The common abbreviation for alternating current is a.c. This is when the flow of electric current changes direction periodically. The mains is a source of alternating current.
Absolute zero is the lowest possible temperature: −273 °C. At this temperature all particles stop moving, and gases exert no pressure at all.
An acceleration is a change in velocity. It is defined as the change in velocity over a given time divided by the time taken for this change.
Air resistance is the force opposing the motion experienced by any object moving through air. Air resistance is a form of drag.
An alpha particle is the same as a helium nucleus: it contains 2 protons and 2 neutrons. It is often written as 'α-particle' or using the symbol, .
Alternating current (often abbreviated to a.c.) is when the flow of electric current changes direction periodically. The mains is a source of alternating current.
An alternator is a type of generator that creates alternating current.
In general, the amplitude of a quantity is a measure of its size. When talking about waves, the amplitude of a wave is the maximum height of a wave measured from its rest position.
Amps or amperes (A) are the SI unit of current. One amp is equal to one coulomb per second.
An analogue signal is one that can change smoothly over a range of different values.
The angle of incidence of a ray is the angle between the incident ray and the normal to the surface.
The angle of reflection of a ray is the angle between the reflected ray and the normal to the surface.
Asteroids orbit the sun but are too small to be called planets. They can be up to 1000 km in diameter.
An atom is the smallest possible unit of a given element. Atoms can combine together to form molecules.
The atomic number of an atom indicates how many protons it contains.
The average speed of an object is the distance travelled in a relatively long time interval divided by that time.
Background radiation is the low-level radioactivity that is around us all the time. There are many sources that contribute to the background radiation including the ground, the air, and space.
The forces acting on an object are said to be balanced if their sum is zero. An object with balanced forces acting on it will not accelerate.
A beta particle is an electron. It is often written as 'β-particle' or using the symbol, .
The Big Bang theory is currently the best explanation for the origin of the universe. The theory states that around 15,000 million years ago all the matter and energy that later became the planets, stars, and galaxies was concentrated into a very small and incredibly dense space. It then started expanding very quickly.
Boyle's Law states that for a fixed mass of gas, at a fixed temperature, the pressure multiplied by the volume is always a constant.
The braking distance is how far a car travels after the driver applies the brakes. This distance depends on the initial velocity and mass of the car and the size of the braking force. The total stopping distance is found by adding the thinking distance to the braking distance.
Brownian motion is the name given to the random movement of small particles floating in still air or suspended in water. This random motion occurs because they are constantly being bombarded by the molecules of the surrounding gas or liquid.
Carbon dating is a technique used by scientists to determine the age of material that was once living tissue. To do this, they measure the ratio of carbon-12 to carbon-14 atoms present in the sample.
The centre of gravity of an object is the point where all the mass appears to be concentrated. An object will balance if it is suspended exactly from its centre of gravity. This is also known as centre of mass.
The centre of mass of an object is the point where all the mass appears to be concentrated. An object will balance if it is suspended exactly from its centre of mass. This is also known as centre of gravity.
The centripetal force is the force that must act on any object that moves in a circle. This force always points towards the centre of the circle. The centripetal force does not change the speed of the object, but does change the direction in which the object is moving.
Comets orbit the sun, following very non-circular paths. They are made of rock and ice.
If two coloured lights combine to form white light, then they are known as complementary colours. For example, blue and yellow are complementary colours.
A concave lens is thinner in the middle than at the edges. It can be used to diverge a beam of parallel light.
A material is said to conduct if electrons can flow easily along it.
Electrical conduction occurs when electrons flow through a material as a result of a potential difference.
Heat conduction is one way in which heat can be transferred. Conduction occurs because vibrating atoms or free electrons in the material pass on their energy to their neighbours.
An electrical conductor is a material through which electrons can flow easily. Metals are good conductors of electricity because they have many free electrons.
Metals are good conductors of heat because they have many free electrons. When a metal is heated some of the electrons move faster and transfer their energy to other electrons and atoms by colliding with them.
The law of conservation of energy states that when energy changes from one form to another, the total amount of energy is always constant. However, some of the energy may be converted to a form in which it is not very useful, for example heat.
The law of conservation of momentum states that the total momentum in a closed system is always constant. This is a useful law when looking at explosions and collisions.
Constructive interference is when two waves combine to form a larger wave.
A contact force is a force that acts between two objects only when they are touching.
