physics mocks: Flashcards
speed:
->the distance travelled per unit time
-if something is changing, it’s accelerating (near the Earth it is constant)
average speed = total distance/total time
s (m/s) = d (m) /t (s)
velocity:
->speed in a given direction
acceleration:
->change in velocity per unit time
acceleration = change in velocity/time taken
a (m/s^2)= v (m/s) -u/t (s)
equation for the final speed:
(final speed)^2 = (initial speed)^2 + (2 x acceleration x distance)
v^2 = u^2 +2as
distance-time graphs:
-the gradient is velocity
-negative gradient = returning back to the starting point
-a horizontal line = stationary
-if the distance = 0, it is back at the starting point
-curved line = velocity is changing, and it is accelerating
velocity time-graphs:
-the gradient is acceleration
-negative gradient = deceleration
-if speed = 0, it is at rest
-horizontal line = constant speed
-area under the line = distance travelled
-curved line = acceleration is changing
vectors:
have magnitude and direction
scalars:
have just magnitude
examples of scalar quantities:
distance, speed, time, energy…
examples of vector quantities:
displacement, velocity, acceleration, force
effects of forces:
-forces can change the speed, shape or direction of a body (measures in newtons (N))
eg: gravitational, electrostatic
friction:
forces between two surfaces which impedes motion and results in heating
eg: air resistance is a form of friction
how to find the resultant force:
-add together if in the same direction
-subtracted if in the opposite direction
Newton’s first law:
states that an object has a constant velocity unless acted on by a resultant force
Newton’s second law:
force = mass x acceleration
f=ma
mass & weight:
-measure of how much matter is in an object, measured in kg
-weight is a gravitational force
weight = mass x gravitational field strength
W = mg
-gfs on Earth is 9.8N/kg
terminal velocity:
-initially, there is no air resistance and the only force acting on it is weight
-as it falls, it accelerates which increases its speed and hence air resistance
-this causes the resultant force downwards to decrease
-therefore, the acceleration decreases
-eventually they are equal, opposite and balance so there is no resultant force
-so, there is no acceleration and the terminal velocity is reached
deformation:
-elastic deformation is when the object returns to its original shape when the load has been removed
eg: a spring being stretched
Hooke’s Law:
states that for a spring, F = kx where F is the force applied to the spring, k is the spring constant, and x is the extension
what does Linear and Non-Linear represent in a Hooke’s Law diagram?
linear (straight line) force extension graph: -Elastic deformation following Hooke’s law:
-the point it stops being linera is called the limit of proportionality. From then on, it does not obey Hooke’s Law, and gradient becomes k
non Linear (curved line) force extension graph:
-deformation not following Hooke’s Law
-after this region, it will fracture
general waves properties:
-transfer energy and information without transferring matter
-the particles oscillate (vibrate) about a fixed point
transverse waves:
-have peaks and troughs
-vibrations are perpendicular to the direction of travel
-eg: light
longitudinal waves:
-consists of compressions (particles pushed together) and rarefractions (particles moved apart)
-vibrations are in the same direction as the direction of travel
-eg: sound
amplitude:
the distance from the equilibrium position to the maximum displacement
wavefront:
the front of a wave, or the same point on each wave
frequency:
-the number of waves passing through a point per second
-frequency of a wave is equal to the reciprocal of the time period
-measured in Hertz (Hz):
frequency = 1/time period
f = 1/t
wavelength:
the distance between two adjacent peaks on a wave
time period:
the time taken for one complete wave to pass a point
speed of wave:
speed = frequency x wavelength
v = f x λ
the Doppler Effect:
-if a wave source is moving towards to an observer, there will be a change in the observed frequency and wavelength due to the Doppler effect
-this is because wavefronts either get bunched together or spaced apart
eg: a siren of an ambulance is high-pitched as it approaches you, and low-pitched as it goes away
reflection:
-all waves can be reflected when they travel from a medium of low optical density (such as air) to one much higher optical density (such as glass)
-the law of reflection says:
angle of incidence = angle of reflection
-frequency, wavelength, and speed are all unchanged
refraction:
-all waves can be refracted, which is when the speed of a wave changes when it enters a new medium
-if the wave enters a denser medium, its speed decreases and it bends towards the normal
-if the wave enters a less dense medium, its speed increases and it bends away from the normal
-in all cases, the frequency stays the same but the wavlength changes. As a result, the velocity must change
electromagnetic waves + spectrum:
-travel at the speed of light in vacuum
-can be reflected, refracted or diffracted
-speed = 3x10 (to the power of 8) m/s
(waves) RMIVUXG
low frequency-> high frequency + energy
longer wavelength<-shorter wavelength
radio waves:
-red
used for radio and television communications
-long wavelength and are reflected by the ionosphere
microwaves:
-orange
used for satellite transmissions and in cooking
-can cause internal heating of body tissues-use an oven
infrared radiation:
-yellow
used in heaters and night vision equipment
-can cause skin burns - protective clothing
visible light
-green
used in fibre optics and photography
ultraviolet light:
-blue
used in fluorescent lamps
-can cause damage to surface cells and blindness - sunscreen + sun glasses
x-rays:
-indigo
-used to observe internal structures of objects and materials + medical applications
gamma radiation:
-violet
used in in sterilising food and medical equipment
can cause cell mutation, leading to tissue damage or cancer
light:
transverse waves and can be reflected and refracted
what is the critical angle?
