4th Form Physics notes (AG) Light Statement: Light travels in straight lines. This is known as µrectilinear propagation¶. Evidence: The formation of shadows provides evidence that t his is the case.
Light and shadows Light travels out from a light source as millions of rays. In the diagram below, some of the rays are prevented from reaching the screen by the opaque object and a shadow is formed.
Object
Shadow
The pinhole camera This provides more evidence for rectilinear propagation. The image is inverted. A small pinhole produces a sharp image, but also a dim one. A larger pinhole produces a brighter, less sharp image. Multiple pinholes produce multiple images.
Reflection Mirrors reflect light rays. We can use a ray box to investigate what happens to a ray when it reflects. o
NORMAL ± Line drawn at 90 to the mirror ANGLE OF INCIDENCE ± angle between normal and incident ray ANGLE OF REFLECTION ± angle between normal and reflected ra y
LAWS OF REFLECTION: 1. Angle of incidence = angle of reflection 2. Normal, incident, and reflected rays are in the sa me plane Remember to label the direction of light with arrows.
Refraction Light travels at different speeds in different MEDIA. This can cause it to change direction. When light enters an µoptically denser medium¶, it slows down and turns towards the normal. When light enters a less dense medium, it speeds up and turns away from the normal. When the incident ray is along the normal, it does not change direction ± but it does change speed. Glass, Perspex, and water are optically denser t han air.
Snells law The REFRACTIVE INDEX (n) is a measure of the optical density of a material. The refractive index of a vacuum or air is 1. The great er the value of n, t he slower the light travels in the material. For light travelling from air into another material, the relationship between refractive index, angle of incidence and angle of reflection is:
. n is constant for f or light passing from air to a
given medium. This is known as Snell¶s Law.
Total Internal Reflection For light passing from glass to air, there is a strong reflected ray, and a weak reflected ray. As the angle of incidence increase, the angle of refraction increases until the refracted ray travels along the boundary. This angle of incidence is called the CRITICAL ANGLE. If the angle of incidence is greater than the critical a ngle, ALL the light light is refracted. The refractive index is related to the t he critical angle according to the following equation: Sin c =
Optical fibres Optical fibres are fine strands of glass. The fibre is covered with µcladding¶ that is less optically dense than the fibre to ensure Total Internal Reflection (TIR) takes place. There is usually a thin protective pr otective coating around the cladding. Glass fibres can be used to transmit data using digital signals. Light entering the end of the fibre undergoes TIR.
No (or very little) energy is lost so as much light leaves the fibre as enters it. Optical fibres are also used in ENDOSCOPY. One bundle of fibres takes light inside the body to illuminate it. A second bundle carries the image out to the surgeon.
Images In a real image, rays from the object pass through it; hence it can be projected onto a screen. In a virtual image, i mage, no rays from the object pass through it; hence it cannot be projected onto a screen.
Properties of the image in a plane mirror y
Virtual ± no rays pass through it (so it can NOT be projected onto a scr een)
y
Same size as the object
y
Laterally inverted ± right is left, but top is still top
y
As far behind the mirror as the object is in front (and a line joining the object and o image crosses the mirror at 90 )
Volume and density The quantity of space occupied by an object object is called its VOL UME. The SI unit for volume is 3 m . This is a large volume so we often use cubic centimetres (cc) or litres (1000 cc) instead. The volume and mass of an a n object are related relat ed by by the property called density. Density =
In symbols: D =
3
3
Density is measured in kg/m , mass is measured in kg, and Volume is measured in m .
Forces prep school information 1. 2. 3. 4.
A force can be thought of as a push or pull of one body on another There are various types of force for ce (e.g. gravitational, gravitational, electrostatic, etc.) We use arrows to show the size and direction of forces. Force is measured in Newtons (N)
Forces new information 5. If the forces of an object are balanced, it does not accelerate or decelerate. The RESULTANT force is zero. 6. If the forces are NOT balanced, it will accelerate or decelerate. The RESULTANT force is NOT zero.
Weight, mass, and gravity The earth¶s gravitational field acts on all objects close to the earth. The resulting force towards the centre of the earth is called the WEIGHT of that object. The weight of an object depends on its MASS, and also on the gravitational field strength (g). Smaller planets have weaker gravitational fields so things weigh less on smaller s maller planets, even though the mass is the same. sa me. Mass is the same everywhere in the universe, but weight is not. Weight (N) = Mass (kg) x Gravitational Fiel d Strength (N/kg) W
=m
xg
µg¶ is the symbol for gravitational field strength. On earth, g = 9.8 N/kg (9.81«)
Pressure y
Pressure =
y
Pressure can be measured in N/m2 or Pascals.
y
A force distributed over a large area creates lower pressure than the same force acting on a small surface area.
y
If objects are said to be µblunt¶, they really have a larger surface area than a µsharper¶ object with a smaller surface area.
In Physics, we usually assume that t hat forces act at points. In real life, this never happens. Forces are always spread over areas. The force divided by the area is PRESSURE. The unit of 2 2 2 pressure is N/m or Pascal. Pressure (N/m ) equals force (N) divided by area (m ). P=
A high pressure is applied when a force is concentrated on a small area. A lower pressure is applied when a force is spread out over a large area.
