OCR Physics P1a-h

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P1a - Heating houses

Temperature = a measure of how hot something is (chosen scale e.g. degrees Celsius)

Heat = a measure of energy (absolute scale e.g. joules)

The amount of energy needed to raise the temperature of an object depends on:

the mass of the object

the change in temp required

material it's made from

Total energy supplied = energy supplied per second x number of seconds

SPECIFIC HEAT CAPACITY:

The amount of energy required to change the temperature of 1kg of material by 1 degree Celsius

Energy (J) = Mass (kg) x Specific heat capacity (J/kg degrees Celsius) x Temp change (degrees Celsius)

SPECIFIC LATENT HEAT:

The amount of energy required to change the state of 1kg of material without changing it's temperature

Energy(J) = Mass (kg) x Specific latent heat (J/kg)

During melting/boiling of water, the energy supplied is used to break intermolecular forces as the water changes state, this explains why the temperature doesn't change

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P1b - Keeping homes warm

Materials that allow energy to flow through them quickly are called conductors e.g. metal is a good conductor

Materials that allow energy to flow through them slowly are called insulators e.g. most non-metals are good insulators

Air is a good insulator because the particles are far apart

Types of insulation:

Fibreglass roof insulation - reduces conduction and convection by trapping layers of air between fibres

Reflective foil on walls - reduces radiation by reflecting heat energy back into the room

Foam cavity wall insulation- reduces conduction and convection by trapping air in the foam

Double glazing - reduces conduction and convection by trapping air between panes of glass

Draught excluder - reduces conduction and convection by keeping as much warm air inside the house as possible

Payback time (years) = cost of installing

annual saving

Cavity wall insulation is made up of an inner and outer wall separated by a space full of air. The heat energy passes through the wall by conduction, heating the air in the cavity. The heat is then carried away from the wall by convection. This heat loss can be reduced by filling the cavity with foam (a good insulator).

Efficiency = useful output energy (J) x 100

total input energy (J)

Conduction = transfer of heat energy through a substance from a hotter region to a cooler region without any movement of the substance

Convection = transfer of heat energy from hotter regions to cooler regions by the movement of particles

Radiation:

Hot objects emit mainly infra-red radiation. Dark matt surfaces emit more radiation than light shiny surfaces at the same temp, because dark matt surfaces are better absorbers.

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P1c - A spectrum of waves

Transverse wave has the following features:

Amplitude - maximum disturbance caused by the wave

Wavelength - distance between 2 corresponding points (from one crest to another)

Frequency - number of oscillations per second

The electromagnetic spectrum:

Radio waves

Microwaves

Infra-red

Visible light

UV

X-rays

Gamma

The wave equation:

Wave speed = Wave length x Frequency

Reflection = when light bounces of a surface

Refraction = when light goes from air to a more dense substance, it slows down and changes direction

Diffraction = when waves pass through a gap and spread out, maximum diffraction occurs when wavelength is same as width of gap

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P1d - Light and lasers

1) If angle of incidence is below critical angle, light is refracted

2) If angle of incidence is above critical angle, light is totally internally reflected

3) If angle of incidence is equal to critical angle, light travels along glass-air boundary

Optical fibres are long, flexible and transparent. In an optical fibre pulse of light are totally internally reflected. They are used to carry binary code.

Advantages of Wireless signals: (e.g. radios, mobile phones and laptops)

Signals available 24 hours a day

No wiring needed

Items can be portable

Various signals:

Light - travels very fast, small loss of signal - cant be used for wireless signalling as it doesn't diffract well

Electrical - Can be sent along wires - signal deteriorates

Radio waves - Can be used for wireless signalling as the waves can diffract - diffraction leads to signal loss

Morse code is a digital signal as the light is either on or off

Lasers produce a a narrow beam of monochromatic light (one colour)

All their light waves:

have the same frequency

are in phase with each other

have low divergence

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P1e - Cooking and communicating using waves