Heat can be transferred by convection. This occurs when warm liquid or gas rises, and creates convection currents.
Conventional current flows from positive to negative around a circuit. However, electrons in the wire actually flow from negative to positive.
A converging beam of light is one that gets narrower until it comes to a point.
A convex lens is a converging lens. Convex lenses are wider in the middle than at the edges. They can be used to focus light to a point.
A convex lens is wider in the middle than at the edge. It can be used to focus light to a point.
Coulombs (C) are the SI unit of charge. One coulomb is the equivalent of 6.24 × 1018 electrons.
The critical angle is the smallest angle of incidence for which all the light is reflected at a boundary and none is refracted.
The critical mass is the minimum amount of a radioactive substance required before a chain reaction will start.
The common abbreviation for direct current is d.c. This is when the flow of electric current is in a constant direction. Batteries are a source of direct current.
The density of an object is equal to its mass divided by its volume. The SI unit of density is kg/m³ although it is also common to use g/cm³.
Destructive interference is when two waves combine to form a smaller wave. Sometimes there is a moment where there is no wave at all.
Diffraction is when waves bend around an object, or through a gap in an obstacle. Diffraction is most obvious when the gap is about the same size as the wavelength.
Diffusion is when the atoms or molecules of a gas or liquid move from a region of higher density to one of lower density as a result of their random motion.
A digital signal has one of two different values. These are often represented by on and off, or by 1 and 0.
A diode is a semiconductor device that permits current to flow in only one direction. Even in the forward direction, a threshold voltage of around 0.7 V is required before the current flows.
Direct current (often abbreviated to d.c.) is when the flow of electric current is in a constant direction. Batteries are a source of direct current.
Displacement is a vector quantity, the magnitude of which is equal to the distance. The measurement '4 miles north' is an example of a displacement.
A diverging beam of light is one that spreads out.
A concave lens is a diverging lens. Concave lenses are thinner in the middle than at the edges. They can be used to diverge a beam of parallel light.
A double insulated appliance does not need an earth wire. It has no exposed metal parts so cannot give an electric shock.
The drag force opposes the motion of any object moving through a fluid (i.e. a liquid or gas).
An earthquake is when the earth releases built-up energy by shaking violently.
When a material behaves elastically, the amount it stretches is proportional to the force applied to it. When the force is removed, the material will return to its original length.
A material behaves elastically up to its elastic limit. For forces greater than this, the material will be permanently deformed.
Electrical current is caused by the flow of charged particles. This is most commonly due to the flow of electrons in a metal wire, but can be due to the movement of any type of charged object.
Electrical energy is used to power many appliances around the home. It can be supplied by batteries or from the mains supply and is easily converted to other forms of energy.
Electrical power is a measure of how quickly a device consumes electrical energy. This is often referred to as the power rating of the device. Electrical power can by calculated using the equation: P = I V
An electromagnet is formed by passing a current through a coil of wire. Unlike normal magnets, electromagnets can be turned on and off.
Electromagnetic induction is the creation of an electrical current in a wire by moving the wire through a magnetic field.
The electromagnetic spectrum (or EM spectrum) is the range of all possible wavelengths of electromagnetic waves, from long wavelength radio waves to short wavelength gamma rays.
Electromagnetic waves form a continuous spectrum of which visible light is only a small part. Gamma rays, X-rays, microwaves, and radio waves are also electromagnetic waves. They are also known as EM waves.
The electromotive force is the force that pushes electrons around a circuit. It can be supplied by a battery and is measured in units of volts. The electromotive force is often abbreviated to EMF.
The electron is a fundamental particle that is present in every atom. It has a negative charge.
Electrostatic induction occurs when a charged object causes a charge in another object, without even touching.
EM is a common abbreviation for the word electromagnetic.
This is the common abbreviation for the electromotive force the force that pushes electrons around a circuit. It can be supplied by a battery and is measured in units of volts.
Energy is the capacity to do work. The SI unit of energy is the joule (J).
The equilibrium position of an object is where it will be if no external forces act on it.
Evaporation is the process of drying when a liquid turns into gas, or vapour. When a wet object dries, it loses a lot of heat because of the evaporation.
Fleming's left-hand rule can be used to work out the direction of the force when a current flows in a magnetic field.
The focal point of a lens is the point at which a parallel beam of light would be focused.
A force is a push or a pull. A force can lift an object, change its shape, or accelerate it.