Stretching springs When a force is applied to a (helical) spring, its length increases. For most materials, the change in length is PROPORTIONAL to the force. This means that: y
If you double the load, the extension is doubled
y
The graph is a straight line through thr ough the origin
When the force is removed, the spring returns to its original length. This is ELASTIC BEHAVIOUR.
Eventually, the spring (or length of wire or rubber band) reaches its elastic limit and equal increases in force produce reducing increases in length. This is INELASTIC (PLASTIC) BEHAVIOUR. Up to the elastic limit: F = kx Where F = Force, x = extension (=current length - original length). k is the spring constant and equals the gradient of the F-x graph.
1
This is Hooke¶s Law for an elastic object [springs and wires]: Load is proportional to extension.
Brownian motion Q: What causes pollen grains to move? A: y
Water is made up of particles (atoms and molecules)
y
Water molecules are too small to see
y
They are in RANDOM motion
y
They collide with the pollen grains
y
These are larger and visible
y
This causes BROWNIAN MOTION
Structure of matter There are three ³states´ of matter. These are SOLID, LIQUID, and GAS. The KINETIC THEORY helps us understand how solids, liquids, and gases behave. This states that matter is made up of tiny particles in constant motion {only at Absolute Zero will particles not be in motion}.
Changes of state Solid to liquid ± melting Liquid to gas ± evaporation or boiling
Solids and liquids similari similarities ties and difference di fferencess Particles in a liquid have a random ra ndom motion within a close-packed structure. structure. Particles in a solid vibrate about fixed positions within a clos e-packed regular structure.
Types of motion VIBRATION / OSCILLATION ± Motion t o and fro about a fixed position 1
This is essentially the stiffness of the spring. It is measured in N/m, i.e. how many Newtons of force it takes to extend the spring in question by a metre.
TRANSLATION ± motion between two distinct distinct positions ROTATION ± CIRCULAR MOTION about a NOMINAL position.
Static Electricity Electrical conductors allow electrons to move about freely within them. Metals are good conductors. Insulators do not allow electrons to move about freely. Plastics are insulators. Insulating materials can be charged by FRICTION. Positive means there are fewer electrons than positive charges. The material has LOST electrons. Neutral means there are equal numbers of electrons and positive charges (earth is neutral). Negative means there are more electrons than positive charges. The material has GAINED electrons. LIKE charges repel, ad UNLIKE charges attract. REMEMBER ± FOR THE TOPIC OF STATIC ELECTRICITY, ONLY ELECTRONS MOVE (in solids ± unlike liquids in electrolyte where positive and negative charge).
Practical uses of electrostatic charge Charge can be supplied by the electricity supply, instead instead of by friction. Practical uses include: 1) Photocopying [aka xerography] xerography] y
Toner (powdered (powdered ink) is attracted t o charged areas on a drum
y
This is then transferred to paper and heated
2) Electrostatic precipitators y
Ash receives negative charge (from negative wires) as it r ises
y
It is attracted to positive plates
3) Inkjet printing 4) Paint spraying
Dangers of electrostatic charge A very large charge may cause sparks and this can be dangerous, e.g. when refuelling an aircraft. Charge can build up due to friction between the fuel and the pipe so the aircraft and the tanker must be earthed 2 during fuelling to avoid sparks.
Shuttling ball experiment A ping-pong ball is coated in conducting paint. It carries negative charges (free electrons) from the cathode (left-hand plate) to the anode (right-hand plate). The ball then shuttles to and fro at a steady rate. If this rate is big enough, the microammeter shows a steady current reading. The higher the rate of shuttling, the larger the current, as more charge is transferred 2
Connected with a large neutral body such as the Earth to neutralise it.
per second. This experiment shows that an electric current (I) is the rate of flow of charges (Q). Hence, current can be defined as the rate of flow of charge. Current (I) =
Charge (Q) = Current (I) x time (t)
Units Current ± ampere ± remember 1A = 1000 mA Charge ± Coulombs Time ± seconds
Measuring current and voltage Current is measured by an ammeter in series. An ideal ammeter has no resistance. Voltage is measured using a voltmeter in parallel. An ideal voltmeter has infinite resistance. Which is a) easier to measure and b) why? [It is therefore easier to measure voltage because, since it is measured in parallel, it is not necessary to disconnect anything.]
Current rule (Kirchhoffs first law) The total CURRENT flowing into any JUNCTION in a circuit is equal to the total current flowing out of the junction. This explains why the current is the same at all points in a series circuit.
Voltage rule (Kirchhoffs second law) The battery voltage is equal to the sum of the other voltages around any LOOP / ROUTE / PATH in a circuit. This explains explains why: wh y: y
Voltages around a series circuit add up to t he battery voltage
y
Voltage is the same across components in parallel
5th
Form Physics Notes (AG)
Calculating Speed Average speed = Speed =
Speed is a SCALAR quantity because it only has magnitude. Velocity is a VECTOR quantity because it has magnitude and direction.
Mass is scalar. Weight, as all forces, forc es, is vector.
