Physics Flashcards
What happens when two objects interact
The forces they exert on each other are equal and opposite
What is a resultant force
When a number of forces acting at a point are replaced by a single force that has the same effect on the motion as the original forces all acting together
What can a resultant force acting on an object cause
A change in its state of rest or motion
What happens if the resultant force acting on an stationary object is zero
The object will remain stationary
What happens if the resultant force acting on an stationary object is not zero
The object will accelerate in the direction of the resultant force
What happens if the resultant force acting on a moving object is zero
The object will continue to move in at the same speed and in the same direction
What is the acceleration of an object determined by
The resultant force acting on the object and the mass of the object
What does the gradient of a distance-time graph represent
Speed
What is the velocity of an object
Its speed in a given direction
What does the gradient of a velocity-time graph represent
Acceleration
What happens when a vehicle travels at a steady speed
The resistive forces balance the driving force
The greater the speed of a vehicle …
The greater the braking force needed to stop it in certain distance
What are most resistive forces caused by
Air resistance
What is the stopping distance of a vehicle a sum of
The distance the vehicle travels during the driver’s reaction time (thinking distance)and the distance it travels under the braking force (braking distance)
What can a drivers reaction time be affected by
Tiredness
Drugs
Alcohol
What may affect driver’s ability to react
Distractions
What happens when the brakes of a vehicle are applied
Work done by the friction force between the brakes and the wheel reduces kinetic energy of the vehicle and the temperature of the brakes increases
What can a vehicles braking distance be affected by
Adverse road and weather conditions (wet and icy) and poor condition of the vehicle
The faster an object moves through a fluid …
… The greater the frictional force that acts on it
Why will an object falling through a fluid initially accelerate
The force of gravity. But eventually the resultant force will be zero and the object will move at its terminal velocity (steady speed)
What is the equation to calculate the weight of an object using the force exerted on it by a gravitational force
W = M * G
W = weight in newtons, N M = mass in kg G = gravitational field strength in newtons per kilogram, N/kg
What may a force acting on an object cause
A change in shape of the object
What will a force applied to an elastic object (e.g. a spring) result in
Stretching and storing elastic potential energy
For an object that is able to recover its original shape, where is elastic potential energy stored
In the object when work is done on the object to change its shape
What is the extension of an elastic object directly proportional to
The force applied, provided that the limit of proportionality is not exceeded
When is work done
When a force causes an object to move through a distance
How are work done, force and distance related by
The equation
W = F * D
W - work done in joules, J
F - force applied in newtons, N
D - distance moved in the direction of the force in metres, m
What is transferred when work is done
Energy
What is power
The work done or energy transferred in a given time
P = E/T
P - power in watts, W
E - energy transferred in joules, J
T - time taken in seconds, s
What is gravitational potential energy
The energy that an object has by virtue of its position in a gravitational field
Ep = m * g * h
Ep - change in gravitational potential energy in joules
M - mass in kg
G - gravitational field strength in newtons per kilogram, N/kg
H - change in height in metres, m
What does the kinetic energy of an object depend on
Its mass and its speed
Ek = 1/2 * m * v2
Ek - kinetic energy in joules
m - mass in kg
v - speed in metres per second, m/s
What is momentum
A property of moving objects
p = m * v
p - momentum in kilograms metres per second, kg m/s
m - mass in kg
v - velocity in metres per seconds, m/s
What is conservation of momentum
The total momentum before an event is equal to the total momentum after the event
What happens when certain insulating materials are rubbed against each other
They become electrically charged. Negative charged electrons are rubbed off on one material and onto the other
When is a material negatively charged
When it gains electrons
When does a material have an equal positive charge
When it loses electrons
What happens when 2 electrically charged objects are brought together
They exert a force on each other
When do objects repel
When they carry the same type of charge
When do objects attract
When two objects carry different types of charge
What kind of charges move easily through some substances e.g. metals
Electrical
What is electric current
A flow of electric charge
What is the size of the electric current
The rate of flow of electric charge
Equation for the size of current
I = Q/t
I = current in amperes, A Q = change in coulombs, C t = time in seconds, s
What is potential difference
Voltage
What is the potential difference between two points in an electric circuit
