Let's see how quickly we can cover everything you need to know for GCSE physics paper two. This is good for higher and foundation tier, double combined and triple separate science. I'll tell you when some of the bigger concepts are for triple, but not what's for higher and foundation, as there's not a lot of difference, honestly. And don't forget to check out the Science Shorts app to help you test your knowledge. Let's go. A force is any push or pull. Forces can be contact forces, that's when objects are physically touching, like when you push a door, or they can be non-contact, like magnetism, electrostatic forces, and gravity. Making this distinction is pretty new to GCSE, and it's a bit silly because even contact forces are due to the electrostatic repulsion between electrons in your skin and the door, for example, but whatever. Contact forces include friction, air resistance, tension, and a normal contact force. Normal means that the force is at right angles to the surface. The important thing is that we can represent forces with vectors. That is an arrow that shows the direction and magnitude of the force. The magnitude is the size of the force, and that's indicated by the length of the arrow. If two forces act on an object, there is a resultant force. We find this by technically adding the vectors. However, if they're going in opposite directions, one must be negative, so in this case, the resultant force would be 3 N to the right, and that's positive if we've decided that positive is in the right direction. If vectors are at right angles to each other, you use Pythagoras to find the resultant. This works because you essentially have a right-angle triangle if you move one of the forces. If forces are balanced, that is, they add up to zero, that means the object will not accelerate. Its velocity won't change. Note that doesn't necessarily mean it's not moving, it just stays at a constant velocity. And if that was 0 m/s to begin with, then it remains stationary. This is Newton's first law of motion, by the way. More on those in a bit. If a measurement or quantity just has magnitude but no direction, it's not a vector, but it's called a scalar instead. Here are some examples of both. Note that displacement is distance traveled with a direction, while similarly, velocity is a vector form of speed. Weight is just another name for the force due to gravity that acts on an object. It's calculated by multiplying the mass in kilograms by gravitational field strength or G. Try not to call it gravity. Here on Earth, that's 9.8 N/kg. Sometimes we just round that up to 10, but you'll be told which to use in a question. That means that 1 kg of mass on Earth has a weight of 10 N or 9.8. Now, if you hold an object up with your hand, you must be pushing up with a force that is equal to its weight in order for the forces to be balanced so it doesn't accelerate. However, it also means that if you lift it upwards at a constant speed, that's also true. To lift something up at a constant speed or even lower it at a constant speed, you must be pushing upwards with a force that is equal to the weight. To start lifting it upwards from rest is a different matter, of course. We can therefore calculate the energy that is used to lift this object using the equation for work done. Work done equals force times distance moved. Work done is just a fancy term for energy transferred by a force. This equation is true for any situation, but in this case, the force is the weight and the distance is the height. So, we could say the gain in energy is equal to mass times G times H. Does that look familiar? It should because that's the exact same equation for calculating gravitational potential energy. That's GPE change to be precise. Forces can also deform an object, change its shape. If you pull on a spring that is fixed at one end, it will stretch or extend. Hooke's law states that F equals K times E, thus force equals spring constant, sometimes called stiffness, times extension. This works for any object that stretches elastically, that is, returns to its original shape once the force is removed. It's also true if an object is compressed instead. As K is a constant, force and extension are directly proportional. That means whatever happens to one happens to the other. So, double the force, we have double the extension. To test this, we can hang slotted masses off a spring, increase the mass, re-measure every time, and you should end up with a straight line of best fit that goes through the origin, 0 0. And this proves this directly proportional relationship. Just make sure your ruler's zero mark is lined up with the bottom of the spring. That way, you can be sure you're only measuring extension rather than the length of the whole spring. Also, make sure you're at eye level with the bottom of the spring when measuring against the ruler to avoid parallax error. The energy stored in a spring is equal to 1/2 KE². If something was attached to the spring and you let go, the object would gain the same amount of kinetic energy, at least in an ideal or closed system, that is, no energy is lost to the surroundings due to heat, for example. A moment is a turning force, like when you turn a nut with a spanner. This is equal to force times distance to the pivot, so the unit just ends up being Newton-meters. Now, this equation might look similar to the work done equation, but this force and distance here are perpendicular to each other, rather than parallel. Like normal forces, if the moments turning clockwise are equal or balanced with the moments turning anticlockwise in the opposite