The amount of radiation that is absorbed or emitted from a surface depends on:

surface temp

colour

texture

Microwaves are used to heat materials, for satellite communications, mobile phones and radar. They:

are absorbed by water and fat molecules which cause them to heat up

can penetrate about 1cm into food

can cause burns when absorbed by body tissue

can travel through glass and plastic

are reflected by shiny metal surfaces

Infra-red rays are used to heat material and in remote controls. They are:

used to heat the surface of the food in surface

reflected off shiny surfaces

absorbed by black objects

Microwaves are absorbed by water and fat molecules in outside layers of food, increasing kinetic energy of the particles. Energy is then transferred to the centre of food by conduction or convection. Infra-red is absorbed by all the particles on the surface of food, increasing their kinetic energy. Energy is then transferred to the centre of food by conduction or convection.

Microwave signals can be lost or affected by:

large obstacles

poor weather conditions

large areas of surface water

the curvature of the earth

interference between signals

Some of the problems can be reduced by:

limiting distance between transmitters

positioning masts on top of hills or tall builidings

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P1f - Data transmission

Infra-red is used in many commonplace devices, for example:

remote controls

automatic doors

short-distance wireless data links for computers or mobile phones

burglar alarms

security lights

Analogue signals = used to transmit data. They very continuously in amplitude.

Analogue signals can have any value within a fixed range of values and are very similar to the sound waves of speech or music

Digital signals = used to transmit data as a series of pulses. They don't vary in amplitude.

They only have two states (on and off), there are no values in-between.

Two or more digital signals can be sent down the same optical fibre at the same time - MULTIPLEXING

Analogue signal - poor signal quality due to interference

Digital signal - high signal quality as interference is easily removed

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P1g - Wireless signals

Advantages of wireless communication:

No connection to a land-line required

Portable, convenient

Disadvantages of wireless communication:

Aerial is needed to pick up the signal

Satellites can be used for global communication:

A signal is sent from a ground station transmitter dish to a satellite receiver dish

A return signal is then sent by the satellite transmitter to a ground receiver dish

The ionosphere is an electrically charged layer in the earth's upper atmosphere. Longer wavelength radio waves are reflected by the ionosphere, enabling radio and television programmes to be transmitted between different places.

DAB Radio = Digital Audio Broadcasting

Advantages:

More stations are available

Less interference with broadcasts from other stations

Disadvantages:

Audio quality is not as good as FM broadcasts

Some areas cannot receive DAB

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P1h - Stable earth

Earthquakes produce shock waves, which can travel inside the earth (seismic waves)

P-waves:

Primary waves

Longitudinal

Travel through solids and liquids

S-waves:

Secondary waves (slower)

Transverse

Travel through solids but not liquids

Seismic waves are detected by seismometers

Factors the contribute to global warming:

Increased energy use in homes and industry

Increased carbon dioxide emissions from fossil fuels

Deforestation

Weather patterns are affected by human activity for example:

Dust from volcanoes reflects radiation from the sun back to space causing cooling

Dust from factories reflects radiation from cities back towards earth causing warming

The sun produces electromagnetic waves, including UV radiation

Prolonged exposure can cause:

sun tan

sun burn

cataracts

premature ageing

skin cancer

People with darker skin tones have a reduced chance of developing breast cancer, as they have an increased amount of melanin in the skin. The melanin absorbed the UV rays and protects the delicate tissues.

To reduce the risk of sunburn or developing skin cancer we can:

stay out of the midday sun

keep skin covered

reduce or avoid use of sunbeds

always use sunscreen

The Ozone layer is a layer of Ozone gas found naturally up high in the earth's atmosphere, which prevents too many harmful UV rays reaching the earth.

The discovery of the hole in the ozone layer had an impact globally:

Legislation was passed in many countries to ban the use of CFCs in fridges

Old fridges and fridge freezers containing CFCs must be disposed of

CFCs are no longer used in aerosol cans