The work done (energy transferred) per coulomb of charge that passes between the points
V = W/Q
V - voltage
W - work done
Q - charge
What are current-potential difference graphs used to show
How the current through a component varies with the potential difference across it
What does a current-potential graph for a resistor at constant temperature look like
The current and voltage are in direct proportion
How can you measure the resistance of a component
Measuring the current through and the potential difference across the component
Equation for voltage
V = I * R
V - voltage
I - current
R - resistance
What does the current through a component depend on
It’s resistance
The greater the resistance …
…the smaller the current for a given potential difference across the component
What is the potential difference provided by cells connected in series the sum of
The potential difference of each cell (depending on the direction in which they are connected)
For components connected in series …
The total resistance is the sum of the resistance of each component
The current is the same in each component
The voltage is shared between the components
For components connected in parallel…
The potential difference across each component is the same
The total current through the whole circuit is equal to the current flowing through the separate components
The resistance of a filament bulb increases as …
…the temperature of the filament increases
The current through a diode flows in …
…one direction only. The diode has a very high resistance in the reverse direction
When does an LED emit light
When a current flows through it in the forward direction
What is a LDR
Light-dependent resistor
The resistance of a LDR decreases as …
… light intensity increases
The resistance of a thermistor decreases …
… as the temperature increases
What is direct current
Current that always passes in the same direction, cells and batteries supply this
What is alternating current
One that’s constantly changing direction
Which supply is the mains electricity
Alternating current
What is the frequency and voltage of mains electricity in the UK
the frequency is 50 cycles per second (50 hertz) and is about 230 V
How are most electrical appliances connected to the mains
Using cable and a three-pin plug
What happens if an electrical fault causes too great a current
The circuit is disconnected by a fuse or a circuit breaker in the live wire
What happens if the current in a fuse wire exceeds the rating of the fuse
It will melt, breaking the circuit
What are some circuits protected by
Residual Current Circuit Breakers (RCCBs)
How do RCCBs operate
By detecting a difference in the current between the live and neutral wires. They also operate much faster than a fuse
Appliances with metal cases are usually …
… Earthed
Some appliances are double insulated and therefore …
… Have no earth wire connection
What do the earth wire and fuse protect
The wiring of the circuit
What happens when an electrical charge flows through a resistor
The resistor gets hot
What is power
The rate at which energy is transferred by an appliance
P = E/t
P = power in watts E = energy in joules t = time in seconds
How are power, voltage and current related by an equation
P = I * V
P - power in watts
I - current in amps
V - voltage in volts
How are energy transferred, voltage and charge related by an equation
E = V * Q
E - energy in joules
V - voltage in volts
Q - charge in coulombs
When is a substance radioactive
When they give out radiation from the nuclei of all their atoms all the time, whatever happens to them
Radioactive decay has a random nature
What is an alpha particle
Helium nucleus - 2 neutrons and 2 protons
What is a beta particle
An electron from the nucleus
What is gamma radiation
A type of electromagnetic radiation
What are types of background radiation
Natural sources:
Rocks
Cosmic rays from space
Man-made sources:
The fallout from nuclear weapons tests
Nuclear accidents
What types of radiation are deflected by both electric and magnetic fields
Alpha and beta
Alpha particles are deflected less than beta particles because of its greater mass and in an opposite direction
What type of field is gamma radiation deflected by
None
What is a half-life of an radioactive isotope
The average time it takes for the number of nuclei of the isotope in a sample to halve
OR
the time it takes for the count rate from a sample containing the isotope to fall to half its initial level
What are the 2 types of fissionable substances in common use in nuclear reactors
Uranium-235
Plutonium-239
What is nuclear fission
The splitting of an atomic nucleus
How does nuclear fission occur
The nucleus must first absorb a neutron
The nucleus undergoing fission splits into 2 smaller nuclei and 2 or 3 neutrons and their energy is released
The neutrons may go on to start to a chain reaction
Nuclear fusion
The joining of the two atomic nuclei to form a larger one
How is energy released in stars
Nuclear fusion
Why do crumple zones and seatbelts work
Increases the time over which a crash occurs. This means the force is lower and less harm to driver/passengers
Why can momentum be positive or negative
Because velocity can be in opposite directions
Why will the momentum in opposite directions after an explosion or crash be the same size but different signs ( - or + )
