Physics Unit 2

Physics Unit 2

FORCES AND THEIR EFFECTS
1. Resultant Forces
  1. Whenever two objects interact, the forces they exert on each other are opposite and equal.
  2. A number of forces acting at a point may be replaced by a single force that has the same effect on the motion as the original forces all acting together. This singe force is called the resultant force.
  3. A resultant force acting on an object may cause a change in its state of rest or motion
  4. If the resultant force acting on a stationary object is: >zero, the object will remain stationary. >not zero, the object will accelerate in the direction of the resultant force.
  5. If the resultant force acting on a moving object is: >zero, the object will continue to move at the same speed in the same direction. >not zero, the object will accelerate in the direction in the resultant force.
2. Forces and Motion
  1. Newton's Second Law: F = m x a
  2. The gradient of a distance-time graph represents speed.
  3. The velocity of an object is its speed in a given direction.
  4. a = (v - u) / t
  5. The gradient of a velocity-time graph represents acceleration.
3. Forces and Braking
  1. When a vehicle travels at a steady speed the resistive forces balance the driving force.
  2. The greater the speed of a vehicle the greater the braking distance needed to stop it in a certain distance.
  3. Stopping distance = thinking distance + braking distance
  4. When the brakes of a vehicle are applied, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increases.
4. Forces and Terminal Velocity
  1. An object falling through a fluid will initially accelerate due to the force of gravity. Eventually the resultant force will be zero and the object will move at its terminal velocity.
  2. W = m x g
  3. (parachutist example)
5. Forces and Elasticity
  1. A force acting on an object may cause a change in the shape of an object.
  2. A force applied to an elastic object such as a spring will result in the object stretching and storing elastic potential energy.
  3. F = k x e
THE KINETIC ENERGY OF OBJECTS ACCELERATING OR DECELERATING
1. Forces and Energy
  1. When a force causes an object to move through a distance, work is done.
  2. W = F x d
  3. Energy is transferred when work is done.
  4. P = E / t
  5. Kinetic Energy = 1/2 x m x v^2
  6. Force is proportional to extension.
2. Momentum
  1. p = m x v
  2. In a closed system, the total momentum before an event is equal to the total momentum after the event. This is called conservation of momentum.
CURRENTS IN ELECTRICAL CIRCUITS
1. Static Electricity
  1. When certain insulating materials are rubbed together they become electrically charged. Negatively charged electrons are rubbed off one material and onto the other.
  2. The material that gains electrons becomes positively charged. The material that loses electrons is left with an equal positive charge.
  3. When two electrically charged objects are brought together, they exert a force on each other.
  4. Two objects carrying the same type of charge repel. Two objects carrying different types of charges attract.
  5. Electrical charges can move easily through some substances, e.g. metals.
2. Electric Circuits
  1. Current: I = Q / t
  2. Potential Difference: V = W / Q
  3. Current-potential difference graphs are used to show how the current through a component varies with the potential difference across it.
  4. The resistance of a component can be found by measuring the current through and potential difference across a component.
  5. V = I x R
  6. The current through a component depends on its resistance. The greater the resistance the smaller the current for a given potential difference across the component.
  7. The potential difference provided by cells connected in series is the sum of the potential difference of each cell.
  8. The resistance of a filament bulb increases as the temperature of the filament increases.
  9. The current through a diode flows in one direction only.
  10. The resistance of a LDR decreases as light intensity increases.
  11. The resistance of a thermistor decreases as the temperature increases.
MAINS ELECTRICITY
1. Household Electricity
  1. Cells and batteries supply current that always passes in the same direction - direct current
  2. Alternating current always changes direction.
  3. Mains electricity is an a.c. supply. In the UK it has a frequency of 50Hz and is about 230v.
  4. Most electrical appliances are connected to the mains using a cable and a three-pin plug.
  5. If an electrical fault causes too great a current, the circuit is disconnected by a fuse or circuit breaker in the live wire.
  6. When the current in a fuse wire exceeds the rating of the fuse it will melt, breaking the circuit.
  7. Some circuits are protected by RCCBs (Residual Current Circuit Breakers)
  8. Appliances with metal cases are usually earthed.
  9. The earth wire and fuse together protect the wiring of the circuit.
2. Current, Charge and Power
  1. When an electrical charge flows through a resistor, the resistor gets hot.
  2. P = E / t
  3. P = I x V
  4. E = V x Q
RADIOACTIVE SUBSTANCES
1. Atoms and Radiation
  1. Some substances give out radiation from the nuclei of their atoms all the time, whatever is done to them. These substances are said to be radioactive.
  2. Ionising Radiation - when a charged particle comes near another atom, it can pull electrons off the atom. This will leave behind ions.
  3. Alpha particle - helium nuclei, 2 neutrons and 2 protons, travels cm in air, stopped by paper
  4. Beta Particle - high speed electron, given out by nucleus, neutron changes into proton and electron, same number of particles in nucleus, mass number doesn't change, travels m in air, stopped by aluminium.
  5. Gamma - unstable nucleus --> new nucleus + gamma radiation, high energy wave, travels lots of m in air, stopped by thick lead and concrete
    1. Gamma has NO IONISING POWER but is still the most powerful form of radiation.
  6. Alpha and beta radiations are deflected by both electric and magnetic fields but gamma radiation is not.
  7. (uses and dangers)
  8. The half-life of a radioactive isotope is 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.
  9. Alpha Decay - 2 protons and 2 neutrons are lost from the mass of an atom in alpha decay.
  10. Beta Decay - a neutron changes into a proton and an electron so the atomic number increases by one.
NUCLEAR FISSION AND FUSION
1. Nuclear Fission
  1. The splitting of an atomic nucleus.
  2. There are 2 fissionable substances in common use in nuclear reactors - Uranium-235 and Plutonium-239.
  3. For fission to occur, the uranium-235 or plutonium-239 nucleus must first absorb a neutron.
  4. The nucleus undergoing fission splits into two smaller nuclei and two or three neutrons and energy is released.
  5. The neutrons may go on to start a chain reaction.
  6. Nuclear bombs: >not controlled so the nuclear fission is much more dangerous. >more neutrons are absorbed by nuclei so more nuclear fission means more energy is emitted.
  7. Nuclear Power Stations: >Boron control rods absorb neutrons and so slows chain reactions down as less neutrons are being absorbed by the nuclei. >less energy emitted = less dangerous.
2. Nuclear Fusion
  1. The joining of two atomic nuclei to form a larger one.
  2. Nuclear fusion is the process by which energy is released in stars.
  3. Stars form when enough dust and gas from space is pulled together by gravitational attraction. Smaller masses may also form and be attracted by a larger mass to become planets.
  4. During the 'main sequence' period of its life cycle a star is stable because the forces within it are balanced.
  5. A star goes through a life cycle. This life cycle is determined by the size of the star.

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	Created by Hannah Robinson about 9 years ago