direction, the object will not turn. That is, if it wasn't turning to begin with. We call this the principle of moments, by the way. You need to know this definition. An application of moments is gears. A small gear can turn a large gear in order to increase the moment produced. You can think of pressure as being how concentrated a force is. The equation is pressure is equal to force divided by area. So, the unit for pressure is Newtons per meter squared. We can also call this unit Pascals or Pa You probably know the deeper you go under water, the greater the pressure. This is due to the weight of the water above your head pushing down on you. We can calculate this pressure by using P = HρG. ρ is just the Greek letter that looks like a P, so it's height times density times gravitational field strength. The density of water is 1,000 kg per m cubed. Gas pressure is a result of the collisions between the gas particles and the surface of the container they're in. You can increase this pressure by adding more gas, reducing the volume, or raising the temperature, which makes the particles move faster, and they collide with the walls of the container with a greater force. These result in the collisions occurring more frequently, while increasing the temperature also results in the particles having more kinetic energy, so they collide with the walls with more momentum, therefore exerting a greater force. The higher your altitude, the less dense the atmosphere is, so pressure decreases. Speed and velocity are measured in meters per second, but velocity also has a direction, so it could be positive or negative, or up or down, left and right. Here are some typical traveling speeds. Speed and velocity are calculated by distance or displacement divided by time. And if you have a distance-time graph, it's the gradient of the graph that gives you the speed or velocity. If it's a curve, just draw a tangent at the point you need to find the gradient for. A speed or velocity-time graph can give you even more information, though. This time, the gradient gives you change in speed divided by time, which is acceleration. Here's the equation: a = v - u / t. That v - u is just change in velocity, final velocity take away initial velocity. The units of acceleration is meters per second squared, and it tells you how quickly speed is changing. If it's a negative gradient heading toward zero, this means the object is decelerating, slowing down. However, this graph can also go into negative values. For example, when a ball is thrown upward and comes back down. In that case, the velocity starts positive and fast, but decreases to zero when it reaches the top, the apex, where it turns around. So then the velocity becomes more negative as it falls. Incidentally, this graph has a constant negative gradient because gravity is accelerating it downwards at a constant rate, even though its direction changes. What you find is that for any object that's falling, its acceleration is 9.8 m/s squared downwards, the same as gravitational field strength, because they're the same thing, actually, but you don't need to know why. A velocity-time graph can give you the distance traveled as well. You get that by calculating the area under the graph. If you have any area under 0 m/s, though, that counts as negative displacement, by the way. That's why the areas of both these triangles in this graph add up to zero. That makes sense, though, doesn't it? Seeing that it's gone back from whence it came. Newton's equations of motion, or SUVAT, are a way of predicting what an object will do if it's accelerating. S is displacement, U is initial velocity, V is final velocity, A is acceleration, and T is time. And the object will usually start at rest, stationary, so U is zero. And like we said, if an object is falling, A is the same as G, that's 9.8 or 10 m/s². For any question involving one of these equations, you write down your variables, put a question mark next to what you're trying to find, and put the values for the other three that you know from the question. You can ignore the fifth unused variable. Depending on what data you're given, you pick the appropriate equation with the four variables in, rearrange it if necessary, then just plug in your numbers. We already know that Newton's first law is this: when there's no resultant force, an object's motion is constant. In other words, no change in velocity. That could be because there are no forces acting, or the forces acting on it are balanced. By the way, inertia is the term we use to describe the tendency for an object's motion to stay constant unless acted on by a resultant force. Newton's second law involves unbalanced forces, that is, there is a resultant force. This is equal to MA, mass times acceleration. That's all Newton's second law is, F = MA. Only one of these laws can be true in any situation. There's either no resultant force, Newton's first law, or there is, Newton's second law. We can prove Newton's second law by doing a practical. We use a trolley on a track being pulled by the weight of slotted masses hanging over a pulley connected by string. We can use light gates or photo gates to measure the acceleration between two points, then change the weight on the string. Just remember that whatever mass you take off the hanger must go on the trolley afterwards, as the force here is accelerating both the trolley and the masses themselves. We draw a graph of force against acceleration, and it should be a straight line through the origin, proving the proportional or directly proportional relationship between force and acceleration. The