Acceleration Acceleration is a vector quantity ± it has magnitude and direction. Deceleration (or retardation) is a negative acceleration. Avge acceleration =
We sometimes simplify this to: Accelerat ion = a=
V = FINAL VELOCITY U = INITIAL VELOCITY [Don¶t useµs¶ for speed because it actua lly represents µdisplacement¶] Unit of acceleration is: m/s/s (not technically correct but ok in exa m) OR m/s2 OR m/s-2.
isplacement-time ime graphs Displacement-t y
Straight line means constant velocity
y
Slope (i.e. gradient) = velocity
y
Curve means acceleration (or deceleration)
y
Instantaneous velocity is the slope of the tangent at that point
Velocity-time graphs y
Straight line means constant acceleration
y
Slope = acceleration
y
Curve means acceleration is changing Area under curve/line = distance travelled
y
N.B. Displacement (s) = distance as a vector quantity.
Newtons 2 nd Law of Motion What are the 2 things that affect the (rate of) acceleration of an object? Resultant/unbalanced force and mass. Force (N) = Mass (kg) x Acceleration (m/s/s) F = ma
If the UNBALANCED (i.e. resultant) force is zero, then t he object will NOT be accelerating. accelerat ing. If the unbalanced force is in the direction of motion, motion, it will accelerate. a ccelerate. If the unbalanced force
is in the opposite direction to the motion, it will decelerate. Why does a ball decelerate on the way up and accelerate on the way down? Because the resultant force (i.e. weight and some not very significant air resistance) is in the opposite (going up) / same (going down) direction to the motion. For a constant mass, (unbalanced) Force is proportional to acceleration. The gradient of a force-acceleration graph is equal to t o the mass. If the unbalanced force is constant, mass is inversely proportional to acceleration.
Stopping distance for cars Stopping Distance = Thinking Distance + Braking Distance. Thinking distance is the distance travelled during the reaction time (i.e. before the brakes are 3 applied). It is affected by speed and also by age, drugs, alcohol , distractions, tiredness, etc. Braking distance is the distance travelled after the brakes are applied. It is affected by mass and speed and also by road conditions, design and maintenance of the brakes, tyres, etc. N.B. Note spelling of BRAKING.
Gravitational Potential Energy (gPE) The change of gPE of an object depends on its mass (m), the gravitational field strength (g), and the change in height (¨h): ¨gPE = mg¨h (Joules)
Kinetic Energy The Kinetic Energy of an object depends on its mass (m) and on its velocity (v): KE = ½.m.v
2
Energy Transfers and the Principle of Conservation of Energy Energy cannot be created or destroyed, but it can be transferred into other forms. There are many situations where KE is transferred into gPE and vice versa. If the amount of one form of energy reduces, then the amount of other types of energy must change by an equal amount. This is because the total a mount of energy remains constant.
Calculations involving KE/PE transfers In problems involving gPE/KE transfers, the key principle is that: The total energy remains constant If the only energy transfers are between gPE and KE: Reduction in gPE = gain in KE Or 3
Although technically alcohol is a drug
reduction in KE = gain in gPE or generally ¨gPE = ¨KE
Worked Example A pendulum of mass 0.1kg is released from a height of 0.1m above its lowest possible point. a) What is the reduction in gPE between release and its lowest position? ¨gPE = mass x g x ¨h = 0.1 x 10 x 0.1 = 0.1 Joules b) What is the maximum speed? KE is
max when gP E E is min ¨KE = ¨gPE KE max E is greatest max is when reduction in gP KEmax = 0.1 Joules Speed is max when KE is max 2 KEmax = ½m v 0.1 = 0.5 0. 5 x 0.1 x v2 v = 1.4m/s c) What is the speed when the height is 0.0 5m below the high point? ¨gPE = ¨KE = 0.5 0. 5 x 0.1 = 0.05 0.05 Joules = ½m v2 2 0.05 0.05 = 0.5 0.5 x 0.1 x v v = 1m/s
WO RK GET SOME DON E! Work is done whenever energy is transferred. In fact work done is EQUAL to the energy transferred. Work done (Joules) = Energy transferred (Joules) GCSE calculations involve MECHANICAL work done. Mechanical work is done when a force moves. Work done (J) = Force (N) x distance (m) W = F.d KE is the work done when« Change in gPE is the work done when« The KE of a body is the work done when the body is accelerated to a velocity v (from rest). The increase in gPE of a body is the work done in raising the height of a body.
POW ER - but dont d ont take all day! The faster a car climbs a hill, the more power it consumes. Power is t he rate of doing work. It is also the rate at which energy is transferred. Power =
=
Energy transferred/time taken
Unit of Power is the Watt (W), 1W = 1J/s [The equation can be written: P =
. This is OK for GCSE but should really be written in
full].
Work, Energy, and Stopping Distances The work done by the brakes is equal to the Kinetic energy transferred (to heat). Work done = Braking force (F) x braking distance (s) 2 KE transferred = Initial KE ± Final E = ½mv ± 0 F.s = ½mv 2
Efficiency Efficiency =
Or
Or
Efficiency has no units but can be expressed as a %. It must be less than 1 (or 100%).
