# Physics Unit 2

Unit 2 Physics

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## Velocity and Acceleration, DT and VT graphs

Speed - refers only to how fast an object is moving

Velocity - must also include the direction specified (e.g. 30mph north)

Acceleration = Change in velocity // Time taken

Distance time graph - Gradient = speed = Verticle // horizontal

Velocity time graph - Gradient = acceleration = Verticle // horizontal

- To find the distance travelled, you work out the area under the graph

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## Mass, weight and gravity, Laws of motion

Gravity - The force of attraction between all masses.

- Everything accelerates towards the ground at apprx 10m/s

- It gives everything a weight and helps keep planets in orbit.

Mass - The amount of stuff in an object. Same on moon or earth.

Weight (N)- The pull of gravity on an object. Would be less on the moon.

Weight = m x g (mass x gravitational field strength) (g=10ish)

Balanced forces mean no change in velocity (stationary/constant speed)

- For a steady speed, there needs to be a zero resultant force

A resultant force means acceleration

F = mass * acceleration        or       a = F/m

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## Laws of motion, Drag and terminal velocity

Reaction forces - if object A exerts a force on object B, object B exerts the exact opposite force on object A.

- Objects still move because of the difference in mass

Friction - acts in the opposite direction of the movement to slow an object down.

*To reduce drag in fluids, you need to make an object as streamlined as possible*

Drag - Increases as speed increases

Terminal velocity - When an object sets off, it has more force accelerating than resistance slowing them down. As speed increases, resistance builds up. This gradually reduces acceleration until the resistance force is equal to the accelerating force. This is terminal velocity.

Terminal velocity is dependant on an objects shape and area

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## Stopping distance, Work done.

Stopping distance = Thinking distance + Braking distance

Thinking distance is affected by how fast you're going and how aware you are.

Braking distance is affected by how fast you're going, how heavy the vehicle is, how good the brakes are and how good the grip is.

Work done = Energy transferred

When a force moves on an object, energy is transferred and 'work is done'.

Energy is supplied by whatever wants to move something.

The work is the actual moving of the object and the energy is transferred into other forms.

Work done = Force x Distance

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## Kinetic and Potential energy, Momentum and Collisi

Kinetic energy is energy of movement and is dependant on mass and velocity.

Kinetic energy = 1/2  x  mass  x  velocity^2

Potential energy is a way in which energy is stored.

Elastic potential energy - Energy stored when work is done on an object to change its shape.

Gravitational potential energy - Energy stored in an object when it is raised to a height against its gravity.

Momentum = mass  x  velocity

Momentum before = Momentum after

Force acting(N) =Change in momentum(kg m/s)  // Time taken for change(s)

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## Static electricity

Build up of static is caused by friction...

when two insulating materials are rubbed together, electrons will be scraped off one and deposited on the other. This will leave a positive static charge on one, and a negative static charge on the other. The way in which electrons are transferred is dependant on the materials. Positive charges never move!

As charge builds up, so does voltage - causing sparks!

The greater the charge, the greater the charge between it and the earth, if the voltage gets big enough then the spark will 'jump' across the gap.

Photocopier - Image plate is positively charged and an image you want photocopying is projected onto it. The whiter parts of image make light fall on the plate and the charge leaks away in those places. The charged bits attract negatively charged black powder, which gets transferred onto the positively charged paper. The paper is heated so the powder sticks..bam, job done.

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## Static electricity continued...Circuits

Clothing - when synthetic clothes are dragged across each other, electrons get scraped off leaving charges on both parts, leading to attraction and sparks when the charges rearrange themselves.

Lightening - Rain and ice bump together in clouds knocking electrons off and leaving a positive charge on the top of the cloud and a negative charge on the bottom. This creates huge voltage and a massive spark.

Current - flow of electrons around a circuit, it will only flow across a component if there is a voltage.  Unit: ampere, A.

Voltage - driving force that pushes current round (like pressure) Unit: volt, V.

Resistance - anything in a circuit that slows the current down. Unit: ohm, .

There's a balance: the voltage is trying to push the current round and the resistance is opposing it. If you increase the voltage, more current will flow but if you increase the resistance, less current will flow.

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## Circuits continued...Resistance

Ammeter - measures current in amps flowing through the component. Must be placed in series but can be put anywhere in the circuit.

Voltmeter - Measures voltage in volts across the component. Must be placed in parallel around the component under test. Voltage = potential difference.

IMPORTANT VOLTAGE-CURRENT GRAPHS

Filament lamp                               Resistor

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## Resistance and V= I x R

Resistance = Potential difference

Diode                                    ----------------------------------------

Current

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LED

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## Circuit symbols and devices continued...

Variable resistor - Resistor whose resistance can be changed by a knob or slider. Great for altering current flow through a circuit.

