The SLOPE on a DISTANCE-TIME GRAPH REPRESENTS THE SPEED OF AN OBJECT.
The VELOCITY of an object is its SPEED IN A PARTICULAR DIRECTION. The slope on a velocity-time graph represents the acceleration of an object. The distance travelled is equal to the area under a velocity-time graph.
Two cars that are travelling in different directions but have the same speed, will have different velocities.
- The GRADIENT of a velocity-time graph REPRESENTS the ACCELERATION.
- The area under a veloctiy-time graph represents the DISTANCE COVERED.
Distance-time graph diagram.
Force, mass and acceleration.
A STATIONARY OBJECT remains stationary if the SUM OF THE FORCES ACTING UPON IT- RESULTANT FORCE- IS ZERO. A moving object with a zero resultant force keeps moving at the same speed and in the same direction.
If the resultant force acting on an object IS NOT ZERO, a STATIONARY OBJECT BEGINS TO ACCELERATE IN THE SAME DIRECTION AS THE FORCE. A moving object speeds up, slows down or changes direction.
Acceleration depends on the force applied to an object and the object's mass.
An object will accelerate in the direction of the resultant force. The bigger the force, the greater the acceleration.
An object will accelerate in the direction of the resultant force. A force on a larg mass will accelerate it less than the same force on a smaller mass.
Doubling the mass halves the acceleration.
Forces and acceleration calculations.
Resultant force (newton, N) = mass (kg) × acceleration (m/s2)
- Air resistance - drag.
- When an object moves through the air, the force of air resistance acts in the opposite direction to the motion. Air resistance depends on the shape of the object and its speed.
- Contact force.
- This happens when two objects are pushed together. They exert equal and opposite forces on each other. The contact force from the ground pushes up on your feet even as you stand still. This is the force you feel in your feet. You feel the ground pushing back against your weight pushing down.
- This is the force that resists movement between two surfaces which are in contact.
- This is the force that pulls objects towards the Earth. We call the force of gravity on an object its weight. The Earth pulls with a force of about 10 newtons on every kilogram of mass.
Weight and friction.
GRAVITY is a force that ATTRACTS OBJECTS WITH MASS TOWARDS EACH OTHER. The weight of an object is the FORCE ACTING ON IT DUE TO GRAVITY. The GRAVITATIONAL FIELD STRENGTH OF THE EARTH IS 10 N/ KG.
The STOPPING DISTANCE of a car depends on two things:
- THINKING DISTANCE.
- BRAKING DISTANCE.
WEIGHT IS NOT THE SAME AS MASS. Mass is a measure of how much stuff is in an object. Weight is a force acting on that stuff.
On Earth, if you drop an object it accelerates towards the centre of the planet. You can calculate the weight of an object using this equation:
weight (N) = mass (kg) × gravitational field strength (N/kg).
Weight and friction 2.
Three stages of falling:
- At the start, the object ACCELERATES DOWNWARDS BECAUSE OF ITS WEIGHT. There is NO AIR RESISTANCE. There is a RESULTANT FORCE ACTING DOWNWARDS.
- As it GAINS SPEED, the objects WEIGHT STAYS THE SAME, but the AIR RESISTANCE ON IT INCREASES. There is a RESULTANT FORCE ACTING DOWNWARDS.
- Eventually, the object's weight is BALANCED BY THE AIR RESISTANCE. There is NO RESULTANT FORCE and the object reaches a STEADY SPEED, called the TERMINAL VELOCITY.
Thinking distance is effected by:
- Influence of alchol.
- Influence of drugs.
Braking distance is effected by:
- The brakes or tyres worn
- The weather conditions are poor, such as icy or wet road.
- The car is more heavily laden, for example, with passengers and luggage.
Kinetic energy and momentum.
WORK DONE AND ENERGY TRANSFERRED ARE MEASURED IN JOULES (J). The work done on an object can be calculated if the force and distance moved are known.
A change in momentum happens when a force is applied to an object that is moving or is able to move. The total momentum in an explosion or collision stays the same.
GRAVITATIONAL POTENTIAL ENERGY: Any object that is raised against the force of gravity stores gravitational potential energy.
ELASTIC POTENTIAL ENERGY: This energy is stored in the stretched or sqaushed object as elastic potential energy.
KINETIC ENERGY: Every moving object has kinetic energy (movement energy). The more mass an object has, and the faster it is moving, the more kinetic energy it has,
MOMENTUM = momentum (kg m/s) = mass (kg) × velocity (m/s)
- Magnitude - an amount because it depends on the object's mass.
- Direction - because it depends on the velocity of the object.
Momentum - higher.
Some INSULATING MATERIALS become ELECTRICALLY CHARGED when they are RUBBED TOGETHER.
IF THE CHARGES ARE THE SAME THEY REPEL.