Centre of gravity, stability, stability, and tipping and toppling The weight of a body acts through its centre of gravity. The stability of an object is related to the angle through which it is ³tipped´ before it ³topples´. Increasing the size of the base and/or reducing the height of the centre of gravity will make an object more stable (because this increases the angle). An object will not ³topple´ (i.e. is stable) providing the centre of gravity is above a point on the base.
MOMENTS Definition: The moment of a force about a point depends on the force and the perpendicular distance from the force to that point.
Moment of a force (N.m) = force (N) x p erpendicular distance to point (m) Principle of Moments: If an object is balanced (i.e. in equilibrium)« Sum of clockwise moments = sum of anticlockwise moments. This is the principle of moments.
Perpendicular distance
Moments calculations presentation Is the body in equilibrium? If so, sum of the clockwise moments = sum of anticlockwise moments. Where are you taking moments about? IN EQUILIBRIUM MOMENTS = 7 MOMENTS ABOUT X
@7
MOMENTS
E.g. In eqbm. 7 M=
7
M
(F1 x d1) = (F 2 x d2 )
Current and Voltage in circuits Current is the rate of flow of charge: I=
(I is in Amps, Q in Coulombs, t in seconds). Voltage is the energy transferred per coulomb of charge: V=
(V in volts, E in joules, Q in coulombs. I = Current in Amp(ere)s (A)
Current rule (Kirchhoffs 1st Law) The total current flowing into a junction is equal to the total current flowing out of the junction.
Voltage rule (Kirchhoffs 2nd Law) The supply voltage is equal to the sum of all the other voltages around any loop in a circuit.
Resistance Resistance reduces the flow of charge in a circuit. The greater the Resistance, the less the current (for a particular voltage). Resistance in a metal wire is caused by collisions between moving electrons and stationary atoms. These collisions are the means of energy transfer (or power consumption). Without resistance, no energy is transferred. Longer wires have more resistance than shorter wires. Thicker wires have less resistance than thinner wires. The resistance increases if the temperature increases. The units of resistance are OHMS. We calculate resistance from the formula: for mula: R (; (;) =
V R
A
V
I
Bulbs, wires, resistors and diodes In a wire at constant temperature (or a resistor), current is proportional to voltage and the resistance is constant. A thinner, longer wire has more resistance. In a filament lamp, the current is not proportional to voltage ± resistance increases a s voltage voltage increases. Diodes allow current to flow in one direction only (+ to -).
I
I
I
VV
V
Resistor
Filament bulb
V Diode
LE Ds and Thermisto T hermistors rs y
Semiconductor devices.
y
More energy on device reduces Resistance (increases c urrent).
R
R
T
L
Power Power is the rate of energy transfer. Electrical Power is: Power = Voltage x Current P (W) = V (V) x I (A)
E.g. I = 2A V = 3V R=
= 1.5 1.5;
P = V.I = 6W
A
V Household Electricity A direct current is always in the same direction but an alternating current changes direction. The electricity in our homes is AC with a frequency of 50 Hz. What do we pay for in our electricity bills? The answer is ENERGY. Electricity companies measure energy in Kilowatt hours (kWh). Energy = Power x time Energy in Joules = Power in Watts x Time in seconds Energy in kW.h = Power in kW x Time in hours
1 UNIT of energy is 1kWh. It is the energy converted when a 1kW appliance operates for 1 hour. The cost of electricity is simply the number of units consumed x the cost per unit. E = V.I.t
Electrical Safety Appliances are fitted with FUSES as safety devices. The ³size´ of the fuse (in Amps) should be just above the normal operating current. Why is this? Energy is supplied into our homes using a LIVE and a NEUTRAL wire. The live wire varies from +230 Volts to -230 Volts 50 times every second sec ond.. The neutral is maintained constantly at zero Volts. Circuits are completed when the live and neutral wires are connected to appliances; current flows and energy is tra nsferred. Why must the live and neutral wire be insulated from each other? A third wire is connected to ³earth´. Usually, no current passes thr ough this wire. wire. Why do we have an earth wire which normally carries no current? FUSES and CIRCUIT BREAKERS prevent FIRE due to electrical faults. Circuits can overheat when too much current flows. A fuse is designed to ³blow´ before overheating of cables, etc. can occur. A circuit breaker uses a simple electromagnet to switch off the current when it is too high. It can be reset. The earth wire together with a fuse (or circuit breaker) prevents ELECTROCUTION. If a metal casing becomes ³live´, a very high current flows ³to earth´. eart h´. This blows the fuse and the appliance stops working. DOUBLE INSULATED appliances don¶t need an earth wire because any metal parts are completely surrounded by an insulating polymer.
Heat Transfer When heat energy is transferred, the temperature of an object may change. Heat energy can be transferred by CONDUCTION, CONVECTION, RADIATION or EVAPORATION.
Conduction Heat causes atoms to vibrate and pass on their energy to neighbouring atoms. If there are free electrons, these can transfer the energy more rapidly by bypassing immediate neighbours. Why are metals good conductors and how are they different from insulators?