A force multiplier is a way of changing the size of a force. Hydraulic systems can be used as force multipliers, by using a liquid to connect pistons with different areas.
Newton's Third Law of Motion states that forces come in equal and opposite pairs. This means that for every force, there is a paired force that has the same magnitude but acts in the opposite direction.
In general, the frequency of an event describes how often it occurs. When talking about waves, the frequency is a measure of how many waves go past a fixed point in a given time.
Friction is a force that acts between objects that are sliding over each other. The direction of the force of friction is always such that it resists the motion.
A fulcrum is the point about which an object turns or rotates. The fulcrum for a door is the hinge. A fulcrum can also be called a pivot.
Fuses are used to limit the current flowing through a circuit or into an appliance. A fuse will melt, or blow, when the current flowing through it exceeds the rating of the fuse.
Gamma radiation (or gamma rays) are a form of electromagnetic radiation. They can be written as 'γ-rays'.
A generator is a device that creates electrical energy.
An satellite in a geostationary orbit around the Earth is always above exactly the same point on the surface. This type of orbit has a period of one day.
The gravitational force acts between all objects that have mass. Your weight is the gravitational force between yourself and the Earth.
In music, harmonics of a note are integer multiples of the original note. They add depth to the note.
Hooke's Law states that the amount a material stretches is proportional to the force applied to it, provided that the elastic limit has not been reached.
The impulse associated with a force is equal to the magnitude of the force multiplied by the duration over which it acts. Impulse is equal to the change in momentum.
An incident ray is a ray travelling towards a surface at which it will be reflected, refracted, scattered or absorbed.
Inertia is the property of an object that makes it continue moving the way it already is. All objects with mass have inertia.
The instantaneous speed of an object is a measure of how fast the object is travelling at a given moment. It is calculated by dividing the distance travelled in a very short time interval near that moment by the time taken.
In electrical circuits, insulation (a non-conductive material such as plastic) is used to isolate current-carrying wires and components from their surroundings. This is an important safety measuring, preventing users from receiving shocks from electric appliances.
Insulation is used to reduce the transfer of heat from a hot object to a cold one.
An electrical insulator is a material that electrons do not flow easily through.
Ionization occurs when an atom becomes loses or gains one or more electrons. The atom will no longer be electrically neutral, but will have an overall charge.
Isotopes of a given element have the same atomic number, but different mass numbers. This means that they have the same number of protons and electrons, but different numbers of neutrons.
Kinetic energy is the energy associated with motion. A fast moving object has more kinetic energy than an otherwise identical slow moving object.
The law of reflection states that the angle of incidence is equal to the angle of reflection.
LDR is the common abbreviation for a light-dependent resistor. This is a semiconductor device in which the resistance decreases as the component is more brightly illuminated.
Light energy is the energy carried by light waves.
A light-dependent resistor is a semiconductor device in which the resistance decreases as the component is more brightly illuminated. The term light-dependent resistor is often abbreviated to LDR.
A logic gate is an electronic component that has at least one input and an output. The output of a logic gate can be determined from its outputs using the relevant truth table.
In a longitudinal wave, the material moves in the same direction as the motion of the wave. Sound is an example of a longitudinal wave.
Around every magnet there is an invisible magnetic field.
The magnetic force is the force that occurs between magnetic objects. The magnetic force can be either attractive or repulsive depending on the two objects.
The mass of an object is an inherent property of the object. It can be changed only by altering the object. The inertia and weight of the object are both due to its mass.
The mass number of an atom is equal to the number of protons plus the number of neutrons in that atom.
Meteors are pieces of rocky debris that drift around space. When they enter the Earth's atmosphere they burn brightly and can be seen as shooting stars.
A meteorite is the name given to the remains of a meteor that is not completely burnt up on the atmosphere but survives to reach the surface of Earth.
A molecule is a group of atoms bonded together.
In the context of forces, a moment is the turning effect of a force. It is equal to the force multiplied by the perpendicular distance to the pivot. A moment is also called a torque.
Momentum is a vector quantity and is equal to mass multiplied by velocity. In a collision or explosion, the total momentum is always conserved.
When a current flows along a wire in a magnetic field, the wire experiences a force. This is known as the motor effect.
The musical pitch of a note is related to the frequency of the sound wave.
The National Grid is the distribution network of electric pylons and cables that is used to transmit electricity across the country, wherever it is needed.
