Download Physics revision facts

Physics revision facts
FACTS FOR HIGHER TIER CANDIDATES ONLY ARE IN italics.
13.1 How can we describe the way things move?
 The slope of a distance-time graph represents the speed
 The steeper the slope the greater the speed
 A stationary object will be a horizontal line
 A object moving at constant speed will be a straight line
 Speed(m/s) = distance(m)/time(s)
 Velocity is speed in a given direction
 Velocity is measured in m/s
 Acceleration is a measure of how quickly an object speeds up or slows down
 Acceleration(m/s2) = change in velocity(m/s)/time(s)
 Deceleration is a negative acceleration – an object that is slowing down
 The slope of a velocity-time graph represents the acceleration of an object
 The steeper the slope the greater the acceleration.
 A horizontal line indicates a constant speed
 The area underneath a velocity-time graph represents the distance travelled
13.2 How do we make things speed up or slow down?
 Forces are pushes or pulls measured in Newtons (N)
 A stationary object exerts a downwards force (weight) and the surface will exert an
upward force (reaction). The 2 forces are equal and opposite
 If resultant forces are equal and opposite they are balanced and an object will be
stationary or move at a constant speed
 If resultant forces are not equal and opposite they are unbalanced and an object will
move in the direction of the bigger force and speed up or slow down
 Friction is a force that occurs when an object moves through a medium. Friction
works against the object.
 When an object travels at a steady speed the frictional forces balance the driving
force.
 Force(N) = mass(kg) x acceleration(m/s2)
 The greater the speed of a vehicle the greater the braking force needed to stop it
 Stopping distance = thinking distance + braking distance
 Thinking distance can be affected by tiredness, drugs & alcohol
 Braking distance can be affected by poor road conditions, weather conditions and
poor vehicle conditions
 Falling objects experience two forces – weight (downwards) & air resistance
(upwards)
 A falling body will accelerate due to weight being the bigger force. Eventually the air
resistance will equal weight and the object will fall at a steady speed – terminal
velocity.
 The weight of a body is affected by its mass and the gravitational field strength.
 The G.F.S on Earth is approximately 10N/kg
 Weight(N) = mass(kg) x G.F.S( N/kg)
13.3 What happens to the movement energy when things speed up or slow down?
 When a force moves an object, work is done resulting in the transfer of energy
 Energy is measured in Joules
 Work done = energy transferred
 Work done(J) = Force(N) x distance moved in direction of force(m)
 Work done against frictional forces is mainly converted into heat energy
 The kinetic energy of an object depends on its mass and speed
 Kinetic energy(J) = ½ x mass(kg) x speed2 ((m/s)2)
13.4 What is momentum?
 Momentum is a measure of the state of motion of an object
 It depends on the mass and velocity of an object
 Momentum(kg m/s) = mass(kg) x velocity(m/s)
 Momentum has both magnitude and direction
 When a force acts on a moving object a change in momentum occurs
- a stationary will start to move
- the momentum of a moving object will increase
- the momentum of a moving object will decrease
 Momentum is conserved in any collision or explosion. The amount of momentum
before the collision or explosion is equal to the amount of momentum after the
collision or explosion
 Force(N) = change in momentum (kg m/s)/ time(s)
 Vehicles use the idea of momentum for safety features such as seat belts, crumple
zones, anti-lock brakes and air bags. In a collision the passengers will continue to
travel at the same speed and direction as before the collision. Safety features try to
minimise injuries
13.5 What is static electricity, how can it be used and what is the connection between
static electricity & electric currents?
 When insulating materials are rubbed together they become electrically charged
 Negative electrons are rubbed off one material and onto another
 The material that gains electrons becomes negatively charged
 The material the loses electrons become positively charged
 Two objects that have the same charge will repel each other
 Two objects that have different charges will attract each other
 Electrical charges can move easily through conductors and not through insulators
 Electrostatic charges are used in photocopiers and smoke precipitators
 Photocopiers – the image is projected onto an positively charged plate. Light causes
the charge to leak away leaving the impression to which negatively charged toner is
attracted. The image is then fixed in place by heat.
 Smoke precipitators – solid smoke particles are positively charged as they pass
through a charged metal grid. Particles are then attracted to negative collecting plates.
 The rate of flow of electricity is called current
 A charged conductor (positive or negative) can be discharged by connecting it to
Earth with a conductor. Electrons will either flow to or from Earth.
 The greater the charge on a body the greater the voltage between the object and
Earth. If the voltage becomes high enough the air molecules can become ionised and
there is a spark as discharge occurs
 Static can be dangerous in some situations and precautions need to be taken to
discharge the discharge.
- petrol stations. Turn off the engine, don’t use mobile phones.