Convection When a FLUID is heated, it expands, becomes less DENSE and rises. Colder, denser fluid sinks to take its place. The process continues as convection currents are established. Why doesn¶t convection happen in solids?
Radiation All objects radiate heat energy (even very cold ones!) but the power radiated depends on the temperature. Radiated heat travels as electromagnetic waves (just like light) ± it travels at the speed of light through a vacuum and can be reflected and focussed. Dull, black surfaces are good emitters and absorbers of heat. Shiny, white surfaces ar e poor emitters and absorbers. What do the following facts tell us a bout heat radiation? A) There is life on Earth B) Thermal imaging cameras can be used in the Ar ctic or the Sahara Describe experiments to show that: A) Heat can be reflected and focussed B) A white surface emits less heat than t han a black one
Waves All wave motion involves OSCILLATION. Waves transfer energy but without any flow of material. There are two types of waves: TRANSVERSE waves have oscillations perpendicular to the motion of the wave (e.g. water [surface], light). LONGITUDINAL waves have oscillations in the direction of motion of the wave (e.g. sound).
Describing waves WAVELENGTH is the distance between any point on a wave and its equivalent point on the next wave. AMPLITUDE is the maximum distance that a point moves from its resting position when a wave passes. FREQUENCY is the number of waves passing any point each s econd. econd. It is measured in Hertz (Hz). It is also the number of complete oscillations per second by a particle in the wave. The amplitude depends on the energy of the wave. The PERIOD of a wave (T) is the ti me for one complete wave to pass measured i n seconds. It
is also the time for one complete oscillation (T = ; f = )
Key Points from the MMSS Exercise Wavefronts are always at 90 degrees to the direction of movement of the t he waves (e.g. the rays for light waves). The frequency of a wave is NOT affected by reflection or refraction.
The Wave Equation v = f P v is speed in m/s. f is the frequency in Hz and P is wavelength in m. This equation works for all typed of wave. Derive the wave equation starting with the formula for speed.
Sound Sound travels as longitudinal waves through a medium ± it cannot travel in a vacuum. Echoes are caused by the reflection of sound. Sounds travel faster in solids than in liquids or gases because the particles are closer together. Sound can be reflected, refracted and diffracted [not on syllabus] (just like any other wave) and it obeys the wave equation. The frequency range for human hearing is 20 Hz ± 20 000 Hz. How could you measure the speed of sound? By measuring the time taken for a sound to travel a known distance, the speed can be calculated (speed =
). This can be applied to echoes (reflections of sound).
The Elec r
ic spect ru ru et ic
y
The di di erent erent t es of el elect ectromagneti romagneticc waves form a conti con tinuous nuous spect spec trum wit with h a range of wavel wavelengt ength and frequency. They transfer energy at a t the same speed in free space [i [ i.e. in a vacuum]
y
They are all a ll transverse waves whi wh ich can be ref lect ected, refract refracted and di diffract ffracted.
y
Elect r
et ic ic waves Dangers
Elect ectromagneti romagneticc
Waves carry energy and cause hea ting ting when absorbed. They can a lso be ioni onising and this can damage liv living tissue tissue and cause cancer in the case of Ult Ultrav raviiolet, x-rays and gamma rays (whi (wh ich are very penet pene trati rating). ng). Infra red and mi microwaves are more li ely to cause burns when absorbed. R adi adio waves carry tivelly safe. very small sma ll amount amounts of energy and are rel re lative Discuss scuss//exp expllain the di differences bet between the harm caused by: by: a) UV ± gamma b) Ir ± mi microwaves
S
e uses of e-m waves
X-ray
machi machines use wavel wavelengt engt hs that hat penet penetrat rate tissue tissue but but do not not penet penetrat rate bon b onee so s o ³x ³x -ray -ra y phot photographs´ show broken bones and frac tures. Concent oncentrated beams of X rays and gamma rays can be used to treat reat cancer by dest destroyi roying abnormal abnorma l cell cells. s. Gamma rays can be used as ³ tracers´ in medi medicine. They can al a lso be used to st ster ilise ilise food and medi medical cal equi equi pment pment. Why is it dangerous to spend too long in the sun? Why is darker sk in less li like kelly to be damaged? Why are very long wavel waveleng engtths needed for l for long di distance communi communica cati tion? on? How do mi microwaves get get around this probl problem? Whatt safe Wha safetty precauti precautions ons shoul shou ld be taken when usi us ing X-rays and Gamma rays? Radiow Radiowaves: aves: Frequency (Hz): < 109 Wavel Wavelength (m): > 0.3 Size scal scale: Mount ountains, buil build ding Uses: y Transmit Transmit R adi adio and TV programmes bet bet ween different fferent places. The longer wavel wavelengt engths radi radiowaves are ref lect ected from the ionosphere, an el e lect ectr icall cally y charged layer i ayer in the Ear th¶s upper at atmosphere Dangers: y No real rea l danger, alt although hough too much TV can make you µsquare eyed¶