the angle of incidence which causes the angle of reflection to be 90º so that the light refracts onto the boundary
< c.a = reflection > c.a = refraction
refractive index = 1/ sin(critical angle)
n = 1/ sin(c)
total internal reflection:
-when the angle of incidence is greater than the critical angle, light is totally reflected within the same medium
-the light must also be travelling from a more optically dense medium into a less optically dense medium (eg: glass to air)
-light will keep reflecting
optical fibres:
-used to transfer information by light, even when bent
-they are used extensively in medicines and communications
sound:
longitudinal waves can be reflected and refracted
-the range of audible frequencies for a healthy human ear is 20 Hz to 20000 Hz
oscilloscope:
an oscilloscope connected to a microphone can be used to display a sound wave and find its the frequency and amplitude
-greater amplitude of sound wave = the louder it is
-greater the frequency of a sound wave = higher pitch
energy transfers: (8)
chemical, kinetic, gravitational, elastic, thermal, magnetic, electrostatic and nuclear
mechanically:
e.g. when gravity accelerates an object and gives it kinetic energy
electrically:
e.g. when a current passes through a lamp and it emits light and heat
by heating:
e.g. when a fire is used to heat up an object
by radiation:
when an object emits electromagnetic radiation
energy:
is always conserved, total energy before = total energy after
efficiency = useful energy output/total energy input x 100%
-sankey diagrams can be used to represent the transfer of input energy into useful energy and wasted energy
conduction:
-main method of thermal energy transfer in solids
-metals are extremely good at conducting heat
-non-metals are poor at conducting heat whilst liquids and gases are extremely poor (insulators)
-substance is heated, then atoms start to vibrate more and bump into each other-transferring energy from atom to atom
-delocalised electrons can collide with atoms helping to transfer vibrations through material and heat better
convection:
-main way that heat travels through liquids and gases (can’t in solids)
-when a fluid (a liquid or a gas) is heated:
-molecules push eachother apart- making fluid expand
-this makes the hot fluid less dense than the surroundings
-hot fluid rises, and the cooler fluid replaces it
-eventually, hot fluid cools, contracts and sinks back down again
-resulting motion = convection current
radiation:
-heat transferred by infrared
-the hotter the object, the more infrared radiation it radiates
-colour of object affects how well it emits and aborbs radiation
-black objects-best at emitting + absorbing radiation
-shiny objects-worst at emtting + absorbing radiation
work:
is done when a force moves something through a distance
-work done = energy transferred
work done = force x distance
W =Fd
conservation of energy:
-when GPE turns into kinetic energy as It accelerates downwards
kinetic energy = 1/2 x mass x speed^2
gpe = mass x gravitational field strength x height
power:
rate at which work is done in a given time
power = work done/time taken
P = W/t
renewable energy:
energy which can be replenished as quickly a it is used
-all have potentionally infinite energy supply
- more costly
-less reliable
non-renewable energy:
-resources can only produce energy some of the time (eg: when it’s windy)
-Fossil fuels: reliable, can produce large amounts of energy in a short time. Produces Green House gases + pollution
-Nuclear: reliable, produces no harm (above) a large amount of energy is produced from a small amount of fuel.produces dangerous radioactive waste
energy transfers in the generation of electricity:
-in burning fossil fuels: chemical energy in chemical bonds
-in nuclear reactors: nuclear energy in atomic nuclei
-in a solar cell: light energy from the sun
-in geothermal nergy: heat energy from the Earth’s core
-in wind energy: kinetic energy from the moving wind
-in HEP: kinetic energy of moving waves
the properties of a circuit:
-current is the flow of charge round the circuit, electrons carry the charge (-) and will only flow through a component if there is voltage across that component (unit: ampere- A)
-voltage (potential difference) is what drives the current round the circuit (unit: volts- V)
-resistance is anything in the circuit which slows the flow. If you add more components to the circuit there will be a higher overall resistance (unit: ohm)
-if you increase the voltage-then more current will flow
-if you increase the resistance-then less current will flow
the ammeter + voltmeter:
the ammeter:
-measures the current flowing through the component
-must be placed in series anywhere in the main circuit, but never in parallel like the voltmeter
the voltmeter:
-measures the voltage across the component
-must be placed in parallel around the component under test-NOT around the variable resistor or the battery!
