Physics GCSE Notes

Force and Motion

Newton’s First Law – an unbalanced force causes change in speed or direction of an object.
Newton’s Second Law – F = ma.
Newton’s Third Law – for every action, there is a reaction. Third Law pairs are equal in magnitude, opposite, of the same type, and act on different objects.
Forces can be contact and non-contact.
Distance and speed are scalar quantities, they do not have a direction.
Displacement and velocity are vector quantities, which include the direction, and can be negative.
Acceleration = change in velocity / time. Acceleration can be positive or negative.
The gradient of a displacement-time graph gives the velocity.
The gradient of a velocity-time graph gives the acceleration, and the area under the curve gives the displacement.
Friction always opposes motion, and causes heating and wearing of surfaces. It is essential for walking and the movement of bicycles, buses or trains (Newton’s Third Law).
Stopping distance = thinking distance + braking distance. Drugs or alcohols increase thinking distance, slippery or icy roads and heavy loads increase braking distance.
Seat belts and crumple zones increase the time of collisions, decreasing the acceleration and therefore decreasing the force exerted on the driver.
Weight is the force of gravity acting on an object. Weight = mass × gravitational field strength. W = mg. g = 10 N/kg.
For an object at free-fall, air resistance increases as the velocity increases. Eventually, terminal velocity (constant speed) is reached when air resistance equals the weight. Streamlined objectives experience less resistance.
Forces can change the shape of materials. For a spring, the extension is proportional to the force up to the limit of proportionality. Hooke’s Law: F = kx, where k is the spring constant (N/m). Past the elastic limit, the spring does not return to its original length.
For a rubber band, the extension is not proportional to the stretching force, and the band always returns to its original size, unless the force is large enough to break the rubber band.
Turning effects or moments can occur when a force is applied to an object that can rotate about a pivot. Moment = force × perpendicular distance from pivot.
Principle of moments – when a system is balanced, sum of clockwise moments = sum of anticlockwise moments.

Electric Circuits

Current (I) is the rate of flow of charge (Q). Q = I × t.
Current is measured by an ammeter placed in series, and has units of amps (A). Current is not used up in the circuit and transfers energy from the voltage source to the compounds.
Voltage (V) is measured by a voltmeter placed in parallel with the voltage source or component, and has units of volts (V). It is a measure of the energy per unit charge transferred from a source or to a component. V = E/Q.
A series circuit has only one current path. The current is the same at all points, and the voltage of the power supply is shared among the components.
A parallel circuit has more than one current path. All components in parallel have the same voltage, and the current is shared among the separate paths.
Resistance (R) is opposition to the flow of current, and is measured in ohms (Ω). Ohm’s Law states that V = I × R. Components that obey Ohm’s Law are called ohmic conductors, and they have constant resistance.
In series, resistances add up: R = R1 + R2 + R3.
In parallel, the total resistance is less than the value of the smallest resistor: 1/R = 1/R1 + 1/R2 + 1/R3.
A diode only allows current to flow in one direction.
Filament lamps do not obey Ohm’s Law because as the voltage is increased, the lamp heats up and resistance increases.
Metals are conductors and they conduct electricity because they have free electrons. When voltage is applied, the free electrons drift slowly from negative to positive, forming an electric current. Electrons gain energy from the battery or power source and lose it to the components.
In conducting gas or molten or dissolved electrolyte, the charge flow is due to the movement of positively and negatively charged particles moving in opposite directions.
Direct current (d.c.) is produced by a battery, and passes in the same direction.
Alternating current (a.c.) is produced by a mains-powered circuit, and changes direction.
All appliances need a live connection, which supplies the voltage from the power source, and a neutral connection, which completes the current circuit.
Appliances with metal parts or cases that could contact the live wire require an earth wire, which protects the user from a short-circuit by providing an alternative current path to the ground instead of through the user. Double-insulated appliances do not need an earth connection because they have a plastic casing with no exposed metal parts so cannot become live.
A fuse is a safety element that melts if the current becomes too large, as in a short-circuit. Fuses must be replaced. Circuit breakers are now used in modern appliances because they can be easily reset and acts faster than a fuse.
Electrical power (P) is the rate of electrical energy transfer. P = I × V = E / t.
Appliances used for heating consume the most energy.
A kilowatt-hour (kW h) is the energy supplied to a 1 kW appliance over 1 hour. 1 kW h = 3,600,000 J.
Static charge can build on an insulator, which can be easily charged by friction e.g. rubbing, which transfers electrons from one insulator to another.
Conductors can only be charged with static when well-insulated from the earth.
Like charges attract, opposite charges repel.
The rapid discharge of static charge build-up on the body, for example by walking across a synthetic carpet and then touching earth, can cause an electric shock.
Electrostatic charge can cause lightning and sparks that can ignite fuel. Therefore, aircraft are earthed before refueling.
In a photocopier, black powder is sprayed and sticks to charged areas of a rubber belt. An image of the sheet of paper being copied is projected onto the belt, and the illuminated parts are discharged. The powder is then transferred to a charged sheet of paper.
In an inkjet printer, ink drops become charged as they pass through the nozzle. Two parallel plates control the movement of the charged ink droplets.
Painting metal panels uses electrostatic induction. Positively charged paint powder are sprayed onto an earthed metal panel. Electrons move from earth onto the panel, and the paint powder are attracted to the panel, ensuring an even coating.

