Physics Revision

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• Created on: 13-05-14 14:23
In what case will a stationary object stay stationary?
If the resultant force is zero
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If the resultant force acting on the subject in not zero, in which direction does the object go in?
The object accelerates in the same direction as the force.
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What does acceleration depend on?
It depends on the force applied to an object and the object's mass
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What is the resultant force?
This is a single force that has tje same effect on the object as all the individual forces acting together
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What happens when all the forces are balanced?
The reultant force is zero
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What happens when the resultant force is zero?
A stationary object remains stationary and a moving object keeps on moving at the same speed and in the same direction.
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What happens when all the forces are not balanced?
An object accelerates in the direction of the resultant force. A moving object speeds up, slows down or changes direction depending on the direction of the resultant force.
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How does size of the force affect an object?
The bigger the force, the greater the acceleration. Doubling the siz of the resultant force doubles the acceleration.
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How does mass affect the force on an object?
A force on a large mass will accelerate it less than the same force on a smaller mass. Doubling the mass halves the acceleration.
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What is the equation for calculating acceleration?
Acceleration = resultant force (Newtons) divided by Mass of the object (Kilograms)
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What does the gradient on a distance time graph represent?
The speed of an object
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What is velocity?
The velocity of an object is its speed in a particular direction.
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What does the slope on a velocity time graph represent?
The acceleration of an object
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What is the equation for working out speed?
Speed = distance travelled divided by Time taken
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Why would cars have a different velocity?
If they are moving at the same speed but in different directions. If they have different speeds in the same direction. If they have different speeds in different directions.
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How do you work out acceleration?
Acceleration= (final velocity-intitial velocity) divided by time taken in seconds
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What two factors does the stopping distance of a car depend on?
Thinking distance and breaking distance
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What forces act on a moving car?
Gravity pulls down on the car, the reaction force from the road pushes up on the wheels, the driving force from the engine, friction between the road and the tyres, air resistance acts on the front of the car
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What is the resultant force of the car?
The resultant force is the sum of all the different forces acting on the car.
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What must happen for a vehicle to slow down?
The driving force from the engine must be decreased, the resistive forces must be increased
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What is stopping distance?
Stopping distance = thinking distance+braking distance
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What is thinking distance?
It takes time for a driver to react to a situation and start to apply the brakes. The car carries on moving during this reaction time. The thinking distance is the distance travelled in this time.
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When does the thinking distance increase?
If the reaction time increases
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Why would the reaction time increase?
It would increase if the driver is tired, distracted, or under the influence of alcohol or other drugs.
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What is braking distance?
The braking distance is the distance taken to stop once the brakes are apllied.
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Why would the braking distance increase?
If the car's brakes or tyres are in poor condition, or if the road and weather conditions are poor. It also increases as the cars speed increases.
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What happens when the brakes are applied?
Work is done by friction force between the brakes and the wheels.This reduces the kinetic energy of the vehicle. Increases the temperature of the brakes.
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What happends when objects are falling through a fluid?
Objects falling through a fluid eventually reach terminal velocity, when the resultant force acting on them is zero and they are moving at a steady speed.
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What is weight?
Weight is a force exerted on an object because of a gravitational force.
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What are the usual two forces affecting a falling object?
The weight of the object. This is a force acting downwards, caused by the object's mass and the earths gravitational fieldn and air resistance.
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What is air resistance?
This is a frictional force acting in the opposite direction to the movement of an object.
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What is the first stage of falling?
At the start, the object accelerates downward because of its weight. There is no air resistance. There is a resultant force acting downwards.
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What is the second stage of falling?
As it gains speed, the objects weight stays the same, but the air resistance on it increases. There is a resultant force acting downwards.
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What is the third stage of falling?
Eventually the objects weight is balanced by the air resistance. There is no resultant force and object reaches a steady speed, called the terminal velocity.