Microw crowaves: aves:
Infrared:
Frequency (Hz): 109 - 3x1011 Wavel Wavelength (m): 0.001 - 0.3 Uses: y Sat ellite llite communi communicati cation, on, as they pass easil eas ily y through the Ear th¶s atmosphere y Cook ing, because microwaves are absorbed by wat wa ter mol molecul ecules, causi causing them to heat heat up Dangers: y A bsorbed by wat wa ter i er in cell cellss where heat heat is rel released my DAMAGE or KILL CELLS
Frequency (Hz): 3x1011 - 3.9x1014 Wavel Wavelength (m): 7.6x10-7 - 0.001 Uses: y Gr ills, ills, toast oasters and heat hea ters y R emot emote cont control rol for TV and VCR ¶s ¶s y O pti ptica call Fi bre bre communi communicati cation on Dangers: y A bsorbed by sk in and FELT as HEAT. Excessi xcessive amount amounts can cause BUR NS
Visible: ible:
Ultraviol traviolet et waves:
Frequency (Hz): 3.9x1014 - 7.9x1014 Wavel Wavelength (m): 3.8x10-7 - 7.6x10-7 Scal Scale si size: Bact acter ia Uses: y Seei eeing y O pti ptica call f i bre bre communi communicati cation on Dangers: y Excessi xcessive amount amounts can damage the reti retina na
Frequency (Hz): 7.9x1014 - 3.4x1016 Wavel Wavelength (m): -9 -7 8x10 - 3.8x10 Scal Scale Si Size: Viruses Uses: y Fluorescent uorescent lamp and secur ity ity codi coding, where surfaces coat coa ted wit with h speci special pai paint absorb UV and emit emit LIGHT Dangers: y below. Darker Passes through sk in t o the TISSUES bel sk in all allows ows less penet penetrati ration on and provi provides more prot protecti ection on y HIGH DOES can KILL NORMAL CELLS and LOW DOSES can cause CANCER
X-rays:
Gamma
rays:
Frequency (Hz): 3.4x1016 - x1019 Wavel Wavelength (m): 6x10-12 - 8x10-9 Scal Scale si size: Atoms Uses: y Produce shadow pi pictures of BONES and METALS, mat mater ials X-rays do not no t easil easily y pass through Dangers: y Pass through SOFT TISSUES, although lthough SOME is ABSORBED
Frequency (Hz): > x1019 Wavel Wavelength (m): < 6x10-12 Scal Scale si size: Nucl uclei Uses: y K illing illing cancer cell cellss y K illing illing bact bact er ia on food and surgi surgical cal inst nstrument ruments Dangers: y Pass through SOFT TISSUES, although lthough SOME is ABSORBED
6th
Form Phys Physics ics Notes (GSM)
Pressure y
y
P=
F
P
Units:
A
or Pascals
P= =
A
= Density of fluid = d
h
(d =
=
)
=
P=dxhxg
A
Kinetic Theory of Matter SOLID
LIQUID
GAS
melt
evaporation
break bonds
or boiling
tight pack vibrate fixed position Ek relatively low
further apart attraction less Ek higher move around
much further apart Ek much higher forces are negligible move around at high speed
If the atoms are [totally] stationary, this is equivalent to E k = 0, and a temperature of 0K, or o -273 C is reached. There is a range of kinetic energies in the atoms of a liquid. During evaporation, some of the fastest molecules leave the surface of the liquid. At the boiling point, all of the particles are effectively evaporating, but at the same temperature.
Gas Pressure The atoms in a gas have a range of speeds and kinetic energies. All of the atoms are moving in random directions. When they hit each other and the walls of the container, they will exert a force, and change direction. The forces due to the individual atoms will be spread over an
area, and since P = , the atoms of the gas will exert a pressure.
The pressure due to a gas is used in both external and internal combustion engines. The energy of the moving atoms is used to drive a piston which in turn can be used to turn the wheels of a steam engine, car, etc. There are three macroscopic properties for a gas ± pressure, volume, and temperature. In this experiment, the pressure and temperature variations are investigated, while the volume is kept constant. The graph is extrapolated to find the temperature at which the pressure would be o zero, i.e. the molecules have stopped moving. Kelvin moved the pressure axis to -273 C, and redefined this as 0 Kelvin. The graph is now a straight line passing through the origin, and therefore: P
absolute
w
temp
= constant
Boyles Law Experiment
Bourdon Pressure Gauge to measure pressure of air
Fixed mass of dry air at constant temperature
Oil Ruler to measure µvolume¶ of air To compression pump
5
3
P / x10 Pa
V / cm
2.5 2.5 2.3 2.1 2.0 1.85 1.85 1.68 1.5 1.5 1.4 1.3 1.0
16 17 19 20.5 20.5 22 25 25 28 30 33 42
/ x10-2
6.3 5.9 5.3 4.9 4.5 4.5 4.0 3.6 3.3 3.0 2.4
y
P x V = constant
y
Px
y y
Pw @P
PxV 40.0 39.1 39.9 41.0 40.7 42.0 42.0 42.0 42.9 42.0
The results show that within experimental error, P x V = constant, and the graph shows that pressure w
(This can also be stated pressure inversely proportional to volume) This is Boyle¶s Law.