gradient actually should give you the total mass of the trolley and masses. Newton's third law, however, is always true. And this is the one that people get confused about. Understandably though, for every action, that means force, there is an equal and opposite reaction force. But this is not referring to balanced forces. It's all about perspective. When we think about the first two laws, we're only really considering one object. For example, the force pulling downwards on the ball is its weight. There's a resultant force downwards. However, if you zoom out and consider what's producing the weight to begin with, we know that the Earth is pulling down the ball, but Newton's third law says the complete opposite is true as well. The ball is also pulling the Earth up. Now, the Earth is so massive that it doesn't really have an effect on it, but it's still true nevertheless. Another example, if we have two ice skaters, if the guy skater pushes on the girl skater, there's an equal and opposite reaction force pushing back on him, too. That's why they both move away from where they were. The overall stopping distance for a car is the result of thinking distance. That's how far you go before you react to seeing the bunny, for example, and the braking distance after you stand on the brakes. If you double your speed, you double your thinking distance because you travel twice as far in the time it takes for you to react. That makes sense. However, doubling your speed quadruples your braking distance because your car needs to lose all of its kinetic energy, which is equal to 1/2 m v squared. So, that means that if you double the v, * 2, if we square that, that's * 4. If you triple your speed, your kinetic energy goes up by a factor of nine. So, that means so does your braking distance. Other factors that affect thinking distance are distractions, alcohol, drugs, whereas braking distance can be affected by the condition of your brakes, the tires, the roads, the weather, etc. Momentum is similar to inertia. You can think of it being a measure of how hard it is to get something to stop. The equation is momentum is equal to mass times velocity. So the unit therefore is kilogram meters per second. Momentum is a vector, which means you can have negative momentum if your velocity is negative. In a collision, total kinetic energy is very rarely conserved. That means that we will lose kinetic energy in the collision, but total momentum is always conserved. That means whatever the total momentum of the objects before the collision was, there must be the same total momentum afterwards as well. Calculations on this can be tricky, but you just have to be careful with your pluses and minuses. You write down m1u1 if there's just one object moving to begin with. Remember u from suvat? It's initial velocity. We can use it here, too. And we add m2u2 if there's a second object moving, too. This then is the total momentum before the collision. This could also be zero, though, if nothing's moving to begin with. Say a cannon about to fire. Then all we have to say is that this is equal to the total momentum afterwards, m1v1 for one object plus m2v2 if the second object is moving after, too. If they're coupled together afterwards, we just say m times v, where m is the total mass of the two. This is one of the few times where it's better to put your numbers in before rearranging, but we leave the variable we're trying to find as its letter. That's what we want to make the subject, making sure that everything traveling to the left, say, has a negative velocity, and you'll be left with one unknown variable. Rearrange to find it, and you'll get your answer. Incidentally, in the case of the cannon, as there's zero total momentum before, the same must be true afterwards, too. The cannonball is moving, though, so that must mean the cannon has the same momentum, but in the opposite direction. They still add up to zero, or we could say the momentums cancel each other out. This is an example of recoil. Newton's second law says that F equals MA, but we know that A is equal to change in velocity over time, delta V over T. So actually, if we multiply that by mass, we find that force is equal to change in momentum over time as well. Or we can say rate of change of momentum. The shorter the time taken for momentum to change, the bigger the force needed or felt. That's why we use seat belts, airbags, and crumple zones in cars. If you're in a crash, your change in momentum is the same, but when we have these safety features, they increase the time taken for this momentum to be lost, so a smaller force is felt, and you're more likely to survive. It's just two ways of looking at forces. The bigger the force, the faster the acceleration or deceleration, and so that also means the faster the momentum changes, too. All waves transfer energy without transferring matter. Oscillations or vibrations are passed along instead of the particles themselves. Longitudinal waves are those in which the direction of the oscillations is parallel to the direction of energy transfer, that is the direction the wave is going. Examples of these are sound waves and seismic P waves. The P stands for primary because they're the faster out of the two. In longitudinal waves, particles bunch up, we call these compressions, and when they're spread out, we call these rarefactions. Transverse waves, on the other hand, are those in which the direction of oscillations is perpendicular to the direction of energy transfer. In other words, they wiggle side to side or up and down. Examples of these are waves on the surface of water, seismic