A negative charge is indicated by the negative sign () and is caused when an object gains electrons. Polythene can become negatively charged when it is rubbed.
The net force is the overall effective force acting on an object, and is found by adding up all the individual forces acting on the object. This is also known as the resultant force.
When an object has neither a positive nor a negative charge, it is said to be neutral. This occurs when the positive and negative charges within the object are balanced.
An object is in neutral equilibrium if it neither returns to its equilibrium position nor falls away from it after being pushed slightly.
The neutron is a fundamental particle that is present in every atom except hydrogen. It has around the same mass as the proton and no charge.
Newton's First Law of Motion states that if a body is at rest it stays at rest, and if a body is moving with uniform velocity in a straight line it continues to do so, unless an unbalanced force acts on the body.
Newton's Second Law of Motion states that the unbalanced force acting on an object is equal to the mass of the object multiplied by its acceleration.
Newton's Third Law of Motion states that whenever object A exerts a force on object B, object B exerts a force on object A with the same magnitude but in the opposite direction. Another way of saying this is that forces always come in equal and opposite pairs.
Forces are measured in units of newtons (N). One newton is the force required to accelerate by one metre per second per second, a mass of one kilogram.
Nichrome is a poor conductor that gives off heat when a current is passed through it. For this reason, nichrome wires are often used to make heating elements in electric fires.
A non-contact force is a force that can act between two objects even when they are not touching.
Non-renewable energy resources include coal, oil, gas, and nuclear fuels. One day we will use up all the non-renewable energy resources on Earth.
The normal to a surface is an imaginary line at right angles to the surface.
Nuclear energy is released by breaking apart or smashing together atoms. This is the energy which powers the sun.
Nuclear fission occurs when a large unstable atom breaks apart via radioactive decay into smaller atoms and nuclear radiation. For example, uranium-235 is unstable and undergoes nuclear fission.
Nuclear fusion occurs when two small atoms are squeezed together to form a heavier atom. For example, in stars, hydrogen atoms undergo nuclear fusion and helium is created.
The nucleus is the very dense, very small centre of every atom.
Ohm's Law states that the current flowing through an ohmic material is proportional to the potential difference applied across the material, provided the temperature of the material remains constant.
An ohmic conductor is a material that obeys Ohm's Law. The resistance of an ohmic conductor is defined as the potential difference across the conductor divided by the current flowing through it.
An optical fibre is a glass fibre along which light travels. As the light undergoes total internal reflection at the edges of the fibre, no light 'leaks' out of the fibre.
An orbit is the path followed by any object moving under gravity. Examples include the path a satellite moves in around the Earth or the path a planet moves in around the sun.
The usual abbreviation for pascal, the unit of pressure, is Pa.
The pascal is the SI unit of pressure. It is equivalent to 1 newton acting over an area of one square metre and is abbreviated to Pa.
A pivot is the point about which an object turns or rotates. The pivot for a door is the hinge. A pivot can also be called a fulcrum.
A positive charge is indicated by the positive sign (+) and is caused when an object loses electrons. Acetate can become positively charged when it is rubbed.
A potential divider is made from two resistors and is used to split an input voltage. The output (or signal) voltage depends on the values of the two resistors (one of which may be a sensor such as a thermistor).
Potential energy is the energy associated with position or state. It is energy which can be converted into another form. A stretched elastic band has more potential energy than an unstretched one. A ball at the top of a cliff has more potential energy than one at the bottom of the same cliff.
Power is the rate of doing work, or the rate of energy transfer. The unit of power is the watt, W.
The power rating of an appliance shows how many joules of electrical energy the appliance uses per second.
Pressure is defined as the force exerted per unit area. The SI unit of pressure is the pascal, which is equivalent to 1 newton acting over an area of one square metre.
The primary colours of light are red, green, and blue. You can make any other colour of light by mixing together the right amounts of the primary colours. White light is made by mixing equal amounts of all three primary colours.
The principal focus of a lens is another name for the focal point the point at which a parallel beam of light would be focused by the lens.
The principle of moments states that a suspended object will be balanced if the sum of the anticlockwise moments acting on it is equal to the sum of the clockwise moments.
The proton is a fundamental particle that is present in the nucleus of every atom. Its mass is around 1800 times greater than that of the electron, and it has a positive charge.
Radioactive decay is the phenomenon whereby an atom spontaneously decays to form one or more different atoms or isotopes. It is always accompanied by the emission of α-particles, β-particles or γ-rays.
