Physics Flashcards
How to charge insulators
Friction
Negatively charged e- are rubbed off on one material and onto the other
What is charging caused by
Gain or loss of electrons
When is a material negatively charged
Gaining e-
When is a material positively charged
Losing e-
Force equations
F= ma
F = momentum/time
F = area* pressure
Work done = force * displacement
Energy eqautions
Kinetic energy = 0.5mv^2
Gravitational Potential energy = mgdeltah
Energy transferred = VIt
Power equations
P = work done/time P = energy transferred / time P = force * velocity
Electrical Equations
Q = It V = IR P = IV = I^2R = V^2/R V = E/Q
Electrical symbols and standard units
R - resistance (ohms) P - power (W, watts) Q - charge (C, coulombs) V - voltage (V, volts) I - current (A, amperes) E = energy, J
SI prefixes
Giga - 10^9 Mega - 10^6 Kilo - 10^3 Hecto - 10^2 Deci - 10^-1 Centi - 10^-2 Milli - 10^-3 Micro - 10^-6 Nano - 10^-9
Uses of electrostatics
Paint sprayers
Dust Precipitators
Defribillators
Photocopiers
Paint sprayers as a use of electrostatic
Spray can charged and charges drops Drops repel (like charge) but attracts object to be spray painted - gives fine spray and even coat
Dust Precipitators as use of electrostatics
Cleans up emissions
Smoke particles get -vely charged by wire grid
Attracted to +vely charge plates and stick together
When heavy enough fall off or knocked off
Risks of static electricity
Charge can build up on clothing made from synthetic materials - cause spark, dangerous near inflammable gases or fuel fumes
Fuel flowing out of filler pipe, paper dragging over rollers, grain shooting out of pipes - lead to spark –> explosion
Role of earthing
Prevents dangerous sparks by providing an easy route for the static charges to travel into the ground
Charge unable to build up
Earthing
Connecting a charged object to the ground using a conductor e.g copper wire
Current
Rate of flow of e- around circuit
Flows from +ve to -ve
Only flows through component if there’s a voltage across it
Voltage
Driving force that pushes current around
Energy that each charged particle transfers passing through a component
Higher voltage, more current
Resistance
Slows down flow of e- (-ve to +ve)
Circuit diagrams
Ammeter, component and resistors placed in series - any order
Voltmeter parallel to component under test
AC vs DC
AC - constantly changing direction, AC of 5Hz = changes direction 5 times (mains supply), gives regularly repeating wave on oscilloscope
DC - current flowing in only direction (cells and batteries), straight line on oscilloscope due to same voltage
Calculating frequency of AC supply (Hz)
1/time period
Diode
Device made from semi conductor material e.g. silicon
Lets current flow freely through it only one direction (high resistance in reverse)
Can convert ac to dc
V-I graph for fixed resistor
y=x
Proportional
V-I graph for filament lamp
S shape
As filament temp increases, the resistance increases
NTC thermistors
Temp dependent resistors
As temp increases, resistance decreases
Useful temp detectors
LDR
Light dependent resistors
Resistance falls with increase in LI
Useful in automatic night lights
Series circuits
Components connected line to line, end to end
Total pd of cells shared by diff components
Current flows from +ve to -ve and is the same everywhere
Total resistance is sum of all resistances
Parallel circuits
Each component is separately connected, removal or disconnection wont affect others
Pd is same across all components
Current shared by diff components
Total resistance is ALWAYS less than branch w/ lowest resistance
Magnetic field
Region where magnets, magnetic materials and wires carrying current experience a force
Field lines go from North to South
Stronger the field, closer field lines
Where’s the magnetic field strongest
North and south poles
Induced magnets
Magnetic materials that turn into magnets when they’re in a magnetic field
Loses magnetism when magnetic field is taken away
Magnetic field encourages electrons to align, forming north and south pole
Which materials can become induced magnets
Nickel
Iron
Steel
Cobalt
Soft magnetic material
Quick and easy to magnetise and demagnetise. Lose magnetic properties quickly when left field e.g iron
Hard magnetic material
Harder to magnetise
Retain magnetic properties for way longer/permanently
V diff to demagnetise e.g. steel
Creating a magnetic field
When current is flowing through a wire a magnetic field is created
Made up of concentric circles perpendicular to wire
Right hand thumb rule can show the direction of the field
Strength of field increases w/ vicinity to wire and increases w/ current
Solenoid
Coil of wire
Magnetic field of solenoid
Outside - same as bar magnet
Inside - strong and uniform
Increasing magnetic field strength around electromagnet
Increasing current
More turns on solenoid