S waves, secondary, they're slower than P waves, and light, and also every other electromagnetic wave, too. We can represent any wave, even longitudinal waves, like this. We call this a waveform. The Y axis is displacement, basically just how far the particles have oscillated from their original position, we call that equilibrium, and the X axis can either be distance or time. The peak of a wave is called the amplitude. That's the maximum displacement from equilibrium, the zero line. If it's distance on the x-axis, one complete wave here gives you the wavelength. We give this the symbol lambda, but it's measured in meters. However, if it's time on the x-axis, one complete wave now gives you the time period instead, capital T for short. This is the time it takes for one complete wave to pass, measured in seconds. Frequency on the other hand is how many waves pass a point every second, and the unit is hertz. So, frequency and time period are the opposite. In fact, they're reciprocals of each other. So, we say frequency is equal to 1 / time period, f = 1 / t. You can often be asked to find frequency from a waveform like this. And so, that means you need to measure the time period, then just find the reciprocal. Easy. The wave equation is this, v = f lambda. Wave speed equals frequency * wavelength. A ripple tank will tell you what frequency the water is being oscillated at. To get the speed of these waves, we measure the distance between 10 peaks, then divide by 10 to get the wavelength, say, and then just use the wave equation to get the speed of the wave. You could also just time how long it takes for a ripple in a tray of water to travel the length of the tray 10 times, then just do total distance divided by time to get the speed that way. The speed of sound waves can be measured by attaching a microphone up to an oscilloscope, for example. If you clap once next to the microphone, the sound can echo off a wall a known distance away, and it comes back to the microphone. Then you can just use the oscilloscope to measure the time it took to travel, then do total distance divided by time. Sound waves cause the eardrum to vibrate, which is converted into a signal that travels to your brain. The human ear can hear frequencies between 20 hertz and 20 kilohertz, 20,000 hertz. Any frequency above this is called ultrasound. Whenever sound waves reach a boundary between two different mediums, materials, some of it goes through. We say it's transmitted, while some is reflected. This is the case when we emit ultrasound into a person's body, and a computer times how long it takes for it to return off different layers, allowing it to build up an image of what's inside. This allows us to scan babies safely. We can also time sound waves in water to build up a picture of what's under a boat or around a submarine. This is called sonar. We've mentioned seismic waves already, but you also need to know that while longitudinal P waves can travel through liquids, transverse S waves cannot. That's how we've come to the conclusion that the Earth has a molten outer core. There's no aftershock felt when an earthquake happens on the other side of the Earth, which suggests there must be a liquid center. When waves reflect off a smooth surface like a mirror, we say that's specular reflection. The angle of incidence will be equal to the angle of reflection. All of these angles are measured from the normal, which is the line we draw perpendicular to the surface. If light is scattered off a rough surface, we call this diffuse reflection instead. EM or electromagnetic waves are special because they don't need a medium to travel through. They're the only waves that can travel through the vacuum of space. There's a range of wavelengths in the EM spectrum, which we split up into these sections. >> Radio waves, microwaves, infrared radiation, visible light, ultraviolet, X-rays, gamma rays. >> If you haven't heard the original version of this, it's a certified banger. Link in description. EM waves are produced when electrons lose energy, and they lose the energy as an EM wave. The higher the frequency, the more energy the wave carries and the shorter the wavelength. The only exception are gamma rays, which are actually emitted by nuclei instead. That means they carry a lot more energy, and that's why they're dangerous. All of these waves, however, can be absorbed by electrons. This allows the retina in our eyes to detect light, for example, phone antennas to receive radio signals, and your face to absorb infrared from the sun and feel heat. UV, X-rays, and gamma rays carry so much energy though that they can cause electrons to leave their atoms. The atoms have been ionized. This can be dangerous if absorbed by DNA in cells as this can cause mutations that can lead to cancer. While some EM waves can be dangerous, we can use all parts of the spectrum for communications, cooking, heating, imaging, medical treatments, and more. When light waves move from one medium to another, say air to glass, their speed changes. In this case, the wave slows down and the wavelength also decreases. Instead of drawing the wave fronts from above like what you see above water, we can just draw a ray to show the direction that the light is moving. A change in medium also results in a change in direction. This is called refraction, that is if it's at an angle to the normal, the line we draw perpendicular to the surface. You can think of light always wanting to get away from the normal, but never write that in an exam. If light slows down, it moves closer to the normal, so that means the angle of refraction