Your reaction time is how long it takes you to react to what you see. In the context of driving, it is how long it takes a driver to react after they see something on the road ahead.
A real image is an image formed when light from an object actually converges. You could place a screen at this point and see an image of the object there.
Reflection occurs when a wave hits a surface and bounces back. Light waves are reflected from mirrors and sound waves reflect to make echoes.
Refraction occurs when a wave moves from one material to another. The speed, amplitude, and wavelength of the wave will change, and its direction of motion may change.
Renewable energy resources include hydroelectric power, solar power, geothermal power, and wave power. These energy resources will never run out.
The resultant force is the overall effective force acting on an object, and is found by adding up all the individual forces acting on the object. This is also known as the net force.
A satellite is any smaller object that orbits another larger object. Satellites of the Earth are both natural (such as the moon) and artificial (such as communications satellites).
A scalar is a quantity that has magnitude only.
Secondary colours are made by choosing two of the primary colours and combining them equally. The three secondary colours are cyan, magenta, and yellow.
A seismic wave is a wave or vibration that passes through the Earth. These can be caused be earthquakes.
A semiconductor is any electrical device in which the resistance decreases as the temperature increases.
The solar system is made up of all the planets and other bodies that orbit the sun. This includes nine planets, a large belt of asteroids, and many comets.
A sonic boom is the loud booming noise created when a moving object travels faster than the speed of sound. It is caused by constructive interference of overlapping sound waves.
Sound energy is the energy carried by sound waves.
Sound is carried by a wave and needs a material to travel through. Sound waves are longitudinal and are produced by vibrating sources such as musical instruments.
The specific heat capacity of a material is the amount of heat required to raise the temperature of 1 kg of the material by 1 °C.
Speed is a measure of how fast something is moving. It is calculated by dividing the distance travelled by the time taken.
An object is in stable equilibrium if it returns to its equilibrium position after being pushed slightly.
Static electric charges are due to stationary charged objects. They occur when an object either loses or gains electrons to become positively or negatively charged respectively.
The terminal velocity is the velocity a falling object reaches when the force of gravity acting on it is exactly balanced by the drag force.
Thermal energy is the energy associated with heat. A hot object has more thermal energy than an otherwise identical cold object.
A thermistor is an electrical component in which the resistance decreases as the temperature increases. Thermistors are often used as heat sensors. A thermistor is a type of semiconductor.
The thinking distance is how far a car travels during the reaction time. The length of the thinking distance depends on the velocity of the car and the reaction time. The total stopping distance is found by adding the thinking distance to the braking distance.
A torque is the turning effect of a force. It is equal to the force multiplied by the perpendicular distance to the pivot. A torque is also called a moment.
Total internal reflection occurs when light passes from a dense material into a less dense material, and the angle of incidence is greater than the critical angle. In this situation all the light is reflected back into the dense material.
Transformers are electrical devices that can change the voltage of a signal. They only work with alternating current.
In a transverse wave, the material moves at right angles to the direction of motion of the wave. Ripples on a pond are an example of transverse waves.
A truth table summarizes the behaviour of a particular logic gate. It lists all possible input combinations and the resulting output.
Ultrasound is very high-pitched sound. It has a frequency which is above the audible range.
An unbalanced force acting on an object will cause it to accelerate.
If the velocity of a moving object is not changing, then it has a uniform velocity.
An object is in unstable equilibrium if it does not return to its equilibrium position after being pushed slightly, but falls away from it.
A force called upthrust acts upwards on any object in a fluid. Upthrust arises because the pressure in a fluid increases with depth. Even objects that sink experience the force of upthrust.
A vector is a quantity that has magnitude and direction.
Velocity is a vector quantity. The magnitude of the velocity of a moving object is equal to the object's speed. The direction of the velocity is the direction in which the object is moving.
The point from which reflected waves appear to come from is called the virtual source of the waves.
The wave source is the point where the waves come from.
The wavelength of a wave is the distance from one peak to the next, or from one trough to the next.
The weight of an object on Earth is the force that occurs due to the gravitational attraction between the object and the Earth. The weight of an object depends on its mass and also on the gravitational field strength.
In the strict language of physics, work is only done when a force moves through a distance, and both the force and distance are in the same direction. The amount of work done is equal to the force multiplied by the distance through which it moves.