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# Physics Cheat Sheet by Prithvi_veeraraju

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### Electr­oma­gnetism

 Electr­omotive force and potential difference - The Electr­omotive Force (e.m.f.) is the name given to the potential difference of the power source in a circuit - It is defined as The electrical work done by a source in moving a unit charge around a complete circuit - The Electr­omotive Force (EMF) is measured in volts (V) Altern­ating Current - An altern­ating current (a.c.) is defined as A current that contin­uously changes its direction, going back and forth around a circuit - An a.c. power supply has two identical terminals that switches between positive and negative - The current is therefore defined as positive or negative, depending on which direction it is flowing at that time - The frequency of an altern­ating current is the number of times the current changes direction back and forth each second - In the UK, mains electr­icity is an altern­ating current with a frequency of 50 Hz and a potential difference of around 230 V The EMF is the voltage supplied by a power supply: 12 V in the above case - The definition of e.m.f. can also be expressed using an equation - Where - E = electr­omotive force (e.m.f.) (V) - W = energy supplied to the charges from the power source (J) - Q = charge on each charge carrier (C) Mains Electr­icity - Mains electr­icity is the electr­icity generated by power stations and transp­orted around the country through the National Grid - Everyone connects to the mains when plugging in an appliance such as a phone charger or kettle - Mains electr­icity is an altern­ating current (a.c.) supply - In the UK, the domestic electr­icity supply has a frequency of 50 Hz and a potential difference of about 230 V - A frequency of 50 Hz means the direction of the current changes back and forth 50 times every second - Mains electr­icity, being an altern­ating current, does not have positive and negative sides to the power source - The equivalent to positive and negative are called live and neutral and these form either end of the electrical circuit Electric Power - Power is defined as The rate of energy transfer or the amount of energy transf­erred per second - The power of a device depends on: - The voltage (pote­ntial differ­ence) of the device - The current of the device - The power of an electrical component (or appliance) is given by the equation: P = IV Induced EMF - An EMF will be induced in a conductor if there is relative movement between the conductor and the magnetic field - It will also be induced if the conductor is stationary in a changing magnetic field - For an electrical conductor moving in a fixed magnetic field - The conductor (e.g wire) cuts through the fields lines - This induces an EMF in the wire - Work is done when charge flows through a circuit - Work done is equal to the energy transf­erred - The amount of energy transf­erred by electrical work in a component (or appliance) depends upon: - The current, I - The potential difference, V - The amount of time the component is used for, t - When charge flows through a resistor, for example, the energy transf­erred is what makes the resistor hot - The energy transf­erred can be calculated using the equation: E = P × t - Where: - E = energy transf­erred in joules (J) - P = power in watts (W) - t = time in seconds (s) - Since P = IV, this equation can also be written as: E = I × V × t Lenz’s Law The direction of an induced potential difference always opposes the change that produces it. - This means that any magnetic field created by the potential difference will act so that it tries to stop the wire or magnet from moving Direct Current - A direct current (d.c.) is defined as A current that is steady, constantly flowing in the same direction in a circuit, from positive to negative - The potential difference across a cell in a d.c. circuit travels in one direction only - This means the current is only positive or only negative - A d.c. power supply has a fixed positive terminal and a fixed negative terminal - Electric cells, or batteries, produce direct current (d.c.) Demons­trating Lenz's Law - If a magnet is pushed north end first into a coil of wire then the end of the coil closest to the magnet will become a north pole - Explan­ation - Due to the generator effect, a potential difference will be induced in the coil - The induced potential difference always opposes the change that produces it - The coil will apply a force to oppose the magnet being pushed into the coil - Therefore, the end of the coil closest to the magnet will become a north pole - This means it will repel the north pole of the magnet