is smaller than the angle of incidence. That's the angle that it hits the surface at. Now, all of these angles are measured from the normal, so that means you must have your protractor with the zero on the normal. Never have it flat on the surface. It's always perpendicular to the surface. Lenses, oh boy, pretty tricky stuff here. Lenses are curved blocks of glass. Also, you have them in your eyes, don't you? They use refraction to make rays of light converge, meet, or diverge, spread out. A convex lens can make rays converge. This is the symbol that we use to represent it. If rays enter parallel to what we call the principal axis, for example, the light from an object very far away, the lens will make the rays converge at this point here. This is called the principal focus. The distance from the center of the lens to this is called the focal length. This doesn't change for a lens, and we can draw it on both sides, and you'll see why in a bit. However, light doesn't usually come from objects infinitely far away, but from objects a little bit closer. The object could be anything, but we often represent it with just an arrow. A convex lens can then project an image using the light that comes from the object. Before our diagram, we only need to consider the light coming from the top of the object. We can do that by drawing two rays. One always goes straight through the center of the lens, and one goes into the lens parallel to the principal axis, then through the principal focus. Where these two rays meet is where the image is formed. That's where you want your screen or retina or camera sensor to be in order to get a clear image formed. You'll also notice that the image is smaller than the object, so we say it's diminished. It's also upside down, so we say it's inverted. Things get a bit trickier when the object is very close to the lens. Now, the rays don't meet, and so we can't get an image formed. It can't be projected. However, if we extrapolate the two rays back behind the lens, they do meet. We can draw the image here. We can say that it's magnified, it's upright, but it's virtual. It's no longer a real image. That means that it can't be projected. This would be what a magnifying glass does, for example. Your eye can then deal with this diverging light to make it focus on your retina, but that means that you see this magnified virtual image, so things appear bigger. Concave lenses always diverge light rays. They always produce a virtual image. With these, our line parallel in goes back through the other principal focus behind the lens. Where it meets the other ray is where the virtual image is. This image is also diminished and upright, as you can see. The magnification of a lens is just the ratio of image height to object height. A magnification greater than one means the image is bigger than the object, less than one, it's diminished. It's smaller than the object. What we perceive as color is a result of different wavelengths of light being emitted by a source or reflected by an object that are then absorbed by the cells in our retina. Most objects will absorb some wavelengths of light while reflecting others. For example, chlorophyll in plants absorb longer red wavelengths of light, which is why leaves appear green. It reflects those shorter wavelengths. This ball looks blue in sunlight because it reflects the blue wavelengths of light. If you only shine red light on it though, it will appear black as that red light will be absorbed and no light is reflected. A black body is a theoretical object that perfectly absorbs and emits all wavelengths of radiation. While there's no such exact thing in reality, it's still a useful concept that we can apply to some objects, like stars in particular. If a body or object absorbs radiation at a greater rate than it's emitting it, its temperature will increase. But, if the temperature increases, that also means it will start emitting radiation at a greater rate, too. A permanent magnet is a metal in which the molecules are permanently aligned in such a way that they produce a magnetic field, which can exert a force on particles in other objects and also electrons. We give the two ends of a magnet the names north and south pole, short for north-facing and south-facing poles, because that's the way they would point to line up with the Earth's magnetic field, say if we made them float. You can use iron filings or mini compasses placed around a magnet to visualize its magnetic field. Magnetic field lines are always complete loops, even though we don't draw them inside the magnet, and they never touch. These ones going out the ends here will eventually loop back around if we carried on drawing them. The direction of magnetic field lines is always from the north pole to the south pole of magnets. An induced magnet is a material, usually a metal, whose particles align temporarily when it's placed in a magnetic field. So, it makes its own magnetic field. That's why an iron nail can be attracted to either the north or south pole of a permanent magnet when placed near it. So, we say iron is magnetic, but it is not a magnet. Cobalt and nickel are also magnetic. Copper and aluminum, for example, are not. Bring two permanent magnets together and they will attract each other if their opposite poles are facing, and they repel if the same poles are facing each other. A solenoid is just a coil of wire that an electrical current can be passed through and it produces a magnetic field, the shape of which is very similar to that of a bar magnet. You can increase the strength of the magnetic field made by increasing the