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What is terminal velocity?
The maximum speed of an object, reached when the forces moving the object are balanced by its frictional forces.
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What is meant by the term mass?
Mass is a measure of how much stuff is in an object
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What is meant by the term weight?
Weight is a force acting on that stuff
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What is weight the result of?
Gravity
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How does the Earth's gravity affect objects?
The Earth's gravity attracts objects towards the centre of the Earth and you feel forces like this as weight.
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What is the equation that links weight, mass and gravitational field?
W= M x G
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Why would a human being weigh less on the moon?
They would weigh less on the moon because the gravitational field strength of the moon is one-sixth of that on Earth. But their mass would stay the same.
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What type of energy would an elastic object store?
Elastic potential energy (when stretched or squashed)
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The extension of an elastic object is directly proportional to what?
The extension of an elastic object is directly proportional to the force applied.
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What might a force acting on an object cause it to do?
A force acting on an object may cause it to change shape.
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In which situation can an elastic object store elastic potential energy?
If they are stretched or squashed
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What happens to an elastic object when its shape changes?
Work is done and it stores elastic potential energy. It is possible to calculate the amount of energy stored when stretching or squashing an elastic material.
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What is Hooke's Law?
When an elastic object is stretched, the increased length is called its extension. The extension of an elastic object is directly proportional to the force applied to it:
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What is the equation for Hooke's law?
F= K x E (F is the force in Newtons, K is the spring constant in Newtons Per Metre, E is the extension in metres
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In what conditions does the equation for Hooke's law work?
The equation works as long as the elastic limit (the limit of proportionality) is not exceeded. If a spring is stretched too much it will not return to its original length when the load is removed.
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What is the spring constant?
The spring constant is different for different objects and materials. It is found by carrying out an experiment. For example, the unloaded length of a spring is measured.
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How does the spring constant work?
Different numbers of slotted masses are added to the spring and its new length is measured each time. The extension is the new length minus the unloaded length.
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What is the unit of measurement for work done and transferred energy?
Joules, J
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In what situation can the work done on an object be measured?
If the force and distance moved are known
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What is power?
Power is a measure of how quickly work is being done.
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When do objects gain gravitational energy?
They gain gravitational energy when they are raised.
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What type of energy do moving objects have?
Kinetic energy
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When is work done?
Work is done whenever a force moves something
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What are everyday examples of work being done?
Everyday examples of work include walking up the stairs or lifting heavy objects.
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What happens every time work is done?
Energy is transferred from one place to another
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What is the equation for calculating the work done?
W=F x D (W is the work done in Joules, F is the force applied in newtons, D is the distance moved in the direction of the force in metres)
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What is the equation for calculating power?
P= E divided by t ( P is the power in Watts, E is the work done, t is the time taken in seconds)
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For example, an electric drill transfers 3000 J in 15 s. What is its power?
Power = 3000 divided by 15 = 200 W
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What is gravitational potential energy?
On earth we always have the force of gravity acting on us. When we are above the Earth's surface we have potential (stored) energy. This is called gravitational potential energy.
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What does the amount of gravitational potential energy depend on?
Mass and height about the ground.
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What happens when an object is lifted?
Work is done against gravitational force. The object gains energy.
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What is the equation for calculating change in gravitational potential energy:
Ep= m x g x h )Ep is the change in gravitational potential energy J, m is the mass in kilograms, g is the gravitational field strength in newtons per kilogram, h is the change in height in metres
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What does the kinetic energy on an object depend on?
Mass and speed
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What is the pendulum?
The pendulum is a simple machine for transferring gravitational potential energy to kinetic energy and back again.
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When does a change in momentum happen?
A change in momentum happens when a force is applied to an object that is moving or able to move.
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What happens to the total momentum is an explosion or a collision.
The total momentum in an explosion or collision stays the same.
