and it applies to all energy transfers, known as the conservation of energy.
E.g. When an object falls, gravitational potential energy is transferred to kinetic energy.
What is a machine?
Something that transfers energy from one place to another or from one form to another.
What is useful energy?
Energy which is transferred to the place and form we want it
e.g. Light from a light bulb.
What is wasted energy?
Energy which is not usefully transferred; not serving it’s purpose
e.g. Heat from a light bulb.
How is energy often wasted?
By friction between parts if a machine as it causes the surroundings to heat up.
How do we calculate the efficiency of an appliance?
Efficiency = (useful energy transferred by the appliance / total energy supplied to the appliance) * 100
In a light bulb, for every 25J supplied, 5J becomes useful light energy.
What’s it’s efficiency?
Efficiency = 5 / 25 (*100)
Efficiency = 20%
What’s the useful and wasted energy from a hairdryer?
Useful: heat
Wasted: sound
What’s a watt, kilowatt and kilowatt hour?
A watt is a unit of power. (W)
A kilowatt is 1000 watts. (kW)
A kilowatt hour is electrical energy supplied to a 1kW electrical device in 1 hour.
What can electrical appliances do?
Transfer electrical energy into whatever firm we need
e.g. An electric drill turns electric energy into kinetic energy.
What is the power of an appliance?
The rate at which it transfers energy.
How many watts are 12 kilowatts?
12,000 W
What is the equation for power?
P = E / t
P: power in watts (W)
E: energy in joules (J)
t: time taken (in seconds) for the energy to be transferred.
What’s the equation for energy?
E = P * t
P: power in watts (W)
E: energy in joules (J)
t: time taken (in seconds) for the energy to be transferred.
What’s the equation for the cost of an electrical appliance?
Total cost = number of kWh * cost per kWh
The price of 1kWh of electrical energy is 9p. How much does it cost to use a 60W electric light for 4 hours?
60 * 4 = 240
240 / 1000 = 0.24
0.24 * 9 = 2.16
2.16 p
What does cost effectiveness mean?
Getting the best value for money.
To compare the cost effectiveness of different appliances, what other costs must we consider?
The cost of buying the appliance The cost of installing the appliance The running cost The maintenance cost Environmental cost The interest charged in a loan to buy the appliance.
What is the payback time?
The time it takes for an appliance or installation to pay for itself in terms of energy savings.
