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IGCSE Physics, Electricity, Magnetism and Electromagnetism, Waves,…
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Electricity
Modelling Circuits
Voltage
The voltage is the 'potential difference' across a component. This is a measure of energy that is transferred between 2 points.
This is measured with a Voltmeter, which has to be placed in parallel to the component you are measuring
Current
In a closed circuit that includes a potential difference, current flows. When current flows, work is done and energy is transferred.
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Charge
Charge is the property of a body which experiences a force in an electrical field. this is measured in coulombs
Components
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Lamp
A lamp consists of a filament, which is heated by the current, as electrons transfer energy as heat through collisions, producing a light.
Resistor
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A variable resistor is a resistor that can change it's resistance. This is often used to change the current in circuits where a fixed cell is used.
A light dependent resistor changes it's resistance based on the light level around it. As the light intensity increases, the resistance decreases
A thermistor is a resistor that changes resistance based on temperature. At low temperature, it's resistance is high, when it heats up, it's resistance lessens.
Diode
A semiconductor diode allows current to flow in one direction only. They are used to convert an alternating current into a direct current. Diodes have a high resistance until a specific voltage, where the resistance lowers and allows lots of current to flow.
Circuits
Series circuit
In a series circuit, electrical components are connected one after another in a loop.
The circuit is just one loop. All electrons will form one current, and an ammeter will measure the same current wherever it is.
In terms of potential difference, all of the source of energy must be evenly distributed, so the total potential difference of the power supply will equal all of the potential difference in all the components.
Resistance will be added up, as the current must flow through both resistors.
Parallel Circuits
In a parallel circuit, all components are connected alongside each other forming loops.
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Since energy is conserved, the energy transferred through the circuit will have to be the same, and since it is conserved, This means that the potential difference will be the sum of all of the different components.
Resistance is not connected in parallel here, so the overall resistance is reduced as the current will flow through either one or the other.
Earthing and Fuses
Fuses are components in electrical systems with a low resistance. They often involve a wire with a specific melting point, and when the temperature gets too high, the wire melts breaking the system
Fuses are cheap, but can only be used once meaning that they have to be used once.
Earthing is the process of grounding your circuit. It is often done in plugs, to stop the object becoming live, as if that happens, the current will flow through you to the ground.
Most electrical objects are made of plastic compared to metal, as even if the insulator around a wire rubs off, the object is still an insulator. This is called double insulation
Circuit breakers serve a similar function to fuses, but they get tripped, and have to be restarted. These are more expensive than fuses, but do not need to be replaced.
Surges are random increases in current, as our home current is AC. We earth things to allow the current to flow away as compared to coming through us.
Mains Electricity
Mains electricity can be produced in power plants. Usually from chemical energy, transferred mechanically to electricity
Mains voltage tends to be from 230-240V. The current is very low, as current can be dangerous. Tends to be 13A. The frequency is often 50Hz
AC Vs DC
Alternating current can flow positively and negatively (both directions). The current is always getting smaller and bigger and changing directions
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Insulators
Insulators do not carry a charge, as they do not have any free-moving electrons to do this. These tend to be non-metals
Insulators can be attracted to a charge, despite the fact that they have neutral charge. This is because the electrons can move, so that they can move to meet the positive charge, while still in orbit , so therefore the nucleus including the electrons is attracted to the charge.
An example of something else happening, is when a positive charge is next to something covalently bonded, the negatively charged nuclei will rotate towards the rod, then attract. Using water as an example, all of the negative oxygens will rotate, and attract slowly.
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Conductors
Anything with Free electrons can carry a charge . This means most metals can conduct, but the only common non metal to conduct is graphite.
Static energy
Dangers
Vehicles can become charged when they are moving and refuelling. . This is because they are 2 insulators rubbing against each other , transferring electrons, therefore charging it negatively
Refuelling can also be dangerous, as if fuel is rubbing against a rubber hose, 2 insulators are transferring electricity, and therefore building up a charge, which could ignite inflammable fuel
To solve these, we can use an Earth wire, or ‘earth’ something. This means that the current is going to the Earth
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Electrons are attracted to the nucleus due to unlike charges, and their high energy levels hold them in orbit and make them travel. Most atoms do not have a charge , as there are the same number of protons as electrons, so that they cancel each other, therefore no charge.
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Waves
Refraction
Refraction when light changes speed as it enters a medium of different optical density, for example air to glass. As it slows down, it bends and exits the medium at a different place
When a wave moves through a denser medium, it shifts towards the normal,
When a wave moves to a faster medium, it shifts away from the normal. Either way, it does not bend
Critical angle is the angle where light refracts at 90 degrees from the normal. The equation is sin-1(1/n) n being the refractive index.
The light inside an object will reflect inside the higher the refractive index is, meaning that the obiect sparkles.