= straight line through (0,0)
= constant x
y=m
(0,0)
x+c
y
Pressure inversely proportional to volume
y
P w T (v constant);
y
P x V = constant (temp. constant) a third law states that
y
V w T (P constant)
y
PV = constant;
y
= constant
@
= constant
= constant
= constant; = constant
IDEAL GAS EQUATION
Radioactivity and Nuclear Physics Atomic nuclei are made up of protons and neutrons. These comprise nearly all of the mass of the atom. The orbiting electrons have negligible mass by comparison. Nuclei are represented A by the Z notation (A = neutrons + protons; Z = protons). Nuclei which contain the same
X
number of protons but different numbers of neutrons are called isotopes. Some of t he isotopes of a given element will be unstable. To become more stable, they emit radioactive radiation ± 4 these include , , and radiation. is a helium nucleus, 2 . is an electron emitted 1 1 0 when a neutron turns into a proton and an electron. 0 1 -1
He n p + e
Gamma radiation is a very high frequency electromagnetic wave. The emission of radioactive radiation always takes place in an attempt to t o improve the stability stability of the t he remaining nucleus.
Background Radiation This is a totally random process and is present everywhere and comes from all directions. Some of the sources of background radiation are building materials (especially granite), cosmic radiation (from the Big Bang), the Sun, etc. Background radiation is greater in mountainous areas because of all the granite rocks. In radioactive experiments, the background count in Becquerels (Bq) should always be subtracted from the actual count to reduce the corrected count. count.
Half-life The half-life of a radioactive sample is the time taken for half of the radioactive nuclei to decay. This decay is totally random and the initial number of radioactive nuclei is immaterial. Its value can vary for different isotopes between fractions of a second and millions of years. The curve is called an exponential exponential decay curve. cur ve.
Rutherford Alpha Particle Scattering Positively charged alpha particles were directed at a thin gold foil. It was expected that the alpha particles would pass straight through the foil. However, about 1 in 2000 bounced back in the original direction. The conclusion was that the atom was not solid as in the kinetic theory model, but was made up of a tiny positive nucleus (the electron shells are a very long way from this nucleus). -15 The nucleus is approximately 10 -15 m in size, the atom is about 10 -10 m, i.e. about 10 5 or 100,000 times bigger.
Geiger, Marsden, Rutherford Expt. (1909) gold foil vacuum
most pass straight through
beam of particles zinc sulphide screen microscope some deviated by large angle U angle U
Nucl ear
Energy
radi radioacti oactive ve decay energy, but but at a slow rat rate. In a nucl nuc lear react reactor, the rat rat e of decay is accel accelerat erated by bombardi bombard ing nucl nuclei with ith other par tic ticles, typi ypicall cally y neut neutrons whi which are not not charged. E.g.: .g.:
ural Natural
23 92
144 90 1 U + 10 n 236 U B a + Kr + 2 n 92 6 36 0+ energy very unst unstabl able
y
S pont pontaneous
y
Process
y
NB:
neut neutrons avail availab ablle for other f issi ssion reacti reactions ons
f issi ssions st star ts reacti reaction on
call ca lled ed a chai cha in reacti reaction on
23
U
must must be above a cer tain cr itica iticall mass el else too many neut neu trons escape
withou ithoutt causi caus ing fur ther f issi ssion y
NB:
238 nat natural ural urani uranium consi cons ists mai mainly of t of two isot sotopes,
(less than 1%)
U (over 99%) and
23
U
Basic layo ut of Advanced Gas Cooled Rea ctor ( AGR ) cont controll rolled ed chai cha in reacti reaction on takes pl place and thermal hermal energy is rel released at at a steady rat rate.The energy from the chai cha in reacti reaction on is used to boil boil wat water. The resulti resu lting ng steam is used to turn turbi urbines whi which dr ive generat generators to produce el elect ectr icity. ity. A
Nucl ear Rea ctor NUCLEAR FU R FUEL ELEMENTS
± Urani Uranium di dioxi oxide, wit with h nat natural ural urani uranium enr iched wit with h ext extra
urani uranium-23 . GR AP APHITE COR E ± Slow neut neutrons are more effecti effec tive ve at causi causing f issi ssion. Graphit Graphitee blocks are used to sl slow down the neut neutrons ± t ± the graphit graphitee act acts as a MODER ATOR . CONTR OL R ODS ± R ate of f issi ssion process cont con troll rolled ed by rai ra ising or lower ing boron bor on-st -s teel eel cont control rol rods. Boron absorbs neut neu trons. When the rods are rai ra ised, more neut neu trons are avail ava ilab ablle to cause f issi ssion and core temperat emperature r ises. The react reactor can be shut shu t down do wn by keep k eepiing the rods lowered.
COOLANT ± Heat from the fission reaction is carried away by carbon dioxide at high pressure. This heat is used to make steam to drive turbines and hence dynamos to generate electricity. WASTE PRODUCTS ± Spent fuel rods are removed from the core and sent to a reprocessing plant. Here, unused uranium is separated from the radioactive waste products together with small quantities of Plutonium-239. This is used as the fuel in fast breeder reactors and in the production of nuclear weapons ± it is the most hazardous substance known.