Adding core of soft iron inside the solenoid - iron becomes induced magnet and magnetic fields combine
How does current flow
Positive to negative
Motor effect
When a current-carrying wire in a magnetic field experiences a force
Factors affecting size of force due to motor effect
Size of current (+ve)
Magnetic flux density (shows strength of magnetic field +ve)
Length of conductor inside the field (+ve)
When will a wire feel the full force
At a right angle to the magnetic field
Experiences some force at other angle but none parallel
Calculating size of the force acting on conductor created by motor effect
When current is at 90 degrees use F=BIl
B - magnetic flux density (tesla -T)
I - current (A)
l - length (m)
Fleming’s left-hand rule
First finger pointing in direction of field
Second finger pointing in direction of current
Point out thumb so it 90 degrees to both fingers - motion
Fleming’s right hand rule
Use thumb to point in direction of current and fingers will tell you directon of field
Construction of dc motor (dynamo)
Loop of wire current flowing in opp directions on either side placed in a magnetic field
Creates moments on both lhs and rhs and the loop rotates, split ring commutator allows it to keep rotating past 90 degress (reverses direction of current) - generates direct current
Factors affecting magnitude of force in dc motors
Size of current
Strength of magnetic field
Put more turns on the coil
Applications of electromagnets
Loudspeakers
Bell
Relay
When is a voltage induced in a conductor (electromagnetic induction)
When a magnetic field changes or a wire cuts magnetic field lines
When can cause a magnetic field to change
The conductor is moving into, or out of, a magnetic field
A magnet is moving towards, or away from, the conductor
The magnetic field is being varied
Factors affecting magnitude of induced voltage
Using a stronger magnet (+ve)
Rate of change of strength of mf (+ve)
Increasing no. turns (+ve)
Speed of movement (+ve)
Factors affecting direction of induced voltage
Direction of movement
Reversed when direction of cutting mf lines reverses, increasing mf in a coil change to one decreasing (and vice-versa)
What can induced voltage produce
Induced current if the conductor is connected in a complete circuit
This current will prodce a magnetic field that opposes the change that whch induced the current
Conductor
Material which allows an electrical current to pass through it easily. It has a low resistance
Ac generator
Device producing a potential diff
Consists of a coil of wire rotating in a magnetic field
Operation of ac generator
Coil is rotated in the magnetic field inducing a current in the coil which flows into an external circuit
Requires 2 split rings
As one side of the coil moves up through the mf, pd is induced in one direction, this reverses when rotation continues and the coil moves down
Creating ac
Factors affecting maximum output voltage (+current)
Rate of rotation (+ve)
Strength of mf (+ve)
Coil has greater area (+ve)
No. turn on the coil (+ve)
Graphical rep of output voltage of ac generator
Sine graph w/ induced potential diff on y and time on x
Why is there no induced voltage when the coil is at 0 and 180 degrees
Coil is moving parallel to the direction of the magnetic field
Applications of electromagnetic induction
Car engines use an alternator to keep the battery charged and an electrical system while engine works
Hydroelectric dams
Step up transformer
Increases voltage of ac
Higher pd and more turns on 2’ coil
Useful as decreases current and resistance so less energy is lost by heating - power lines
Step down transformer
Decreases voltage of ac
Higher pd and more turns on 1’ coil
Reduces pd of supply before reaching hmes
Components of a transformer
Ac input leading to primary coil
Iron core w/ mf
Secondary coil leading to ac output
Uses generator effect
Transformer eqns
Vp/Vs = np/ns
V - potential diff
n - no. turns
VsIs = VpIp (power output at 2’ = power input at 1’)
I - current
Consequence of 100% efficiency
Total transfer of electrical power
Need for high voltage in electrical power transmission
Higher voltage, lower current –> lower resistance losses –> lower energy losses
Types of forces
Weight Normal contact Drag (air resistance) Friction Magnetic Electrostatic Thrust Upthrust Lift Tension
Hooke’s law
F = ke
F - force (N)
k - spring constant (N/m)
e - extension (m)
Spring constant
Measure of the stiffness of a spring up to its limit of proportionality or elastic limit
Higher k, stiffer spring
Limit of proportionality
Point beyond which Hooke’s law is no longer true when stretching a material
Elastic limit
Furthest point a material can be stretched/deformed while being able to return to its previous shape, becomes inelastic after
Force extension graphs