Total internal reflection occurs in things with a high refractive index, meaning that it reflects.
This is used in fibre optics, which are thin strands of glass used in communication, medicine and as sensors. They can carry light over long distances and paths.
They are made of a glass core, encased in cladding preventing the light from escaping.
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This can also be used in prisms, where the AOI is larger than the critical angle. Prisms like this are used in medical equipment, and blinkers like bike light.
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EM spectrum
The EM spectrum is a group of waves. All electromagnetic waves travel at the same speed in a vacuum and free space.
The higher the frequency, the higher the energy transferred.
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Astrophysics
Gravitational field strength is the field of gravity that our planet emits. On the earth, it tends to be approximately 10N.
Planets further away from the sun travel more slowly because the further the distance, the smaller the gravity.
All orbits that orbit other objects do so because the gravitational force between the object and the orbited object. For an example, satellites do this
Objects in our solar system are usually orbiting other objects. Those with smaller mass tend to travel at a higher speed, which means they have a lesser orbital period (time taken for an object to complete an orbit).
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As the object is travelling in a circle, it is constantly changing direction. This means it is constantly accelerating. This means that it is constantly changing velocity.
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The Big Bang
Initially, all of the matter in the universe was contained in a small space, which was very dense, therefore hot.
Then, It exploded, expanding constantly. This is still occuring now. Tbjs is known as the big-bang theory.
Red Shift is a phenomenon like the doppler effect, as when an object is moving away from the observer, the wavelengths are stretched tinting it red.
This provides evidence for the expansion, as the colour spectrum of a distant star is pushed further to the red side, compared to our sun, giving evidence for an expanding universe.
Alongside this, scientists discovered that there are microwaves coming from every direction in space.
Some believe in the steady state theory, which suggests that matter is constantly created, keeping the universe at the same density as it expands.
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Life Cycle of Stars
They begin as a nebula, a cloud of dust and gas.
Gravity pulls this together, forming a protostar. The temperature rises as the star gets denser, and particles collide with each other.
The star enters a long stable period, where the outward pressure balances the gravity pulling inwards. This is called a main sequence star, and tends to last several billion years.
Then, the hydrogen begins to run out. The star then grows to become a red (super_ giant. It is red because the surface cools. Fusion of helium occurs, due to lack of hydrogen making heavier elements in the core.
Small to medium stars then become unstable, shedding tis outer layer of dust and gas, leaving behind a hot, dense solid core. This is known as a white dwarf.
As it cools down, it emits less energy, eventually becoming a dead core known as a black dwarf.
Bigger stars glow brightly again, as they undergo more fusion, expanding and contracting making heavy element. Eventually, they explode, distributing these elements.
The explosion removes gas and dust, leaving a dense core known as an neutron star, or if it is dense enough a black hole
When the temperature increases enough, hydrogen nuclei have nucler fusion, forming helium. This gives out lots of energy, keeping the star hot.
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Radioactivity
Uses and Risks
all radiation has its uses and dangers. All radiation can enter living cells, and ionise them, leading to tissue damage.
Lower doses can cause damage, but not kill the cells, mutating them.
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Uses
Certain radioactive isotopes can be injected into people, and can be read by a Geiger-Muller tube to see how they move.
Because they are gamma, they pass out of the body easily
Radiotherapy can be used to kill cancers,
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Atomic Structure
The current model of the atom involves a core of neutrons and protons, no charge and positive respectively. This core is then orbited by electrons(negative) in shells.
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Fission and Fusion
Nuclear fission is a type of nuclear reaction that is used to release energy from large, unstable atoms, by splitting them
To do this, a neutron is fired at a nucleus, which then splits, creating 2 lighter elements approximately the same total mass as the initial isotope. Alongside this, 2 neutrons are released
If the neutrons are travelling fast enough, they can hit another nucleus causing a chain reaction.
The kinetic and thermal energy can be used to heat water, turning a turbine, making energy.
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Nuclear fusion is a type of nuclear reaction, where 2 light nuclei collide to form a heavier nucleus.
The heavier nucleus produced does not have as much mass as the 2 light nuclei, where some of the mass is released as energy
This is a resistor. They always have a constant amount of resistance, therefore the gradient stays the same. It can change based on the resistance
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This is a diode. A diode will only allow current to flow through once a specific amount of voltage is put through. Most diodes allow around 0.6v of resistance through them.
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Energy
Energy is never consumed. It can be transferred and stored, but never created or destroyed.
When a system(object) changes, energy is always transferred. It can be transferred into, or away from. A closed system is a system where zero energy enters or leaves.
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It can be also transferred by doing work. Work can be done with force, or electricity, or any energy.
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This is a lightbulb. The lightbulb gets hot, and the temperature affects it’s resistance. The higher the temperature, the higher the resistance.
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