Investigation Investig ation of Factors Affecting the Strength of an Electromagnet A
Paperclips Strength of electromagnet depends on: 1. Current 2. Number of turns / length 3. Nature of core (best with soft iron)
Domain theory of Magnetism
N S
S
demagnetised
N
S
S
N
S
N
S
N
S
N
S
N
S
N
S
N
S
N
S
N
S
N
partially magnetised N fully magnetised
Flemings Left hand rule Thi This is used to predi predict the directi rection on of moti motion on of a conduct conduc tor whi which is carryi carrying a current current through a magneti magneticc f ield. The foref inger and second f inger of the lef t hand are set set at r ight gh-t angl angles to each ot other and to the thumb. Foref inger i e l d ( NS)
Second
f inger
u r r e n t
Thumb r u s t
(+-) The interacti eraction on of t of the magneti magneticc f ield due to a current current and a second magneti magneticc f ield is used in numerous devi devices, e.g. e.g. elect ectr ic mot motor, loudspeaker, anal ana logue televi evision, et etc.
Loudspeakers An
el elect ectr ic current current is fed into the coil coil.. There is an interacti eraction on bet between this el elect ectromagneti romagneticc f ield and the permanent permanent magneti magneticc f ield due to the pol pole pi pieces. Usi Using Fl Flemi eming¶s lef t hand rul rule, the coil coil will move in and out out as shown on the di diagram. Musi usic is made up of of a ser i ser ies of alt a lterna ernati ting ng patt patterns, erns, and these cause the cone to move backwards and forwards. Thi Th is movement movement causes the surroundi surrounding ai a ir mol molecul ecules to move backwards and forwards, so that hat a sound wave is produced.
CURR EN ENT
Elect romagnet ism Maxwell axwell,, Faraday, Fl Flemi eming.
+ current current goi going away
current current towards
Maxwells
Right Hand Corkscr ew Rule
Righ Rightt thumb in di directi rection on of conventi conventiona onall current current (+-); -); f ingers of r ight ght hand in the directi rection on of magneti magneticc f ield.
coil coil face current current anti anti-c -cllockwi ockwise
ric Moto r The Elect ric Thi This uses the interacti eraction on of an el elect ectromagneti romagneticc f ield in the coil coil with ith the permanent magneti magneticc f ield from the two permanent permanent magnet magnets. The current current is reversed every ha lf revol revolution tion by the split split r ing commut commutator. Thi This ensures conti continua nuall rot rotation tion in one di directi rection on because it makes sure that hat the current current always f lows eit either her cl clockwi ockwise or anti anticclockwise in the coil coil.. The rat rate of t of turni urning and the power of t of the mot motor can be changed by us ing st stronger magnet magnets, larger current currents, and a sof t iron core. In a practi practica call mot motor, there will will be more than one coil coil,, so that hat the moti motion on is made smoot smoother. permanent permanent magnet magnet
carbon brushes
armat armature
split split r ing commut commutator
Electromagnetic induction When there is relative motion between a conductor and a magnetic field, a voltage (or electromotive force, or emf) is induced and a current will flow. The size of the induced emf and current depends on: ± 1. The rate at which the magnetic field is cut 2. The strength of the magnetic field 3. The length of wire affected by the field Michael Faraday discovered this around 1830.
S
N
galvanometer (sensitive ammeter)
When the magnet is brought towards the coil, the induced current flows in such a direction that it opposes the motion (i.e. a north pole is induced in the coil). The galvanometer deflects to the right. When the magnet is withdrawn from the coil, a south pole is induced, and the galvanometer deflects to the left. The induced current flows in such a direction as to oppose the motion of the magnet. This is called Lenz¶s Law, and it ensures that the Principle of Conservation of Energy is obeyed. Fleming¶s Right Hand Rule can be used to predict the direction of the current.
Electromagnetic Electromagne tic braking
induced magnetic force
strong magnet
aluminium tube
weight
When the magnet is moving the aluminium tube is a conductor in a moving magnetic field. As a result, an emf and current are induced in the aluminium tube. These are in a direction to oppose the motion. The resultant force (weight-induced force) is much smaller than the weight alone, and so the magnet falls at a slow, steady speed. This type of electromagnetic braking has various uses, e.g. electric trains, some theme park rides, and protection systems in lifts. Another use is in speedometers.
AC Generator This is very similar to the motor, except that it does not involve a battery. The coil is turned by some external force, e.g. steam, in a gas-fired power station, the turning blades of a windmill, etc. When the coil is in the position shown, it is cutting the magnetic field at rightangles, and so the maximum emf and current are induced. When the coil has moved through 90o, it is then moving parallel to the magnetic field, i.e. no longer cutting the field, and the induced emf and current will be zero. It can be shown that the emf and current are sinusoidal or cosinusoidal [sin/cos waves].
brushes N
- V+ S
slip rings U
V
Transformer theory Current and voltage are only induced in the secondary coil when the magnetic field is changing in the soft iron core. This requires AC in the primary coil. It can be shown that: ±
=
or
Also, by conservation of energy: ± input power = output power I p x V p = Is x Vs Transformer equation:
=
(=
)
=