- refuelling planes. The fuel tanker and the plane are linked with a copper conductor
13.6 What does the current through an electrical circuit depend on?
 Circuit symbols
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Current is measured in amps (A) using an ammeter which is connected in series
Potential difference (voltage) is measured in volts (V) using a voltmeter which is
connected in parallel
The current in a series circuit is the same at any point. The potential voltage in a series
circuit is the split up across the components. The total resistance is equal to the
resistance of each component
The current in a parallel circuit is the split up across components. The potential
voltage in a parallel circuit is the same at any point
Resistance is a measure of how hard it is for current to flow
Resistance is measured in ohms (Ω)
Resistance(Ω) = potential difference (V) / current (A)
The current through a resistor is directly proportional to the voltage
In a filament lamp as the temperature increases the resistance of the lamp increases
A diode only allows current to flow in one direction. It has a very high resistance in
the reverse direction so no current flows
The resistance of a Light Dependent Resistor (LDR) decreases as light intensity
increases
The resistance of a thermistor decreases as the temperature increases
13.7 What is mains electricity and how can it be used safely?
 Direct current (d.c) always flows in the same direction. Cells and batteries use d.c
 Alternating current (a.c) changes direction continuously. Mains supply is a.c
 The number of complete cycles of reversal per seconds is called the frequency.
 For UK supply the frequency is 50Hertz (Hz) and the voltage is 230-240V
 In a 3 pin plug the live wire is brown (on the right), the earth wire is yellow & green
(at the top) and the neutral wire is blue (on the left)
 The neutral wire carries the current away from the appliance
 The live wire carries the current to the appliance
 The inner cores of the wires are made of copper which is a good conductor
 The outer layers are made of flexible plastic which is a good insulator
 The pins are made from brass which is a good conductor
 Determine the time period & frequency of a supply from an oscilloscope trace
 Frequency (Hz) = 1 / time period (s)
 To find the time period measure the horizontal distance between 2 peaks. This tells
you the time period of the signal in divisions. Multiply the number of divisions by the
timebase per division
13.8 Why do we need to know the power of electrical appliances?
 Current is the flow of charge measured in amperes
 When charge flows through a resistor electrical energy is transformed into heat energy
 The rate at which energy is transformed is called power
 Power (W) = energy transferred (J) / time (s)
 Power can also be calculated by power (W) = current (A) x voltage (V)
 The amount of electrical charge which passes any point in a circuit is measured in
coulombs (C)
 Charge (C) = current (A) x time (s)
 As charge passes through a device energy is transformed. The amount of energy
transformed depends on potential difference. The greater the potential difference the
more energy transformed per coulomb
 Energy transformed (J) = potential difference (V) x charge (C)
13.9 What happens to radioactive substances when they decay?
 An atom is made up of protons, neutrons & electrons
 Protons have a mass of 1 and a charge of +1
 Neutrons have a mass of 1 and a charge of 0
 Electrons have a mass of almost 0 and a charge of -1
 The mass number of an element is the number of protons + the number of neutrons
 The atomic number of an element is the number of protons
 Atoms of the same element which have the same number of protons but different
numbers of neutrons are called isotopes
 Atoms may lose or gain electrons to form charged particles called ions
 Radioactive isotopes are atoms with unstable nuclei which decay and give out
radiation
 Alpha decay – an unstable nucleus emits an alpha particle, which is a helium nucleus
made up of 2 protons and 2 neutrons. The mass number of the unstable nucleus will
decrease by 4 and the atomic number by 2
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Beta decay – the unstable atom decays by changing a neutron into a proton and an
electron is emitted. The mass number of the unstable stays the same and the atomic
number increases by one.
 Ionisation is the process by which radioactive particles collide with neutral atoms and
become charged due to electrons being knocked out of their structure
 Naturally occurring radiation is known as background radiation. It poses not threat to
health. Comes from atmospheric gases, medical, nuclear industry, cosmic rays,
gamma rays from rocks, soil & building materials and from food
 Rutherford & Marsden fired alpha particles at gold foil. Most particles passed
straight through the foil. Some particles were deflected slightly and some were
rebounded back towards the source. Rutherford concluded that all atoms consist
largely of empty space with a small, dense positive core that he called the nucleus and
around this nucleus the negative electrons were spaced out. This model is called the
nuclear atomic model and replaced the original “plum pudding” model – where an
atom consisted of a positive sphere of matter in which electrons were set in.
13.10 What are nuclear fission & nuclear fusion?
 Nuclear fusion is the joining of atomic nuclei to form a larger atomic nucleus. This
releases more energy that it uses which makes it self sustaining.
 This is how stars release energy. In the core of the sun hydrogen is converted to
helium by fusion.
 Nuclear fission is the process of splitting atomic nuclei. It is used in nuclear reactors
to produce energy to make electricity.
 The two fissionable substances in common use in nuclear reactors, Uranium 235 &
Plutonium 239.
 For fission to occur the nucleus must first absorb a neutron. This makes the nucleus
unstable. It splits into two smaller nuclei and additional neutrons. Energy is also
released.
 These neutrons may then go on to start a chain reaction.
 You need to be able to sketch a chain reaction