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What is momentum?
A moving object has momentum. This is the tendency of the object to keep moving in the same direction. It is difficult to change the direction of movement of an object with a lot of momentum.
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What is the equation for calculating momentum?
p=m x v (p is the momentum in kilogams per second, m is the mass in kilograms and v is the velocity in m/s)
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Velocity is speed in a particular direction, what does this mean for momentum?
The momentum of an object also depends on the direction of travel.
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In what conditions could the momentum of an object change?
If the object speeds up or slows down. If the object changes direction.
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Why is momentum conserved?
As long as no external forces are acting on the objects involved, the momentum stays the same in explosions and collisions. We say that momentum is conserved.
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When do some insulating become electrically charged?
When they are rubbed together.
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What is an insulator?
Material that is a poor conductor of electricity or heat.
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What happens when the insulating materials are rubbed together?
Negatively charged particles called electrons move from one material to the other, the material that loses electrons becomes positively charged, the material that gains electrons becomes negatively charged, both materials gain an equal amount of char
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What is the only way to check that an object is charged?
To see if it repels another charged object. This is because charged objects will also attract small uncharged objects
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What are series connections?
Components that are connected one after the other on the same loop of the circuit are connected in series. The current that flows across each component connected in series is the same.
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What are parallel connections?
Components that are connected on separate loops are connected in parallel. The current is shared between each component connected in parallel.
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When does resistance in a wire increase?
If the length of the wire increases, if the thickness of the wire decreases
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What happens when the current is flowing through a resistor at a constant temperature?
It is directly proportional to the potential difference across it.
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What is direct current?
If the current flows in only one direction it is called direct current.
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What is alternating current?
If the current constantly changes direction, it is called alternating current. Mains electricity is an AC supply.
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How much is the UK's mains supply?
230 V
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Why does mains electricity cables wires contain a core of copper?
Because copper is a good conductor of electricity
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Why are the outer layers of the cables plastic?
Because plastic is a good electrical insulator.
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What is the blue wire within a plug?
Neutral wire
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What is the brown wire within a plug?
Live wire
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What is the green and yellow wire within a plug?
Earth wire
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What are two feature of a plug?
The case is made from tough plastic or rubber, because these materials are good electrical insulators. The three pins are made from brass, which is a good conductor of electricity.
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What are three other features of a plug?
There is a fuse between the live wire and the terminal. The fuse breaks the circuit if too much current flows. The cable is secured in the plug by a cable grip.
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What do fuses do?
Fuses protect electrical circuits and appliances.
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What does the fuse do when there is a fault?
The fuse breaks the circuit if a fault in an appliance causes too much current flow. This protects the wiring and the appliance if something goes wrong.
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How does a fuse work?
The fuse contains a piece of wire which melts easily. If the current going through the fuse is too much, the wire heats up until it melts and breaks the circuit.
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What are the most common ratings of fuses in plugs?
3A, 5A, 13A
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What does RCCB stand for?
Residual current circuit breakers
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What do RCCB's do?
Protect some circuit. The detect a difference in the current between the live and neutral wires. RCCB's work a lot faster than fuses.
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What does the earth wire do?
The earth wire creates a safe route for the current to flow through if the live wire touches the metal casing of an appliance.
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How does the earth wire work?
A strong current surges through the earth wire because it has a very low resistance. This breaks the fuse and disconnects the appliance.
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Why do some appliances not have an earth wire?
They have plastic casings, or they have been designed so that the live wire can't touch the casing. As a result, the casing can't give an electric shock even if the wires become loose.
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What are filament lamps?
Filament lamps contain a thin metal filament that glows when electricity passes through it. Most of the energy is transferred as heat energy instead of light energy.
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What are isotopes?
Isotopes are atoms that have the same number of protons, but different number of neutrons.
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What was the early model of the atom called?
