- The steeper the line on a distance-time graph, the greater the speed it represents.
- Speed (metre/second, m/s)= distance travelled (metre, m)
time taken (second, s)
- Velocity is speed in a given direction.
- Acceleration is change of velocity per second.
- A body travelling at a steady speed is accelerating if its direction is changing.
- The slope of the line on a velocity-time graph represents acceleration.
- The area under the line on a velocity-time graph represents distance travelled
- To carry out calculations involving:
- The slope on a distance-time graph represents speed.
- The slope on a velocity-time graph represents acceleration.
- The are under the line on a velocity-time graph represents the distance travelled.
Speeding up and slowing down 1
- When two objects interact, they always exert equal and opposite forces on each other.
- The unit of force is the newton.
Remember that if a body is accelerating it can be speeding up, slowing down or changing direction. If a body is accelerating there must be a resultant force acting on it.
Rusultant force = mass x acceleration
(newtons, N) = (kilograms) x (metres/second)2
Speeding up and slowing down 2
- The thinking distance is the distance travelled by the vehicle in the time it takes the driver to react.
- The braking distance is the distance the vehicle travels under the braking force.
- The stopping distance = the thinking distance + the braking distance.
- The weight of an object is the force of gravity on it.
- An object falling freely accerlerates on about 10m/s2.
- An object falling in a fluid reaches a terminal velocity.
Work, energy and momentum 1
- Work done = energy transferred.
- Work done (joules) = force (newtons) x distance moved in the direction of the force (metres).
- Elastic potential energy is the energy stored in an elastic object when work is done on the object.
- The kinetic energy of a moving object depends on its mass and its speed.
- Kinetic energy ) J = 1/2 mass x speed2.
- Momentum (kg m/s) = mass (kg) x velocity (m/s).
- Momentum is conserved whenever objects interact, provided no external forces act on them.
Work, energy and momentum 2
- Momentumhas size and direction.
- When two objects push each other, they move apart with equal and opposite momentum.
- The more time an impact takes, the less the force exerted.
- Force (newtons) = change of momentum (kilogram metre/second)
time taken (seconds).
- Like charges repel; unlike charges attract.
- Insulating materials that lose electrons when rubbed become positively charged.
- Insulating materials that gain electrons when rubbed become negatively charged.
- Electrical current is the rate of flow of charge.
- A metal object can only hold charge if it is isolated from the ground.
- A metal object is earthed by connecting it to the ground.
- If a metal object gains too much charge, it will produce sparks.
- A spark from a charged object can make powder grains or certain gases explode.
- To eliminate staic electricity,
a. use antistatic materials, and b. earth metal pipes and objects.
Currect electricty 1
- Every component has its own agreed symbol.
- A circuit diagram shows how components are connected together.
- A battery consists of two or more cells connected together.
- Resistance (ohms) = potential difference (volts)
- The current through is a resistor at constant temperature is directly proportional to the potential difference across the resistor.
- Filament lamp: resistance increases with increase of the filament temperature.
- Diode: 'forward' resistance low; 'reverse' resistance high.
- Thermistor: resistance decreases if its temperature increases.
- LDR: resistance decreases if the light intensity on it increases.
Current electricity 2
- For components in series circuits:
- a. the current is the same in each component.
- b. the potential differences add to give the total potential difference.
- c. the resistances add to give the total resistance.
- For components in parrallel circuits:
- a. the potential difference is the same across each component.
- b. the total current is the sum of the currents through each component.
- c. the bigger the resistance of a component, the smaller its current is.
Mains electricity 1
- Alternating current repeatedly reverses its direction.
- Mains electricity is an alternating current supply.
- A mains circuit has a live wire which is alternately positive and negative every sycle and a neutral wire at zero volts.
- Cables consist of two or three insulated copper wires surrounded by an outer layer of flexible plastic material.
- Sockets and plugs are made of stiff plastic materials which enclose the electrical connections.
- In a three-pin plug or a three-core cable, the live wire is brown, the neutral wire is blue, the earth wire is yellow/green. The earth wire is used to earth the metal case of mains appliance.
- A fuse contains a thin wire that heats up and melts, cutting the current off, if too much current passes through it.
- A circuit breaker is an electromagnetic switch that opens (i.e. 'trips') and cuts the current off if too much current passes through it.
Mains electricity 2
- The power supplied to a device is the energy transfer to it each second.
- Electrical power supplied (warrior) = current (amperes) x potential difference (volts).
- An electrical current is the rate of flow of charge.
- When charge flows through a resistor, electrical energy is transferred as heat.
- Charge (coulombs) = current (amperes) x time (seconds).
- Energy transferred (joules) = potential difference (volts) x charge flow (coulombs).
Nuclear physics 1
- alpha decay- nucleus loses 2 protons and 2 neutrons and 2 protons and 2 neutrons are emitted as an alpha particle.
- beta decay- a nuetron in the nucleus changes into a proton and an electron is created in the nucleus and is instantly emitted.
- Alpha particles in a beam are sometimes scattered through large angles when they are directed at a thin metal foil.
- Rutherford used the measurements from alpha scattering experiments to prove that an atom has a small positively charged central nucleus where most of the mass of the atom is located.
Nuclear physics 2
- Nuclear fission occurs when a uranium-225 nucleus or plutonium-239 nucleus splits.
- A chain reaction occurs in a nuclear reactor when each fission event causes further fission events.
- In a nuclear reactor, one neutron per fission on average goes on to produce further fission.
- Nuclear fusion occurs when two nuclei are forced close enough together so they form a single larger nucleus.
- Energy is released when two light nuclei are fused together.
- A fusion reactor needs to be at a very high temperature before nuclear fusion can take place.