Transcript
2 Force is measured in newtons, N Every action has an equal but opposite reaction SI unit of force is the newton Weight always acts down Resultant force is the combined force of the action and reaction When the resultant is zero, an object continues doing what it was doing- an object at rest remains at rest, a moving object keeps moving in the same direction at the same speed Resultant force is measured in Newtons If resultant force is not zero, acceleration is occurring Deceleration is just negative acceleration If there is no acceleration, resultant force is zero Acceleration=velocity change Changing direction is acceleration even if going at constant speed, as acceleration=velocity change and velocity is a vector Force=mass*acceleration Acceleration slows as resultant force decreases If a vehicle moves at constant speed, the resultant force on it is zero The greater the speed, the greater the deceleration needed to stop it This increased the required braking force Stopping distance is the distance the vehicle travels during the driver's thinking distance, or reaction time plus the braking distance Thinking distance is increased when a driver is tired, drinking or drugged Braking distance is increased by condition of car (tyres and brakes), weather, condition of road, and environment (hills) In freefall, the force acting on an object is gravity Gravity causes object's near the surface of Earth to accelerate at about 10m/s The force of gravity is called weight F=ma W=mg ^Same formula Gravitational field strength is measured in N/kg When objects fall through liquid, they exert a drag force This force grows, until it is equal to the weight of the falling object- at this point, acceleration stops, as the object is at terminal velocity Hanging weights from a spring makes it strength The increased in length is the extension Materials that behave this way are elastic Inelastic objects do not regain their srcinal shapes when the deforming forces acting on them are removed The limit of proportionality is the point up to which force applied and length of extension are directly proportional The rule stating this is called Hooke's Law F=ke K= spring constant in N/m, e= extension in m The stiffer a spring, the greater it's spring constant Work is done when an elastic object is stretched, storing elastic potential energy which is released when the stretching force is removed 3 When an object moves, a force must have applied to it Work is done when a force moves an object When work is done and an object is moved, the energy used is transferred to the moved object As such, energy transferred is equal to work done, energy transferred = work done Both energy transfer and work are measured in joules Work=force x distance Power is the rate of transfer of energy P=energy/time Power is measured in watts and energy in joules All moving objects have momentum Objects with greater mass & velocity have greater momentum Momentum=mass * velocity, p=mv When objects meet, the total momentum prior to their interaction is the same as the total momentum after the interaction (assuming no external forces are present) This is called conservation of momentum Thus, total momentum change is zero Momentum is a vector, having both size and direction With two objects at rest, momentum is zero In an explosion, the objects move apart with equal and opposite momentum As such, total momentum remains zero When a force acts on an object, it's momentum changes The longer the impact, the smaller the change in momentum (this is why you have crumple zones in cars, etc) Impact time is the time taken for impact to occur 4 When two insulating materials rub together, electrons are transferred from one material to the other Electrons are negatively charged so the material which gained electrons becomes negatively. The material which lost electrons becomes positively charged. This is called charging by friction Objects with opposite charges attracts each other, while objects with similar charges will repel each other Current=charge/time Current through a component is measured with an ammeters, placed in series with the component P.d. is measured across a component with a voltmeter placed in parallel with the component. p.d. measured in volts, current in amps Resistance is the measure of the opposition to current flow, in ohms Ohm's law states that if a resistor is kept at constant temperature, the current and potential difference are directly proportional A component which obeys Ohm's law is called an Ohmic conductor 5 Cells and batteries supply current which circulates in one direction around a circuit. This is called direct current. The current supplied through the mains circulates one way, then in the reverse direction. This is called alternating current. The mains supply of electricity in the UK is at a frequency of 50Hz. This means it changes direction 50 times every second. The voltage of this supply is 230V. The neutral wire in a plug from the mains is kept at 0V The UK uses a threepin plug connection to the mains supply plug sockets The outer cover is made of insulating plastic or rubber The plug's pins are brass, which is a good conductor, but also resilient to rust and physical damage The longest pin is the topmost one, which is the earth Brown->live Blue->neutral Green+yellow->earth Appliances with metal cases connect to the earth wire, earthing the device to prevent it from harming the user Devices with plastic cases, or devices where no metal components on the inside are touching any metal components on the outside are said to be double insulated. These appliances do not connect to the earth wire. Thicker cables are used to carry more current Fuses are fitted in series with a live wire If a fault develops in the appliance, a large current flows to earth, through the fuse. This blows the fuse, destroying the circuit and making the appliance safe to touch. Fuses have different ratings based on how much current they can take before melting. The fuse rating must be slightly higher than the normal current through an appliance, to prevent it blowing instantly, but not so high as to take too long to blow in case of a fault. Circuit breakers can replace fuses. These are electromagnetically operated switches, which fulfill the same role as a fuse, but don't need to be replaced after use. RCCB or residual current circuit breakers cut off the current if the current in the live wire differs from the current in the neutral. This makes them faster than fuses or normal circuit breakers. Appliances transform electrical energy into other forms of energy The rate at which this process occurs is called the power Power=energy/time Power is measured in watts, energy in joules Power=current * potential difference The current is the rate of flow of charge Charge=current * time Charge is measured in coulombs, or C When charge flows through an appliance, electrical energy is transferred to the appliance. Resistors function by transforming this energy into heat energy. Energy=potential difference * charge 6 There are three types of radioactive decay, alpha, beta and gamma In alpha decay, the nucleus loses 2 protons and 2 neutrons, emitting these as an alpha particle With beta decay, one neutron in the nucleus becomes a proton and an electron. The electron is emitted from the nucleus. Protons and neutrons have relative masses of 1,while the relative mass of an electron is negligible. Atoms have an equal number of protons and electrons- thus, no overall charge If an atoms gains or loses an electrons, it becomes an ion, and gains a charge Protons+neutrons=mass number Protons=atomic number Beta decay results in the atomic number going up by one Alpha decay results in the mass number dropping by 4 and the atomic number by 2 Gamma radiation does not result in a change in atomic or mass number. Gamma rays have no charge or mass Ionisation creates ions. It occurs when nuclear radiation travels through a material. It collides with the atoms of the material, causing those atoms to lose electrons and become ions. This can damage or kill cells. Alpha particles are strongly ionising, but have poor penetration as they are large particles, stopped by paper or skin Beta particles penetrate further but are less ionising, they are blocked by a sheet of aluminium or a few metres of air Alpha and beta particles are deflected by electric and magnetic fields, as they both have a charge. They have opposite charges and so are deflected in opposite directions. Gamma rays are electromagnetic waves. As such, they can travel a long way through a material without colliding with an atom. As such they are weakly ionising, but very penetrating. It takes several centimetres of lead or several metres of concrete to stop it. Gamma rays are unaffected by electrical and magnetic fields. Radioactivity of a sample can be measured by measuring the count rate The radioactivity of a sample decreases over time (and thus so does the count rate). This time varies depending on the isotope A half-life is the time taken for the count rate of an isotope to half itself, or the time it takes for the number of unstable nuclei in the isotope to half Alpha radiation is used in smoke alarms- it doesn't penetrate skin, making it non-harmful for humans, but is blocked by smoke. Thus, it can be used to detect smoke. Beta radiation is used to monitor paper thickness, or metal foil thickness. Gamma and beta radiation is used as a tracer in medicine. The isotope is taken into the parents body and can be tracked by it's emission of radiation. Must have short half-life to limit harmful exposure. 7 Nuclear fission occurs when a nucleus is split