IF THE CHARGES ARE OPPOSITE THEY ATTRACT.
IF ONE IS CHARGED AND THE OTHER IS NOT THEY ATTRACT.
Flow chart diagram of how a photocopier works.
Electrical circuits can be represented by CIRCUIT DIAGRAMS.
Components can be connected in SERIES, OR IN PARALLEL. The characteristics of the CURRENT and POTENTIAL DIFFERENCE (VOLTAGE) are DIFFERENT IN SERIES AND PARALLEL CIRCUITS,
For a circuit to work:
- There must be a COMPLETE CIRCUIT.
- There must be NO SHORT CIRCUITS.
Circuits 2 - Series and parallel connections.
Series connections. Components that are connected one after another on the same loop of the circuit are connected in series. THE CURRENT THAT FLOWS ACROSS EACH COMPONENT CONNECTED IN SERIES IS THE SAME.
Parallel connections. Components that are connected on separate loops are connected in parallel. The current is shared between each component connected in parallel.
Circuits 3 - Current and voltage in series and par
When two or more components are connected in series, the same current flows through each component.
When two or more components are connected in series, the total potential difference of the supply is shared between them. This means that if you add together the voltages across each component connected in series, the total equals the voltage of the power supply.
When two or more components are connected in parallel, the total current flowing through the circuit is shared between the components.
When two or more components are connected in parallel, the potential difference across them is the same. This means that if a voltage across a lamp is 12V, the voltage across another lamp connected in parallel is also 12V.
Resistance and resistors.
RESISTANCE is measured in OHMS. It can be calculated from the POTENTIAL DIFFERENCE ACROSS A COMPONENT AND THE CRRENT FLOWING THROUGH IT. The total resistance of a series circuit is the sum of the resistances of the components in the circuit.
Resistors, filament lamps and diodes produce different current-potential difference graphs. The resistance of thermistors depends on the temperature, while the resistance of light-dependant resistors (LDRs) depends on the light intesity.
An electric current flows when electrons move through a conductor.
The moving electrons can collide with the atoms of the conductor. This makes it more difficult for the current to flow, and causes resistance.
You can calculate resistance using this equation :
POTENTIAL DIFFERENCE (volt, V) = CURRENT (amps, A) x RESISTANCE (ohm).
Resistance and resistors 2 - Changing the resistan
Series circuits - When components are connected in series, their total resistance is the sum of their individual resistances.
Resistor at constant temperature: The current flowing through a resistor at a constant temperature is directly proportional to the potential difference across it.
Resistance and resistors 3 - The filament lamp and
The filament lamp does not follow Ohm’s Law. Its resistance increases as the temperature of its filament increases. So the current flowing through a filament lamp is not directly proportional to the voltage across it.
The DIODE: The diode has a very high resistance in one direction. This means that current can only flow in the other direction.
Resistance and resistors 4 - Thermistors and LDRs.
Thermistors are used as temperature sensors - for example, in fire alarms. Their resistance decreases as the temperature increases:
- At low temperatures, the resistance of a thermistor is high and little current can flow through them.
- At high temperatures, the resistance of a thermistor is low and more current can flow through them.
LDRs (light-dependent resistors) are used to detect light levels, for example, in automatic security lights. Their resistance decreases as the light intensity increases:
- In the dark and at low light levels, the resistance of an LDR is high and little current can flow through it.
- In bright light, the resistance of an LDR is low and more current can flow through it.
The UK mains electricity supply is about 230V. The more electrical energy used, the greater the cost.
Electrical supplies can be DIRECT CURRENT (D.C.) OR ALTERNATING CURRENT (A.C.).
The cable: A mains electricity cable contains two or three inner wires. Each has a core of copper, because copper is a good conductor of electricity. The outer layers are flexible plastic, because plastic is a good electrical insulator. The inner wires are colour coded:
- BLUE = NEUTRAL.
- BROWN = LIVE.
- GREEN AND YELLOW STRIPES = EARTH.
The features of a plug are:
- 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.
Diagram of the inside of a plug.
Mains electricity 2 - Earthing.
The earth wire creates a safe route for the current to flow through if the live wire touches the casing.
You will get an electric shock if the live wire inside an appliance, such as a cooker, comes loose and touches the metal casing. However, the earth terminal is connected to the metal casing so that the current goes through the earth wire instead of causing an electric shock. A strong current surges through the earth wire because it has a very low resistance. This breaks the fuse and disconnects the appliance.
Mains electricity 3.
The circuit breaker does the same job as the fuse but works in a different way.
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. The fuse contains a piece of wire that melts easily. If the current going through the fuse is too great, the wire heats up until it melts and breaks the circuit.
The more energy that is transferred in a certain time, the greater the power. A 100W light bulb transfers more electrical energy each second than a 60W light bulb.
power (watts) = current (amps) x potential difference (volts).