current or adding more turns in the coil. A current flowing through even a straight piece of wire will produce its own magnetic field. We draw the field lines as concentric circles around it using our right hand to remember which way the field goes. We use the letter B as a shorthand for a magnetic field, by the way, as well as in the equation coming up. The motor effect is when such a wire is in another magnetic field and it experiences a force. The equation is F bill, F = B I L, where F is force, I is current in amps, L is length of the wire in the magnetic field and B is the magnetic flux density, essentially the magnetic field strength. This is measured in Tesla. Note that this equation only works as it is if the current and magnetic field lines are perpendicular to each other. So, that means that if the wire runs parallel to the field lines, it won't experience a force. However, to find out the direction of the force on the wire, we use Fleming's left-hand rule. Your thumb is force, first finger is field, middle finger is current. Make a gun shape with them where your middle finger is on the trigger, so they're all perpendicular and bam, freeze, FBI. Just twist your wrist to line up your fingers with the current and the field, always north to south pole, and the direction your thumb is pointing is the direction of the force on the wire. In this case, it's upwards. To measure the size of this force in reality, we put the magnet on a balance. Due to Newton's third law, the magnet will also be pushed down with the same force. Calculate the force from the simulated mass measured, use an ammeter to get the current and a ruler to measure the length of the wire, and you can calculate the magnetic flux density between the poles of your magnet by rearranging the equation. Electric motors, of course, employ the motor effect by using a coil of wire that experiences opposite forces on both sides, causing it to turn. However, the current must be reversed every half a turn, otherwise it would just stop in the vertical position in this case, so that's why we have what we call a split-ring commutator to reverse the current every half turn. To make a motor turn faster, you can increase the current, use a stronger magnet, or add more turns to the coil. So, essentially, there's more wire experiencing the force. A loudspeaker is, in essence, just a motor that goes back and forth instead of round and round. The varying current, due to the signal from your phone, say, will cause the coil to vibrate back and forth, and that's attached to the speaker cone, which then produces sound waves in the air. So, a magnet will cause a current-carrying wire to move, but the opposite is also true. A wire that's moved through a magnetic field will result in a current being induced in it, if it's in a complete loop, of course. The electrons will move. To be more precise, we should say a potential is induced in it, essentially voltage. This is called the generator effect, sometimes known as the dynamo effect. A generator itself looks like a motor. You turn the coil, and a potential will be induced in the coil. This is basically how power stations work. The steam made from burning fuels or nuclear fission turns the turbine, which turns this coil in the generator. A generator doesn't need a split-ring commutator, it still works. All that it means is that it's an alternating PD that's induced, or alternating current, AC, is produced. If you do add a split-ring commutator, it just gets rid of the alternating part of the current, so you've now got DC coming out of it instead. Lumpy DC, but DC nevertheless. Technically, this is now a dynamo. To increase the output of a generator or a dynamo, just turn it faster, or, similar to a motor, add more turns to the coil, or use a stronger magnet. I say turn it faster, but it's not that easy. You see, the current induced in the coil also produces its own magnetic field, and this opposes the turning that led to it being produced to begin with. So, that's why it requires energy to keep it turning. And that makes sense, doesn't it? We can't get energy for free. Similar to a loudspeaker being a back-and-forth motor, a microphone is a back-and-forth generator. Sound waves move the diaphragm back and forth, which is attached to a coil that moves back and forth around a magnet, and then this induces a potential in the coil. That signal then travels through the wires to the phone, recorder, or whatever. Transformers are used in the national grid to change the voltage at which electricity is transmitted through the overhead cables. The current from a power station is so high that too much energy would be lost due to the resistance in the cables if it just went straight into them. Therefore, a step-up transformer increases the voltage before it enters the grid. This in turn reduces the current, so less energy is lost as heat due to resistance. The reason one goes up when the other one goes down is because electrical power is equal to voltage or PD times current, V times I. In an ideal world, the power in and out of the transformer should be the same. That would mean it's 100% efficient, so V and I would be inversely proportional. We can therefore say that V times I for the primary coil is equal to V times I for the secondary coil. This is the basic makeup of a transformer. The primary coil is connected to the power station, say in this case, and the secondary coil is connected to the overhead cables. There are more turns on the secondary coil here, which means that it's a step-up transformer. The voltage will increase, the current will decrease. More