Diode - Device made from semi-conductor material like silicon. Lets current flow freely through in one direction, but not the other.

Light dependent resistor - In bright light, resistance falls and in darkness it is highest. Useful for burglar detectors or automatic night lights.

Thermistor - temperature dependant resistor. In hot conditions the resistance drops, in cool conditions the resistance goes up. Useful as temp detectors.

Ammeters - always connected in series, even in a parallel circuit

Voltmeters - always connected in parallel with a component, even in a series circuit.

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## Series circuits, Parallel Circuits

In series circuits, components are connected along one wire end to end between positive and negative of power supply. If only one component or wire is removed, the whole circuit breaks.

Potential difference is shared so V = V1 + V2 + V3 etc.

Current is the same anywhere so A1 = A2

Resistance adds ups so R = R1 + R2 + R3

Cell voltage adds up so 2 lots of 1.5 will give 3

In parallel circuits, each component is separately connected to the supply so one can be disconnected without the circuit breaking.

Potential difference is the same so V1 = V2 = V3

Current is shared so A = A1 + A2 + A3

Resistance is tricky, the current flowing through each component depends on its resistance. The total resistance is always less than the branch with the least resistance.

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## Series and parallel circuits, Mains electricity

Christmas fairy lights are wired in series which is why they're such a pain - when one goes they all go!The only advantage is that the bulbs can be tiny as the 230v is shared out between them.

Everything in a car is connected in parallel so that each component can be turned on and off separately. And, everything always gets the full voltage of the battery.

Mains supply is AC, battery supply is DC

UK mains supply is apprx 230v. It is AC (alternating current) meaning the current is constantly changing direction. Frequency of AC is 50Hz.

By contrast, cells and batteries supply direct current (DC) meaning the current keeps flowing in the same direction.

Oscilloscope - A cathode ray oscilloscope is like a snazzy voltmeter.

An AC supply will show up as a regularly repeating wave.

A DC supply will show a straight line.

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## Mains electricity, Fuses and Earthing

Plugs - They must be wired right to avoid hazards.

Blue wire is neutral, green/yellow is earth, brown is live.

Plug wiring errors - Blue and brown wrong way round, earth not connected, base wires showing.

Fuses prevent electric shocks - To prevent surges of current and shocks, a fuse should be placed within the circuit. If the current gets too big, the fuse wire heats up and the fuse blows, breaking the circuit preventing shocks. They should be rated as near as possible but just above the normal operating current.

Earthing prevents fire and shocks - The earth pin is connected to the case via the earth wire. If a fault develops in which the live somehow touches the case, then because the case is earthed, a big current flows in through the live, through the case and down the earth wire. The surge in current blows the fuse, cutting off the live supply.

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## Energy and Power in circuits, Charge, voltage and

Energy is transformed in circuits....

Motion: Motors, Light: Light bulbs, Heat: Hair-dryers/kettles, Sound: Speakers.

All resistors produce heat when a current flows through them because the electrical energy is turned into heat energy. The more current, the more heat.

Energy = Power  x  Time     (E=P x t)

Power = Voltage  x  Current   (P=V x I)

Total charge = Current x Time  (Q=I x t)

Energy transformed = Charge x Potential difference   (E=Q x V)

When an electrical charge goes through a change in voltage, the energy is transferred.

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## Atomic structure

Rutherford's plum pudding...

Isotopes are different forms of the same element. Isotopes have the same number of protons but a different number of neutrons. So they have a different mass number but the same atomic number. Unstable isotopes are radioactive so decay and turn into other elements and give out radiation.

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Alpha particles are helium nuclei.  Big heavy and slow moving. Don't penetrate far and are strongly ionising.

Beta particles are electrons. Move quite fast and are quite small so penetrate moderately and are moderately ionising. For every Beta particle emitted, a neutron turns into a proton in the nucleus.

Gamma rays are very short EM waves. Opposite of Alpha particles - no mass and teeny tiny. Penetrate a long way and are weakly ionising. Gamma emission never changes the proton or mass numbers of the nucleus.

Food, air, building materials, explosions, or dumped nuclear waste.

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## Nuclear fission and fusion

Nuclear fission - the splitting up of a big atomic nuclei.

Nuclear reactors - A controlled chain reaction where the atomic nuclei is split up and releases energy in the form of heat. The heat is then used to heat water to drive a steam turbine.

Nuclear fusion - Two light nuclei come together to create a larger nucleus. Fusion releases a lot of energy and doesn't leave behind a lot of radioactive waste. Problem is, it can only happen at really high temperatures. No materials can stand that temperature so fusion reactors are hard to build, requiring hot hydrogen being contained in a magnetic field rather than a physical container.

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