ac & dc:
-the Uk mains electricity supply is approximately 230 volts
-it is an a.c supply (alternating current), which means the current is constantly changing direction
-cells and batteries supply d.c (direct current), which means that the current keeps flowing in the same direction
Ohm’s Law: formula linking voltage, current and resistance
voltage = current x resistance
V = IR
- steeper line = lower the resistance
-a straight-line = constant + resistance
-graph curves = resistance is changing
current-voltage graphs:
1) Wire: the current through a wire (at constant temperature) is proportional to voltage
2) Different (fixed) Resistors: the current through a resistor (at constant temperature) is proportional to voltage. Different resistors have different resistances, hence the different slopes
3) Metal Filament Lamp: as the temperature of the metal filament increases, the resistance increases, hence the curve
4) Diode: current will only flow through a diode in one direction
Light-Emitting Diodes: LEDs
-LEDs emit light when a current flows through them in the forward direction. They have lots of practical applications
-they are used for the numbers on digital clocks, in traffic lights and in remote controls
-unlike a light bulb, they don’t have a filament that can burn out
-LEDs, like lamps, indicate the presence of current in circuit. They are often used in appliances to show that they are switched on
Light-Dependent Resistors: LDRs
-LDRs are a special type of resistor that changes its resistance depending on how much light falls on it
-in bright lights, the resistance falls and in darkness, the resistance is highest
-this makes it a useful device for various electronic circuits, e.g: burglar detectors
-in graphs the same + a non-ohmic resistor
thermistors:
-a thermistor is a temperature-dependent resistor
-in hot conditions, the resistance drops and in cool conditions, the resistance goes up
-thermistors make useful temperature detectors, e.g: car engine temperature sensors, thermostats and fire alarms
fixed resistors + wires: graph
-current through a fixed resistor or a wire increases as the p.d across it increases
-current is directly proportional to p.d for a fixed resistor (or a wire)
-this is because the resistance of the fixed resistor (or wire) stays constant
-current on y-axis and p.d on x-axis
-ohmic resistor
filament lamps: graph
-current + voltage is not directly proportional because the resistance of the filament lamp increases as the temperature of the filament increases
-the higher temperature causes the atoms in the metal lattice of the filament to vibrate more
-this causes an increase in resistance as it becomes more difficult for free electrons (the current) to pass through
-resistance opposes the current, causing the current to increase at a slower rate
-non-ohmic resistor
diodes:
-allows current in one direction only called: forward bias
-in the reverse direction, the diode has a very high resistance, and therefore no current flows called: reverse bias
diodes: IV graphs
-when the current is in the direction of the arrowhead current symbol, this is forward bias
-shown by a sharp increase in p.d and current on the right side of the graph
-when the diode is switched around, this is reverse bias
-shown by a zero reading of current or p.d on the left side of the graph
-non-ohmic resistor
resistors:
-two types: fixed resistors, variable resistors
-fixed resistors have a resistance that remains constant
-variable resistors can change the resistance by changing the length of wire that makes up the circuit
-a longer length of wire has more resistance than a shorter length of wire
thermistors: graph
-the resistance changes a lot for small changes in temperature
-resistance decreases with increasing temperature
-non-ohmic resistor
series circuits: properties
-in series circuits, the different components are connected in a line, end to end, between the +ve and -ve of the power supply (except for voltmeters, which are always connected in parallel, but they don’t count as part of the circuit)
-if you remove or disconnect one component, the circuit is broken and they all stop working. This is generally not very handy, and in practice only a few things are connected in series, e.g: fairy lights
series circuits: actual circuit
-the current is the same everywhere. I1 = I2 = I3 = … The size of the current depends on the total potential difference and the total resistance of the circuit (I = Vtotal divided by Rtotal)
-the total resistance is the sum of the resistance of each component in the circuit- Rtotal = R1 + R2 + R3 + ….