### Nuclear and Atomic Physics

 - Atoms are the smallest particles in the world and they cannot be broken down into smaller parts since they are the smallest. - in Nuclear Physics there are 3 particles: - Alpha - Beta - Gamma Gamma These are the most penetr­ating and are stopped only by many centim­etres of lead. They ionise a gas even less than beta particles and are not deflected by electric and magnetic fields. Ionizing effect of radiation - A charged electr­oscope discharges when a lighted match or a radium source (held in forceps) is brought near the cap and this causes radiation Half Life The rate of Decay is unaffected by temper­ature but every radioa­ctive element has its own definitive decay rate, expressed by it’s half life. This is the average time for half the atoms in a given sample to decay. Alpha These are stopped by a thick sheet of paper and have a range in air of only few centim­etres since they cause ionisation in a gas due to frequent collisions with gas molecules. Background Radiation A type of radiation that is being recieved from the surrou­ndings like e.g. Underg­round, In the sky etc. Beta These are stopped by a few millim­etres of aluminium and some have a range in air of several metres and their ionizing power is much less than that of alpha particles. As well as being easily deflected by electric fields, they are more easily deflected by magnetic fields.

### General Physics

 Momentum - An object with that is in motion has momentum which is defined by the equation (Momentum = mass x velocity). Kinetic energy - The kinetic energy, E, of an object (also known as its kinetic store) is defined as: K The energy an object has as a result of its mass and speed - This means that any object in motion has energy in its kinetic energy store - Kinetic energy can be calculated using the equation: EK = ½ × m × v2 Collisions The total momentum before a collision = The total momentum after a collision - Before the collision: - The momentum is only of mass m which is moving - If the right is taken as the positive direction, the total momentum of the system is m × u - After the collision: - Mass M also now has momentum - The velocity of m is now -v (since it is now travelling to the left) and the velocity of M is V - The total momentum is now the momentum of M + momentum of m - This is (M × V) + (m × -v) or (M × V) – (m × v) Gravit­ational potential energy - The gravit­ational potential energy, E, of an object (also known as its gravit­ational store) is defined as: P The energy an object has due to its height in a gravit­ational field - This means: - If an object is lifted up, energy will be transf­erred to its gravit­ational store - If an object falls,­ energy will be transf­erred away from its gravit­ational store - The GPE of an object can be calculated using the equation: ΔEP = mgΔh Impulse - When a resultant (unbal­anced) force acts on a mass, the momentum of that mass will change - The impulse of a force is equal to that force multiplied by the time for which it acts: impulse  =  force × change in time impulse = FΔt - The change in momentum of a mass is equal to the impulse provided by the force: impulse = change in momentum impulse = FΔt  = Δp - Change in momentum can also be described as: Δp = Δ(mv) Δp = mv − mu - Where: - m = mass in kg - v = final velocity in m/s - u = initial velocity in m/s - Therefore: impulse = FΔt  = Δp = mv − mu Work done - Work is done when an object is moved over a distance by a force applied in the direction of its displa­cement - It is said that the force does work on the object - If a force is applied to an object but doesn’t result in any movement, no work is done Work is done when a force is used to move an object - The formula for work done is: Work done = force × distance W = fd Energy - Energy is a property that must be transf­erred to an object in order to perform work on or heat up that object - It is measured in units of Joules (J) - Energy will often be described as part of an energy system - In physics, a system is defined as: An object or group of objects - Therefore, when describing the changes within a system, only the objects or group of objects and the surrou­ndings need to be considered - Energy can be stored in different ways, and there are changes in the way it is stored when a system changes - The principle of conser­vation of energy states that: Energy cannot be created or destroyed, it can only be transf­erred from one store to another - This means that for a closed system, the total amount of energy is constant Efficiency of energy transfer - The efficiency of a system is a measure of how well energy is transf­erred in a system - Efficiency is defined as: The ratio of the useful power or energy transfer output from a system to its total power or energy transfer input - If a system has high effic­iency, this means most of the energy transf­erred is useful - If a system has low effic­iency, this means most of the energy transf­erred is wasted Pressure - Pressure is defined as The concen­tra­tion of a force or the force per unit area - For example, when a drawing pin is pushed downwards: - It is pushed into the surface, rather than up towards the finger - This is because the sharp point is more concen­trated (a small area) creating a larger pressure Liquid Pressure - A fluid is either a liquid or a gas When an object is immersed in a fluid, the fluid will exert pressure, squeezing the object - This pressure is exerted evenly across the whole surface of the fluid and in all directions - The pressure exerted on objects in fluids creates forces against surfaces - These forces act at 90 degrees (at right angles) to the surface The pressure of a fluid on an object creates a force normal (at right angles) to the surface - The pressure of a fluid on an object will increase with: - Depth within the fluid - Increased density of the fluid ### Calcul­ating Pressure in Liquids - The pressure is more accurately the difference in pressure at different depths h in a liquid, since the pressure changes with the depth - The pressure due to a column of liquid can be calculated using the equation Δp = ρgΔh or in simple words —> \$Rho Gravity Height\$
revision notes