Directly proportional until limit of proportionality - rate slows down (non-linear extension and inelastic deformation)
What happens when a spring is extended/ compressed
Work is done
Provided there’s no inelastic deformation work done = elastic potential stored
Elastic potential energy
E = 1/2 k x^2
E - energy (J)
k - spring constant (N/m)
e - extension/compression (m)
Mass
Property that resists change in motion (inertia)
What happens at terminal velocity
Object moves at a steady speed in constant direction because the resultant force acting on its 0
Stages of falling through a fluid
Object accelerates downwards (gravity)
As speed increases as does frictional forces
At terminal velocity weight is balanced by frictional forces
Inertia
Tendency of an object to continue in its current state (at rest or in uniform motion)
Newton’s 1st Law
Object remains in same state of motion unless a resultant force acts on it
Examples of Newton’s 1st law
Runner experiences same air resistance as thrust
Object at terminal velocity experiences same air resistance as weight
Newton’s Second Law
Resultant force = m x a
a is proportional to resultant force and inversely proportional to mass
Inertial mass
Ratio of force over acceleration
Measure of how diff it is to change velocity
Factors affecting air resistance
Speed
Surface area (+ve)
Air flow - turbulent (+ve) vs streamlined
Equation for momentum
mass * velocity
Force is the rate of change of momentum (kgm/s)
Equation for work done
Force * distance
What happens hen force moves an object
Energy is transferred and work is done
Calculating % efficiency
Useful output/ total input * 100
Factors affecting rate of conduction
Temp diff Cross-sectional area Length (distance heat must travel) Substance between 2 objects (better/worse thermal conductor) Time
Heat vs temp
Temp is a measure of how hot something is - degrees
Heat is a form of energy - joules
Flows between things of diff temps
Transfer of heat
Conduction
Convection
Radiation
Good conductors
Metal
Poor conductors
Insulators - non metals and gases
Fluids
Anything that can be made to flow
When does convection occur
When particles w/ a lot of heat energy in a liquid or gas move and take the place of those w/ less heat energy
Why does convection occur
Liquids and gases expand when heated (higher kinetic energy) –> take up more vol
Also less dense so rises
What allows convection currents to work
Diff in differences of density of heated particles and cooler ones
Thermal radiation
Electromagnetic waves in the infrared region
Requires waves not particles - works through a vacuum
Radiation properties of dull, matt or rough surfaces
Good absorption and emission
Radiation properties of shiny surfaces
Poor absorption and emission
Good reflectors
Factors affecting radiation
Type of surface
Size - thin and flat > fat
Eqn for spp heat capacity
thermal energy / (mass * deltat)
Internal energy of a system
Total energy that its particles have in their kinetic and potential energy stores
What happens when a system is heated
Transfers energy to its particles (gain kinetic energy), increase in internal energy
Leads to change in temp or state
What does size of temp change depend on
Mass of substance
Spp heat capacity
Energy input
Latent heat
Energy needed to change state of a substance
Spp latent heat
Energy needed to change 1kg of a substance from one state to another w/out changing it’s temp
Spp latent heat of fusion
Spp latent heat for changing between a solid and liquid
Spp latent heat of vaporisation
Spp latent heat for changing from liquid –> gas
Spp latent heat eqn
E = mL
E - energy for change in state (j)
m - mass (kg)
L - spp latent heat (j/kg)
Gas temp
Increase in temp –> transfer of energy into the ke stores of the particles
Higher temp, higher avg energy (higher avg speed)
Outward gas pressure
Total force exerted by all particles in the gas on a unit area of the container walls
Increasing gas pressure
Incresed temp –> more ke –> more collisons
Decreased vol
Relationship between pressure and vol of gases
PV = constant
P - pressure
V - vol
Inversely proportional - valid for a gas of fixed mass at a constant temp
Density
Measure of how close together the particles in a substance are
Mass/vol
How does depth affect pressure of liquids
As depth increases as does no. particles above that point
Weight adds to pressure experienced at that point
Eqn for hydrostatic pressure
p = h rho g
p - pressure (Pa)
h - depth (m)
rho - density (kg/m^3)
g - gravitational field strength (N/kg)
Measuring density of irregular solid
Measure mass w/ balance
Fill eureka can (displacement can) just below spout and place solid inside can
Collect water that pours out and that is the solids vol and you can use rho=m/v
Measuring density of liquid
Place measuring cylinder on balance and zero it
Add 10 ml and record total vol and mass
Repeat until cylinder is full