parallel circuits: properties
-in parallel circuits, each component is seperately connected to the +ve and -ve of the supply (except ammeters, which are always in series)
-if you remove or disconnect one component, it will hardly affect the others at all
-this is obviously how most things must be connected, for example in cars and in household electrics
parallel circuits: actual circuit
-the potential difference is the same across all branches. V1 = V2 = V3 = etc
-current is shared between branches. Itotal = I1 + I2 + I3 + etc.
-there are junctions where the current either splits or rejoins. The total current going into a junction equal the total current leaving it, as charge can’t just dissapear or appear
-if two identical components are connected in parallel then the same current will flow through each component
formula linking charge, current and time
charge = current x time
Q = It
formula linking energy transferred, charge & voltage:
energy transferred = charge x voltage
E (in joules) = Q x V
formula linking energy transferred, charge, current & resistance:
energy transferred = charge x current x resistance
E = Q x I x R
wires in a plug:
-there are three wires in a plug-live,neutral and earth
-only the live and neutral wires are usually needed, but if something goes wrong, the earth wire stops you getting hurt
-live wire (brown): provides a path along ehich the electrical energy from the power station travels
-neutral wire (blue): completes the circuit by carrying the current back to the original power source
-earth wire (green + yellow): to protect the user by providing a path for the current to escape without passing you/user
appliances being earthed and insulated:
-all apliances with metal cases must be “earthed” to reduce the danger of electric shock, “earthing” just means that the case must be attached to an earth wire, an earthed conductor can never become live
-if the appliance has a plastic casing and no metal parts showing then it’s said to be double insulated
-the plastic is an insulator, so it stops a current flowing-which means you can’t get a shock, anything with double insulation doesn’t need an earth wire-just a live and neutral
circuit breakers:
-circuit breakers are an electrical safety device used in some circuits, like fuses, they protect the circuit from damage if too much current flows
-when circuit breakers detect a surge in current in a circuit, they break the circuit by opening a switch
-a circuit breaker can easily be reset by flicking a switch on the device, this make them more convenient than fuese-which have to be replaced once they’ve melted
electrical power
-> electrical power is the rate at which an appliance transfers energy
-an appliance with a high power rating transfers a lot of energy in a short time
-this energy comes from the current flowing through it. This means that an appliance with a high power rating will draw a large current from the supply
electrical power (measured in Watts) = current x voltage
P=IV
formula linking energy transferred, current, voltage and time
Energy transferred = Current x voltage x time
E = I x V x t
magnets:
-all magnets have two poles-north and south
-a magnetic field is a region where magnetic materials (e.g: iron) experience a force
-magnetic field lines are used to show the size and direction of magnetic fields, they always point from NORTH to SOUTH
-placing north and south poles of two permanent bar magnets near each other creates a uniform magnetic field between the two magnets
magnetic field patterns:
1) compasses and iron filings align themselves with magnetic fields
2) you can use multiple compasses to see magnetic field lines coming out of a bar magnet or between two bar magnets
3) you could use iron fillings to see magnetic fiel patterns. Just put the magnet(s) under a piece of paper, scatter iron fillings on top, and tap the paper until the iron fillings form a clear pattern
magnetism:
-magnets affect magnetic materials and other magnets
-like poles repel each other and opposite poles attract
-both poles attract magnetic materials
-when magnetic materials are brought near to a magnet, that material acts as a magnet
-this magnetism has been induced by the original magnet
-the closer the magnet and the magnetic material get, the stronger the induced magnetism will be