turns, higher voltage. The coils are wrapped around a soft iron core. Get this into your head right now, though. There is, or should be, no electricity or current in the core. Instead, the electricity is wirelessly transmitted from one coil to the other, much like how an electric toothbrush is charged or wirelessly charging a phone. How is this? Well, it's because the alternating current in the primary coil produces a magnetic field, and the iron core acts like a guide for this. We use iron for the core, by the way, as it's easily magnetized and demagnetized. In other words, it's a good guide for the magnetic field. This magnetic field then induces a voltage and current in the secondary coil. In order for a current to be induced though, a wire must experience a change in a magnetic field, which is why we must use AC. If we use DC in the primary coil, it would make a magnetic field, but it would be static. It wouldn't be changing, and that can't induce a current in the secondary coil. The ratio of turns in the coils is equal to the ratio of the voltages. So, if the secondary coil has double the turns, it has double the voltage and therefore half the current compared to the primary coil. So, we can say NP over NS equals VP over VS. You can also flip the whole thing when it comes to rearranging to find VS or NS. A step-down transformer at the other end of the cables steps the voltage back down to a safer PD of 230 V, which means that it must have fewer turns on the secondary coil this time. Our solar system of the Sun with eight planets orbiting it with an asteroid belt as well between Mars and Jupiter. Our Moon and other planets' moons are natural satellites. Our solar system is one of many found in our galaxy, which we call the Milky Way. It's believed that stars are the result of dust and gas particles in clouds. We call such a cloud a nebula being attracted to each other due to gravity. The cloud becomes more dense and hotter until fusion starts to occur. A star will remain stable so long as the outward pressure from fusion and the force of gravity pulling inward remain balanced. We say it's in the main sequence stage of its cycle. When a star dies, the outward pressure increases, which causes it to expand, turning it into a red giant if it's a star of similar size to our Sun, or a super red giant for stars much bigger than our Sun. Once all the fuel for fusion has run out, a red giant will then collapse into a white dwarf and then a black dwarf once it's cooled. A super red giant, however, explodes. We say it's gone supernova, leaving a very dense neutron star at the center or an even more dense body that causes a black hole. The outer layers of the supernova move away forming new nebulae, nebulas, from which new stars could be made. Nuclei fuse together to make heavier elements. Some of these could only be made as a result of the huge amount of energy released from a supernova. Our moon is a natural satellite, but we can make artificial satellites. Some satellites orbit in a circle around the Earth like geostationary satellites. These sit above the same spot above the equator. As such, that means the time period of their orbit is 24 hours, and they're generally used for communication. They move at a constant speed, yet their direction is constantly changing. So, technically, their velocity is also changing, which means they're accelerating towards the Earth. They don't get any closer though. It's like they want to get closer to the Earth, but they're going too fast to do that. Any force that results in circular motion is called a centripetal force, and that always acts towards the center of its orbit. We can draw the velocity at any point as a tangent. Think about it this way, if the Earth suddenly disappeared, that's where the satellite would fly off. That means that centripetal force and velocity are always at right angles to each other. They're perpendicular. Other satellites travel in elliptical orbits, and they're used for reconnaissance, weather, GPS, for example. The closer they get to the Earth, the faster they move, and vice versa. We have a good idea of what wavelengths of light are emitted from stars. However, when we look at distant stars and galaxies, these wavelengths appear longer. They've been shifted towards the red end of the spectrum. We say the light has been red shifted, similar to when the pitch of an ambulance siren drops when it's moving away from you. Like, "Nee-naw, nee-naw, nee-naw, nee-naw." This shows that galaxies are moving away from us, and this is the case in every direction we look. Not only that, the light waves from more distant galaxies are even more red shifted, suggesting they're moving faster away from us. We say they're receding faster at a faster rate. This implies that if we go back in time, all of these galaxies appear to have originated from the same point in space. This is therefore used as evidence for the Big Bang Theory, and it suggests that the observable universe is still expanding. The other piece of evidence for it is CMBR, cosmic microwave background radiation. Outer space might look dark, but we can detect microwave radiation being emitted from very far away from all directions. This could be emitted as a result of matter still cooling down, so it seems we're looking at the edge of the Big Bang, which in essence means it's still going. So, I hope you found that helpful. Leave a like and a comment if you did, and click on the card to take you to the playlist for all of the papers, and don't forget to check out the Quiz Shorts app to help you test your knowledge, and I'll see you next time.