Waves

Waves are vibrations or oscillations that can transfer energy and information without the transfer of physical material.
Transverse waves – vibrations are perpendicular to the direction of wave travel.
Longitudinal waves – vibrations are parallel to the direction of motion, to give regions of compression and rarefaction.
Wavelength (λ) – length of one complete cycle of a wave motion.
Frequency (f) – number of vibrations each second, measured in hertz (Hz). Increasing the frequency increases the pitch of sound. Humans can detect sound from 20–20,000 Hz. Sounds above the maximum frequency that humans can detect are called ultrasound.
Amplitude (a) – the greatest displacement from the rest position. Loudness is determined by amplitude.
Wave equation – wave speed = frequency × wavelength. v = f × λ.
Plane waves reflect off a plane barrier by the law of reflection: angle of incidence = angle of reflection. Plane waves reflected from a concave barrier are converged towards the focus, as in a satellite dish.
Light reflected from a plane mirror produces an image which is virtual, behind the mirror at the same distance as the object is in front, upright, and the same size as the object.
Circular waves are reflected as if they came from a point behind the barrier, the position of the image.
Reflections of sound are called echoes, which are used for measuring distances and produces images of inside the body e.g. sonar or ultrasound scan.
Refraction – waves change speed and direction when they pass from one material into another or where the density of the medium changes. The wavelength decreases, but the frequency remains the same. Some reflection also occurs at the interface.
Refraction at the air-water boundary can cause a virtual image to be created that makes objects appear shallower than they really are. Water waves slow down when they pass from deep into shallow water.
Light waves are refracted towards the normal when travelling into a denser medium, and vice versa. The refractive index n of a material = sin i / sin r.
Total internal reflection occurs when the angle of incidence is greater than the critical angle. Here, all the light is reflected and none is transmitted. This feature is used in reflecting prisms and optical fibres, for communications or in an endoscope.
Diffraction is the spreading out of waves when they pass through a small gap that is a similar size to the wavelength of the wave.
The electromagnetic spectrum contains radio waves, microwaves, infra-red, visible light, ultraviolet, X-rays, and gamma rays, in order of increasing frequency/energy and decreasing wavelength.
Waves of shorter frequency are more penetrating. X-rays are used for imaging and gamma rays are used for radioactive tracing. Ultraviolet rays from the Sun can cause skin cancer.
All objects emit infra-red radiation and absorb it from their surroundings. It is used in toasters and ovens, and in remote control devices.
Microwaves have the right wavelength to be absorbed by water in food in a microwave oven.
Television (0.6 m) and radio programmes (3 to 100’s of m) are broadcast using long-wavelength radiowaves. These are diffracted more around hills and buildings, reducing “shadow” causing poor reception, but are low frequency which limits the information transmitted. For long distances, shorter wavelengths are preferred to minimize diffraction and attenuation of the signal due to loss of energy.
Radiowaves carry signals by amplitude modulation (AM) or frequency modulation (FM), where the signal is represented by changes in amplitude or frequency of the wave, respectively.
Satellites use shorter wavelength microwaves to transmit information to regional transmitter to minimize diffraction. To receive satellite signals directly to a television, a set-top box is required to convert microwaves into longer wavelength waves.
Optical fibres carry information using pulses of light or infra-red radiation, which are higher frequency than radiowaves and are thus able to carry more information.
Signals travelling along wires or fibres are attenuated and pick up noise resulting in distortion of the signal. Signals are amplified at several stages during their transmission because of attenuation. For analogue signals, both the signal and the noise are amplified. Digital signals can be restored to their original condition by regeneration.
Earthquakes produce three types of wave. L waves are long-wavelength waves that travel around the Earth’s crust. P (primary) waves are faster, longitudinal waves that can travel through both solids and liquids. S (secondary) waves are slower, transverse waves that can only travel through solids. S waves are not detected in the shadow region on the opposite side of the Earth, because part of the core is liquid.
Energy released from the core by nuclear reactions cause the continental drift of the tectonic plates. Resistive forces between the plates prevent motion, until the internal forces due to the compression and stretching of the material overcome the friction, causing a “jerk” leading to an earthquake.
When plates move towards each other, an ocean plate moves under a continental place, called subduction. The ocean plate partially melts, forming metamorphic rocks under high pressure and temperature and pushing up mountains in the continental plate, which can lead to volcanoes.
When plates move away from each other under the ocean, new rock is formed from magma flowing out of the crack. The new rocks are rich in iron and become magnetised by the Earth’s magnetic field, which reverses once every few thousand years.