Calculate density for each measurement and find avg
Representation of waves
Displacement on y and distance on x axis
Crests/peaks and troughs show maximum +ve and -ve displacement fom rest
Amplitude
Heigh of peak
Max displacement from eqm
Wavelength
Peak to peak or trough to trough
Frequency
No. complete waves passing in one second
1 Hz = 1 wave per second
1/period
Waves
Vibrations transferring energy by causing particles (or fields) to vibrate
Types of wave
Transverse e.g water ripples, EM waves, seismic S waves
Longitudinal e.g sound, seismic P and ultrasound waves
Longitudinal waves
Vibrations are parallel to direction of wave travel
Show areas of compression and rarefaction
Compressions
Regions of high pressure due to particles being close together
Occurs when particles in the medium are pushed closer as the wave passes
Particles move backwards and forwards between compressions
Rarefactions
Regions of low pressure due to particles being spread further apart
Occurs when particles in the medium are pulled further apart as the wave passes
Transverse waves
Vibrations are at right angles to direction of wave travel
Energy is transferred from left to right
Particles move up and down as the wave is transmitted through the medium
Wave period
Time taken to complete one cycle
Inversely proportional to frequency
Calculating wave speed
Distance/ time OR
Frequency * wavelength
When does reflection occur
At a surface
Refraction
Change in direction of a wave at a boundary of 2 transparent materials
Can cause optical illusions as the light waves appear to come from a diff position to actual source
When does light bend towards the normal
When light goes from a less dense medium to a more dense medium
When does light bend away from the normal
When light goes from a more dense medium to a less dense medium
Effects of refraction
Freuency remains the same
If waves slows, wave length decreases (proportional)
Effects of reflection
Wavelength, freq and speed stay the same
Doppler effect
Observed frequency of source is less or more than the true frequency
Faster observer approaches or recedes from the source, the greater the shift in freq. and wavelength
Production of sound waves
Vibrating source - these vibrations can travel through solids, liquids and gases
Speed travels 330m/s in air
Cannot travel in a vacuum - no particles to carry vibrations
Frequency of sound waves
High freq = high pitch
Low freq = low pitch
Amplitude of a sound wave
High amp = loud (more energy)
Low amp = quiet
Range of human hearing
20 Hz to 20kHz
Range of frequencies that’ll cause the ear drum to vibrate
Echoes
Reflection of sound waves at a surface
Ultrasound waves
Have a frequency higher than 20,000 H
Partial reflection
When ultrasound meets a boundary between substances some is reflected and some transmitted (and possible refracted)
Uses of ultrasound
Med - Prenatal scanning, breaking kidney stones
Industry - finding fault in materials and echo scanning
Cleaning jewellery - vibrations caused by waves shake apart dirt
How ultrasound scanning works
Time between emission of waves and detection of partially reflected ultrasound waves can be interpreted and used to determine locations of boundaries and form images of structures hidden from view
Echo sounding
Sonar used by boats and submarines, where sound waves help identify depth of water or location of objects in deep water
Properties of electromagnetic waves
Transverse waves
Travel at the speed of light
Components of EM spectrum
From lowest to highest energy and freq and longest to shortest wavelength
Radio waves Microwaves IR Visible light UV X-rays Gamma
Uses of radio waves
TV signals - long wavelength means they travel further in Earth’s atmosphere
Uses of microwaves
Cooking - waves absorbed by water molecules casing vibrations (heat)
Mobile phones - wavelength penetrates our atmosphere
Uses of IR
Optical fibre communication (TV remotes)
Uses of visible light
Seeing - only part of spectrum we can see
Uses of X-rays
Medical images of bones - absorption produces an image
Uses of gamma radiation
Killing cancer cells - highly penetrative
Sterilising food
Which parts of the EM spectrum can be harmful
The higher the frequency of the radiation, more likely its going to cause damage Microwaves IR UV X-rays Gamma
Hazards of microwaves
Internal heating of body tissues
Hazards of IR
Can cause skin to burn as is felt as heat
Hazards of X -rays and gamma rays
Damage cells causing mutations (cancer) and cell death
EM spectrum
Continuous spectrum of all the possible wavelengths of EM waves
Uses of UV
Energy efficient lamps and sun tanning lamps
Hazards of UV
Premature skin aging
Increased risk of skin cancer
Cataracts
Radiation dose
Measure of the risk of harm due to exposure to radiation
Measure of Sieverts
1000mSv = 1 Sv
Nuclide
More generic term for isotope