The different directions
What are protons and neutrons held together by
Strong forces which balances the repulsive electrostatic force between the protons
When is a nucleus stable
When the electrostatic force is balanced
What affects braking distance
Speed
Quality of brakes
Quality of tyres
How good the grip is (road surface, tyres, weather conditions)
When an object falls, what is its potential energy converted into
Kinetic energy
Kinetic energy gained = potential energy lost
What is the limit of proportionality
The maximum force that the elastic object can take and still extend proportionally
What do brakes do
Reduce the kinetic energy by transferring it into heat (and sound)
How are cars designed to convert kinetic energy safely in a crash
Crumple zones
Side impact bars
Seat belts
Air bags
How do crumple zones help passengers in a car crash
Crumple up on impact
Converts car’s kinetic energy into other forms of energy as the car changes shape
Increase the impact time, decreasing force produced by momentum
How do side impact bars help passengers in a car crash
Strong metal tubes fitted into car door panels
Help direct the kinetic energy of the crash away from the passengers to crumple zones
How do seat belts help passengers in a car crash
Stretch slightly, increasing time taken for wearer to stop - reduces forces acting on chest
Stretching absorbs kinetic energy of the wearer
How do air bags help passengers in a car crash
Slow you down more gradually
Prevent you from hitting hard surfaces in the car
Diode
A special device made from semiconductor material e.g. silicon
Used to regulate the voltage in circuits
Lets current flow freely through it in only one direction
When are parallel circuits used
In cars so everything can be turned on and off separately and gets the full voltage from the battery
What can an oscilloscope screen show
Electricity supplies
What does a DC source look like on an oscilloscope trace
A straight line as it is always at the same voltage
What does an AC source look like on an oscilloscope trace
A regularly repeating wave. From that you can work out the period and frequency of the supply
Equation for frequency
1/time period (s)
What’s a time period
The time taken to complete a full cycle (peak to peak or trough to trough)
Hazards in the home
Long cables Frayed cables Cables in contact with hot or wet things Water near sockets Damaged plugs Too many plugs in one socket Appliances without their covers Lighting sockets without bulbs in
Live wire
Brown coloured
Alternates between +ve and -ve voltage
Neutral wire
Always at 0V
Electricity flows in and out through the live and neutral wires only
Earth wire
Protects the wiring
Works together with a fuse to prevent fire and shocks
Why is the earth wire connected to the metal casing
So it can carry the electricity to earth should something go wrong and the live or neutral wires touch the metal case
Wiring of plugs
Neutral wire - left pin
Earth wire - top pin
Live wire - right pin (attached to fuse)
What do different appliances need
Different amounts of energy
Thicker cables have less resistance, so they carry more current
What are metal parts of plugs made from and why
Copper or brass because they are very good conductors
What is the case, cable grip and insulation made from and why
Rubber or plastic because they are really good insulators and flexible too
What is ‘earthing’
When the case is attached to an earth wire
An earthed conductor can never become live
When is an appliance double insulated
If it has a plastic casing and no metal parts showing
These don’t need an earth wire
Two-core cables
Cables that only carry live and neutral wires
Things affecting radiation dose
High altitude
Being underground
Working in mines or the nuclear industry
Working as a radiographer in hospitals
Order of ionisation (most to least)
Alpha
Beta
Gamma
Order of penetration (most to least)
Gamma
Beta
Alpha
What happens when a beta particle is emitted
A neutron turns to a proton in the nucleus
How do smoke detectors use alpha radiation
A weak source of alpha radiation is placed in the detector, next to two electrodes
The source causes ionisation and a current flows between the electrodes
If there’s a fire then smoke will absorb the radiation - so the current stops and the alarm sounds
What type of isotopes can be taken into the body
Gamma or beta emitters so that the radiation passes out of the body
Ones with a short half-life so that the radioactivity inside the patient quickly disappears
Which type of rays are used in radiotherapy
Gamma as they can kill all living cells
Sterilisation of food and surgical instruments with radiation
Exposure to a high dose of gamma rays kills all microbes, keeping food fresher
Alternative to medical instruments being boiled
Irradiation doesn’t involve high temperature so fresh fruit and plastic can be sterilised without damage
How are stars initially formed
Clouds of dust and gas, and the force of gravity makes the dust and gas spiral in together and produces a protostar. Temperature rises and pressure rises inside the star
What’s the voltage of Earth
0V
What does a neutron decay to
A proton and a beta particle
Nebulae
Clouds of dust and gas
Nuclear fusion vs. Nuclear fission
Fusion releases more energy
Less radioactive emissions
Uses ‘cleaner fuel’ (hydrogen)