Power is a measure of how quickly energy is transferred. The unit of power is the watt (W). You can work out power using this equation:
Direct and alternating current.
DIRECT CURRENT : If the current flows in only one direction it is called direct current, or d.c. Batteries and cells supply d.c. electricity, with a typical battery supplying maybe 1.5V.
ALTERNATING CURRENT: If the current constantly changes direction, it is called alternating current, or a.c.. Mains electricity is an a.c. supply, with the UK mains supply being about 230V. It has a frequency of 50Hz (50 hertz), which means it changes direction, and back again, 50 times a second.
Energy in circuits - higher.
Charge, current and time: Electrical charge is measured in coulomb, C. The amount of electrical charge that moves in a circuit depends on the current flow and how long it flows for.
CHARGE (coulomb, C) = CURRENT (AMPS, A) x TIME (second, s)
Energy transferred, potential difference and charge: For a given amount of electrical charge that moves, the amount of energy transformed increases as the potential difference (voltage) increases.
ENERGY TRANSFORMED (joule, J) = POTENTIAL DIFFERENCE (volt, V) x CHARGE (coulomb, C).
Atoms and isotopes.
An atom is made from a nucleus surrounded by electrons. The nucleus contains protons and neutrons.Isotopes are atoms that have the same number of protons, but different of neutrons.
The nuclei of some isotopes are unstable. They emit radiation and break down to form smaller nuclei.
An early model of the atom was the plum pudding model. It was disproved by Rutherford's scattering experiment and replaced by the nuclear model.
The number of electrons in an atom is always the same as the number of protons, so atoms are electrically neutral overall. Atoms can lose or gain electrons. When they do, they form charged particles called ions:
- if an atom loses one or more electrons, it becomes a positively charged ion.
- if an atom gains one or more electrons, it becomes a negatively charged ion.
Atoms and isotopes 2.
Isotopes - the number of protons in the nucleus of an atom is called its atomic number:
- The atoms of a particular element all have the same number of protons.
- The atoms of different elements have different numbers of protons.
The total number of protons and neutrons in an atom is called its mass number.
The proton number is shown below the chemical symbol, and the mass number is shown above.
Isotopes are the atoms of an element with different numbers of neutrons. They have the same proton number, but different mass numbers.
Atoms and isotopes - Radioactive decay and the plu
The nuclei of some isotopes are unstable. They can split up or 'decay' and release radiation. Such isotopes are called radioactive isotopes or radioisotopes.
When a radioactive isotope decays, it forms a different atom with a different number of protons.
When an atom emits alpha or beta radiation, its nucleus changes. It becomes the nucleus of a different element. This is because the number of protons in the nucleus determines which element the atom belongs to.
A scientist called Rutherford designed an experiment to test the plum pudding model. It was carried out by his assistants Geiger and Marsden. A beam of alpha particles was aimed at very thin gold foil and their passage through the foil detected. The scientists expected the alpha particles to pass straight through the foil, but something else also happened.
Some of the alpha particles emerged from the foil at different angles, and some even came straight back. The scientists realised that the positively charged alpha particles were being repelled and deflected by a tiny concentration of positive charge in the atom. As a result of this experiment, the plum pudding model was replaced by the nuclear model of the atom.
Background radiation is all around us. We can do little to avoid it. Most background radiation comes from natural sources, while most artificial radiation comes from medical examinations, such as X-ray photographs.
Radiation comes from radioactive substances including the ground, the air, building materials and food. Radiation is also found in the cosmic rays from space.
- Animals - All animals emit natural levels of radiation.
- Rocks - Some rocks give off radiactive radon gas.
- Soil and plants - Radioactive materials from rocks in the ground are absorbed by the soil and hence passed on to plants.
Nuclear reactors use a type of nuclear reaction called nuclear fission. Another type of nuclear reaction - nuclear fusion - happens in the Sun and other stars.
Fission is another word for splitting. The process of splitting a nucleus is called nuclear fission.
Uranium or plutonium isotopes are normally used as the fuel in nuclear reactors, because their atoms have relatively large nuclei that are easy to split, especially when hit by neutrons.
When a uranium-235 or plutonium-239 nucleus is hit by a neutron, the following happens:
- the nucleus splits into two smaller nuclei, which are radioactive
- two or three more neutrons are released
- some energy is released
The additional neutrons released may also hit other uranium or plutonium nuclei and cause them to split. Even more neutrons are then released, which in turn can split more nuclei. This is called a chain reaction. The chain reaction in nuclear reactors is controlled to stop it going too fast.
Nulear fission diagram.
Nuclear fusion involves two atomic nuclei joining to make a large nucleus. Energy is released when this happens.
The Sun and other stars use nuclear fusion to release energy. The sequence of nuclear fusion reactions in a star is complex, but overall hydrogen nuclei join to form helium nuclei.