Used when referring to nuclei of DIFF elements and isotopes are used when referring to sev. diff nuclides of the SAME element
What does an unstable nucleus cause
Emissions. These are random
Types of emission
Alpha
Beta
Gamma
Alpha particles
4 He 2 nuclei so atomic no . 2 and atomic no. 4
Relatively big and heavy and slow moving (0.1c)
Strongly ionising
Don’t penetrate far and stopped v quickly
Deflected by magnetic and electric fields (attracted to -ve)
Beta particles
e- so increases atomic no. +1
V small and move quickly (0.8c)
Penetrate moderately before colliding
Moderately ionising
For every beta particle emitted a neutron converts to a proton
Deflected by magnetic fields and electric fields (attracted to +ve)
Gamma radiation
Photon w/ no mass and no charge V quick (c) Penetrate long way into materials Weakly ionising - tend to pass through atoms After an alpha or beta emission, nucleus sometimes has extra energy to get rid of so emits a gamma ray
Sources of background radiation
Radon gas Cosmic rays Medical X - rays Rocks and building materials Food
When do alpha and beta particles experience a deflecting force
When they move in magnetic fields - provided their motion isn’t parallel to the field
beta deflects more - lower mass, lower charge
Uses of ionising radiation
Smoke detectors - alpha
Tracers in med - short half-life gamma emitters
Radiotherapy - gamma rays
Sterilisation of food and surgical instruments - gamma rays
Radiotherapy
Used in conjunction w/ chemo to kill cancerous cells
High dosage of gamma rays directed carefully to treat cancers
Smoke detectors
Weak source of alpha radiation placed in detector, close to 2 electrodes
Causes ionisation and a current flows
Smoke absorbs radiation, current stops –> alarm sounds
Tracers in med
Iodine-131 is absorbed by thyroid and gamma rays can detect and indicate whether thyroid is working correctly
Only gamma and beta emitters w/ short half lives can be taken into the body - so radiation can pass out of the body
Sterilisation
High dose of gamma rays will kill all microbes
Irradiation allows food and plastic items to be sterilised w/ out boiling
If the radioactive source is inside the body …
alpha is most dangerous - easily absorbed by cells
beta and gamma less likely to be absorbed and will usually pass through it
If the radioactive source is outside the body ..
beta and gamma are most dangerous - penetrate skin and damage cells inside
alpha unlikely to react to living cells inside the body
Charging by induction
Neutral object placed near a charged object can become charged
Types of charging by induction
Magnetic - unmagnetized iron next to magnet
Electrostatic
Sparking
Occurs when the air between 2 objects becomes ionised by a large voltage and therefore starts conducting
Milliamp
1 * 10^-3
Microamp
1 * 10^-6
Kilohm
1 * 10^3
Megohm
1 * 10^6
Identifying poles on solenoids
If the current is circulating clockwise - south pole
Anti-clockwise - north pole
Electromagnet vs permanent magnet
E - can be switched on/off, vary strength of mf, can reverse polarity, made from soft magnetic material
Permanent - opposite
Electromagnet
Many turns of insulated wire wound onto soft iron
Current creates external magnetic field
Typical voltage output of a power station
11 kV or 33 kV
Transmission voltage in UK
275 kV or 400kV
SUVAT eqns
v = u +at
s = 1/2 (u+v)t
v^2 - u^2 = 2as
g on the Moon
1.6 N kg-1
g on Jupiter
26 N kg-1
g on the Sun
280 N kg-1
Convection vs conduction
Conduction - Can occur in solids, heat is transferred by microscopic motions of individual particles
Convection - Cannot occur in solids, heat is transferred by macroscopic motion of large no. particles
Converting density
1 g cm-3 = 10^3 kg m ^-3
1 kg m-3 = 10-3 g cm -3
Mechanical waves
Sound Ultrasound Seismic waves Water waves Waves on a string Wavs on a slinky
Incident energy =
Reflected energy + transmitted energy + absorbed energy
Source and observer approaching one another
Shorter wavelength
Higher freq
Source and observer moving away from one another
Longer wavelength
Lower freq
Using reflection to measure distances
d = vt/2
t - time for pulse to travel and return
v - speed of sound in medium
Absorption of EM waves
EM waves transfer energy from source to absorber
When absorbed, energy transferred to matter that absorbs them
Can cause heating, e- at surface to vibrate at freq. of waves, ionisation
Red light
Long wavelength
Low freq
Violet light
Short wavelength
High freq
c
Speed of light
3 * 10 ^8 m/s
Penetrating ability of alpha
Blocked sheet of paper and human skin
Penetrate few cm in air
Penetrating ability of beta
Typically blocked by thin metal
Not blocked by human skin
Penetrate up to sev. m in air
Penetrating ability of gamma rays
To block it to large extent requires sev cm of very dense material e.g. lead
Can penetrate up to hundreds of m in air