Earth and Universe

Gravity keeps a planet in orbit around the Sun and a moon around a planet. The gravitational force increases with the mass of each object, and decreases with distance by the inverse square law.
There are 9 planets in the Solar System, and most planets have natural satellites. Planets further from the Sun have longer orbital times due to an increasing orbital distance and a weaker gravitational force.
Artificial satellites orbit around Earth and other planets for navigation, surveillance, communication, seeing into space and monitoring the weather. Geostationary satellites are located above the equator, have an orbit time of 24 hours and stays above the same point on the Earth’s surface.
Asteroids are small rocks that orbit the Sun, and most lie in the asteroid belt between Mars and Jupiter.
Comets orbit the Sun with periods ranging from years to millions of years. The orbits of comets are highly elliptical and they can orbit in any direction or plane. Comets consist of ice and dusts and they are visible when they pass close enough to the Sun for the ices to vaporise and glow brightly.
Meteors are debris left by a comet path, these particles become heated and glow as they fall to the Earth.
Our Sun is one of the two hundred billion stars that form the Milk Way galaxy, a collection of stars held together by gravity, and there are more than a thousand billion known galaxies in the Universe.
SETI is the search for extraterrestial intelligence. Robots are used to analyze nearby planets, and telescopes are used to search for radio signals from aliens. However, the immense size of the universe means that direct exploration is very limited, and no evidence for any other life form has been found.
Stars are born from clouds of dust, hydrogen and helium which contract due to gravity, and become heated. Eventually, the temperature is sufficient for nuclear fusion reactions to begin, entering the main sequence. Hydrogen nuclei fuse together to form helium nuclei, releasing heat and light energy.
Our Sun is currently in its main sequence. When hydrogen runs out, the Sun will cool and expand to become a red giant, while the core will contract and heat up to allow the fusion of helium nuclei to form carbon and oxygen. The Sun will then contract, losing its outer layers and becoming a hot dense body called a white dwarf. Finally, as nuclear reactions stop, the star cools down and becomes a black dwarf.
More massive stars become red supergiants after their main sequence, and fusion reactions in the contracting core produce heavier elements such as iron. The star is generating energy again and becomes a blue supergiant. When nuclear reactions are finished, the star contracts and finally explodes in a supernova that flings off the outer layers to form a dust cloud, and leaving behind a dense neutron star.
Our Solar System is likely to have formed from the gas and dust from exploding supernova due to the presence of heavier elements in the inner planets and the Sun.
Spectra of light from stars moving away from the Earth appear red-shifted, known as the Doppler effect. Distant stars are receding most rapidly, suggesting that the Universe arose from a “Big Bang” and has been expanding since. Background microwave radiation that fills space is left over from the explosion.
The age of the Universe is about 15 billion years old. The fate of the universe depends on the speed at which galaxies are moving apart and the mass it contains. If there is enough mass for the gravity to stop the expansion, the Universe will contract into a “Big Crunch”. Otherwise, the Universe will continue to expand. If there is exactly enough mass, the Universe will stop expanding and enter a steady state.