The plum pudding model
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Where does most background radiation come from?
Most of it comes from natural resources
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Where does most artificial radiation come from?
Comes from medical experiments, such as X-ray photographs
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It can damage cells and make them cancerous
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Can be used in medicine to trace where certain chemicals collect in the body, used in industry to control measuring equipment.
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What are some natural sources of background radiation?
Cosmic rays, rocks and soils, living thins
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What are some artificial sources of radiation?
Radioactive waste from nuclear power stations, radioactive fallout from nuclear weapons testing and medical x-rays.
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What happens to photographic film when it absorbs radiation?
It goes darker. The more radiation the film absorbs, the darker it is when it is developed.
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What do people who work with radiation wear?
Film badges, which are checked regularly to monitor the levels of radiation absorbed.
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What is the Geiger- Muller tube?
It detects radiation. Each time it absorbs radiation, it transmits an electrical pulse to a counting machine.
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How does the counting machine connected to the Geiger-Muller tube work?
It makes a clicking sounds or displays the count rate. The greater the frequency of clicks, or the higher the count rate, the more radiation the Geiger-Muller tube is absorbing.
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How many types of main radiation are there?
There are three main types of radiation emitted from radioactive particles. These are alpha, beta and gamma radiation.
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Alpha radiation consists of alpha particles. An alpha particle is identical to the nucleus of a helium atom, which comprises two protons and two neutrons.
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Beta radiation consists on high energy electrons emitted from the nucleus. These electrons have not come from electron shells or energy levels around the nucleus. They have come from when a neutron splits into a proton and an electron.
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What happens when a neutron splits?
When a neutron splits, it splits into a proton and an electron. The electron then shoots out of the nucleus at high speed.
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Gamma radiation is very short wavelength- high frequency- electromagnetic radiation. This is similar to other types of electromagnetic radiation such as visible light and x-rays, which can travel long distances.
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How far can each type of radiation travel?
Radiation can be absorbed by substances in its path. Alpha radiation travels only a few centimetres in air, beta radiation travels tens of centimetres in air and gamma radiation travels many metres.
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What happens to the radiation as it travels?
All types of radiation become less intense the further the distance the radioactive material, as the particles or ray become more spread out.
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Why does the width of a substance affect radiation?
The thicker the substance, the more the radiation is absorbed. The three types of radiation penetrate materials in different ways.
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How does alpha particles penetrate substances?
Alpha radiation is the least penetrating. It can be stopped or absorbed by just a sheet of paper/
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How does beta radiation penetrate substances?
Beta radiation can penetrate air and paper. It can be stopped by a thin sheet of aluminium.
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How does gamma radiation penetrate substances?
Gamma radiation is the most penetrating. Even small levels can penetrate air, paper or thin metal. Higher levels can only be stopped by many centimetres of lead or many metres of concrete.
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What is the charge on each of the types of radiation?
Alpha particles are positively charged, beta particles are negatively charged and gamma radiation is electrically neutral.
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What does the charges mean for the radiation types?
It means that alpha radiation and beta radiation can be deflected by electric fields, but gamma radiation is not deflected.
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What would beta particles be attracted towards?
A positively charged plate
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What would alpha particles be attracted towards?
A negatively charged plate.
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Why can magnetic fields deflect alpha and beta radiation?
Because they consist of charged particles. Just as with electric fields, gamma radiation is not deflected by magnetic fields.
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What causes the cell do become cancerous after being in contact with radiation?
If the DNA in the nucleus of a cell is damaged. The cell then goes out of control, divides rapidly and causes serious health problems.
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How can radiation be used to help treat people?
Very high doses of radiation can kill the cell completely. We use this property of radiation to kill cancer cells, and also harmful bacteria and other micro-organisms.
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If the radioactive source is inside the body, which is most dangerous?Why?
Alpha radiation is the most dangerous because it is easily absorbed by cells. Beta and gamma radiation are not as dangerous as they are less likely to be absorbed by a cell and will usually pass through it.
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If the radioactive source is outside the body, which is most dangerous? And Why?
Beta and gamme are the most dangerous because they can penetrate the skin and damage the cells inside. Alpha radiation is not as dangerous because it is unlikely to reach living cells inside the body.