### Properties of Waves

 - Waves transfer energy and inform­ation - Waves are described as oscill­ations or vibrations about a fixed point - For example, ripples cause particles of water to oscillate up and down - Sound waves cause particles of air to vibrate back and forth - In all cases, waves transfer energy without trans­fer­ring matter - For water waves, this means it is the wave and not the water (the matter) itself that travels - For sound waves, this means it is the wave and not the air molecules (the matter) itself that travels - Objects floating on water provide evidence that waves only transfer energy and not matter Frequency - Frequency is defined as: The number of waves passing a point in a second - Frequency is given the sy­mbol f and is measured in Hertz (Hz) Types of Waves - Transverse - e.g. vibrations of guitar string. - Longit­udinal - e.g. Tsunami waves. Wave Speed - Wave speed is the speed at which energy is transf­erred through a medium - Wave speed is defined as: The distance travelled by a wave each second - Wave speed is given the symbol, ν, and is measured in metres per second (m/s), it can be calculated using: \$wave speed = frequency × wavele­ngth\$ Features of a wave - When describing wave motion, there are several terms which are important to know, including: - Crest (Peak) - Trough - Amplitude - Wavelength - Frequency - Wave speed - Wavefront Wavefront - Wavefronts are a useful way of picturing waves from above: each wavefront is used to represent a single wave - The image below illust­rates how wavefronts are visual­ised: - The arrow shows the direction the wave is moving and is sometimes called a ray - The space between each wavefront represents the wavelength - When the wavefronts are close together, this represents a wave with a short wavel­ength - When the wavefronts are far apart, this represents a wave with a long wavel­ength - Wave speed is defined as: The distance travelled by a wave each second - Wave speed is given the symbol ν and is measured in metres per second (m/s) - Wave speed is the speed at which energy is transf­erred through a medium - Transverse and longit­udinal waves both obey the wave equation: V = f x lambda Crests & Troughs - A crest, or a peak, is defined as: The highest point on a wave above the equili­brium, or rest, position - A trough is defined as The lowest point on a wave below the equili­brium, or rest, position** - Where: - v = wave speed in metres per second (m/s) - f = frequency in Hertz (Hz) - λ = wavelength in metres (m) Amplitude - Amplitude is defined as: The distance from the undist­urbed position to the peak or trough of a wave - It is given the symbol A and is measured in metres (m) - Amplitude is the maximum or minimum displa­cement from the undist­urbed position Transverse Waves - Transverse waves are defined as: Waves where the points along its length vibrate at 90 degrees to the direction of energy transfer Wavelength - Wavelength is defined as: The distance from one point on the wave to the same point on the next wave - In a transverse wave: - The wavelength can be measured from one peak to the next peak - In a longit­udinal wave - The wavelength can be measured from the centre of one compre­ssion to the centre of the next - The wavelength is given the symbol λ (lambda) and is measured in metres (m) - The distance along a wave is typically put on the x-axis of a wave diagram Longit­udinal Waves - Longit­udinal waves are defined as: Waves where the points along its length vibrate parallel to the direction of energy transfer

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