Radioactivity

Radioactive nuclei are unstable and will emit radiation when they change to a more stable form. All three types of radiation can be detected by a Geiger-Muller tube or counter.
Isotopes of an element are nuclei that consist of the same number of protons, but different number of neutrons. Many isotopes are radioactive.
Alpha (α) radiation consists of helium nuclei. It is very ionising, but not penetrating. It is stopped by paper or a few centimetres of air. It is dangerous if it gets inside your body (e.g. through ingestion).
Beta (β) radiation consists of fast-moving electrons. It is ionising and more penetrating than alpha radiation. It is stopped by a few millimetres of aluminium.
Gamma (γ) radiation are energetic electromagnetic waves, they are weakly ionising and very penetrating, only stopped by thick lead or concrete.
The half-life of a radioactive nuclide is the time taken for the radioactivity of the sample to fall to half of its initial activity. The half-life of radioisotopes can range from a few nanoseconds to millions of years.
Alpha radiation is used in smoke detectors – smoke particles block the internal detection of alpha particles and the alarm is triggered.
Beta and gamma radiation are used in medical imaging. The radioactive isotope or tracer is ingested or injected into the patient and imaging the radioactivity emitted from the body allows doctors to follow the path of the radioisotope around the body in order to diagnose diseases such as blocked blood vessels.
Carbon-14 has a half-life of about 5700 years. The amount of carbon-14 in dead material decreases gradually over time. Carbon-containing materials can be dated to up to 50,000 years (carbon dating).
Uranium-238 decays eventually to lead-206 has a half-life of 4500 million years. This is used to date very old rocks, or the age of the Earth (estimated to be about 4500 million years).

Energy

When a force (F) moves an object a distance x, the work done (W) is the energy transferred. W = Fx.
The conservation of energy states that energy cannot be created or destroyed. It can only be transferred.
Kinetic energy is related to velocity (v) and mass (m): KE = ½mv2.
Gravitational potential energy is related to mass (m), height (h) and the gravitational field strength (g): GPE = mgh.
Other types of energy include elastic potential energy, chemical potential energy, nuclear energy, thermal energy, electrical energy, and radiant energy (which includes light and other electromagnetic radiation).
The efficiency (e) of an energy transfer is the percentage of the energy input that is transferred to the desired output. Some of the energy is wasted into heat, light, etc.
Electricity is generated by electromagnetic induction. When magnet moves through a coil, a voltage is induced. In a power station, rotating electromagnets driven by turbines generate alternating current in thick copper bars around the electromagnet. Bicycle dynamos also generate electricity from movement. The size of the induced voltage depends on the strength of the magnetic field and the speed of rotation.
A d.c. motor utilizes the reverse principle, the motor effect: a current in a magnet field experiences a force. A current in a rectangular coil in a magnetic field will rotate continuously when commutator is used to reverse the direction of the current the coil passes though the vertical position. The size of the force depends on the magnitude of the correct and the strength of the magnetic field. A loudspeaker utilized alternating current to generate vibrational movement in its cone, which produces sound.
A transformer is used to change the size of an alternating voltage. An alternating current in the primary coil produces a changing magnetic field which is concentrated in the iron core. The secondary coil experiences the changing magnetic field and an alternating voltage is induced in it. The ratio of the number of turns in the coils is equal to the ratio of voltages. A step-up transformer increases voltage, but the current is reduced by the same factor, so the total power remains constant (assuming 100% efficiency). A step-down transformer decreases voltage. A transformer does not work with direct current because there is no changing magnetic field.
High currents cause energy loses due to heating of wires, which increases resistance. Thus, electricity from a power station is stepped up to 400 kV for transmission in the electrical grid to reduce current, and is stepped down in stages for consumers e.g. 240 V for homes.
Countries have historically relied on fossil fuels for energy – coal, gas and oil. Coal is burnt to convert water into steam, which drives the turbines in generators. Gas is burnt to produce hot exhaust gases which drive the turbines directly. Fossil fuels are non-renewable and create air pollution. Carbon dioxide is a greenhouse gas and sulphur dioxide contributes to acid rain. Sulphur dioxide can be removed by passing the gas through a slurry of limestone in water.
Nuclear energy is produced from the fission of uranium and plutonium, creating steam and driving turbines. However, there is the risk of a meltdown causing release of radioactive material, as well as the problem of long-term storage of radioactive waste.
Renewable energy sources generate less pollution than fossil fuels, but are more expensive and less reliable. Wind can be used to drive turbines, but wind turbines are unsightly and take up a large area.
Fast-slowing rivers, streams or waterfalls can be made to turn water turbines, known as hydroelectricity. Tidal energy converts the in-and-out movement of the tides into electrical energy, while wave energy is harnessed from the constant up-and-down motion of ocean waves.
Geothermal energy is extracted from underground rocks by water pumps. This can be used as a source of hot water, or if the rocks are hot enough, to produce steam to generate electricity.
Solar panels are used to heat water for domestic use. The panel is covered with a glass panel which allows visible and UV light from the sun to pass through, but absorbs re-emitted infra-red radiation. The pipes and the interior of the panel are black to increase absorption. Photovoltaic cells convert solar energy into electricity directly (e.g. calculators), but their efficiency is often low.