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The nuclei of radioactive atoms are unstable. They break down and change into a completely different type of atom. This is radioactive decay.
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What is a half-life?
The possibility to measure how long it takes for half the nuclei of a piece of radioactive material to decay.
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What are the two definitions of a half-life?
1.) The time it takes for the number of nuclei of the isotope in a sample to halve. 2.) The time it takes for the count rate from a sample containing the isotope to fall to half its starting level.
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What are some examples for how radiation is used?
In smoke detectors, for sterilising medical instruments, for killing cancer cells, for dating rocks and material, in chemical tracers to help with medical diagnosis, for measuring the thickness of materials a paper factory.
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What are tracers?
Doctors use radioactive chemicals called tracers for medical imaging.
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How do tracers work?
Certain chemical concentrate in different damaged or diseased parts of the body, and the radiation concentrates wit it. Radiation detectors placed outside the body detect the radiation emitted and with the help of computers, build an image of body.
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When traces are used in this way, why is it not normally harmful?
It has a short half-life and so decays before it can do much damage. It is not poisonous.
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Because these types of radiation readily pass out of the body, and they are less likely to be absorbed by cells.
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What do nuclear reactors use?
They use a type of nuclear reaction called nuclear fission.
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What is nuclear fission?
The process of splitting a nucleus.
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What two substances are used as fuels for nuclear reactors?
Uranium or plutonium
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What need to happen in order to fission to take place?
The uranium or plutonium must first absorb a neutron. When this happens, the nucleus splits into two smaller nuclei. Two or three neutrons are released. Some energy is released.
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What happens to the additional neutrons released in nuclear fission?
They may be absorbed by uranium or plutonium nuclei, causing them to split. Even more neutrons are released which in turn split into more nuclei. This is called a chain reaction.
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What is nuclear fusion?
Nuclear fusion involves two atomic nuclei joining together to make a large nucleus. Energy is released when this happens.
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When does nuclear fusion happen?
The sun and other stars use nuclear fusion to release energy. The sequence of nuclear fusion reaction in a star is complex, but overall hydrogen nuclei join to form helium nuclei.
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When do stars form?
Stars form when enough dust and gas clump together because of gravitational forces. Nuclear reactions take place to keep the star hot.
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When do planets form?
Planets form when smaller amounts of dust and gas clump together because of gravitational forces.
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Why do stable stars such as the sun do during their lifetime?
They change during their lifetime to form other types of stars, such as red giants, white dwarfs, neutron stars and black holes. The fate of a star depends on how much matter it contains.
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What is the first stage of a stars formation?
Stars form from massive clouds of dust and gas in space. As the mass fall together it gets hot. A star is formed when it is hot enough for hydrogen nuclei to fuse together to make helium. fusion process releases lot of energy which keeps core hot.
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What is the second stage in a stars formation?
During this stable phase in the life of a star, the force of gravity holding the star together is balanced by the high pressure due to the high temperature. The sun is at this stable phase in its life.
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What is the third stage in a stars formation?
When all hydrogen has been used up in the fusion process larger nuclei begin to form and the star may expand to become a red giant
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What is the fourth stage in a stars formation?
When all the nuclear reaction are over small star, like our sun may begin to contract under the pull of gravity. It becomes white dwarf which fades and changes colour as it cools.
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What is the fifth stage in a stars formation?
A larger star with more mass will go on making nuclear reactions, getting hotter and expanding until it explodes as a supernova.
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What does an expanding supernova do?
An expanding supernova throws hot gas and dust into space leaving a neutron star which eventually shrinks into a black hole.
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What pulls the dust and gas together in space?
Gravity
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What is the life cycle of stars the same size as the sun?
Main sequence star----- red giant----white dwarf----black dwarf
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What is the life cycle of stars much bigger than the sun?
Main sequence star---- red super giant----supernova--- neutron star or black hole
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Why is there nuclear fusion in stars?
The temperatures and pressures inside a star are so great that nuclear fusion can happen. Stars have enough hydrogen to maintain their energy output for millions of years
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Other cards in this set

Card 2

Front

If the resultant force acting on the subject in not zero, in which direction does the object go in?

Back

The object accelerates in the same direction as the force.

Card 3

Front

What does acceleration depend on?

Card 4

Front

What is the resultant force?

Card 5

Front

What happens when all the forces are balanced?