Physics

Unit 2

Velocity & Acceleration

Forces

Work and Energy

Momentum

Static Electricity Circuits

Mains Electricity

Power in electrical appliances

Radioactive decay

Fission and fusion

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Velocity & Acceleration

Distance-time graphs:

They show the distance an object, e.g. a car has moved in a certain amount of time.

VideosAcceleration Part 1Acceleration Part 2Speed and Velocity

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Acceleration= change in velocity (m/s)

time taken for change (s)(m/s²)

Velocity-time graphs• Velocity is speed in a given direction

• Acceleration is change of velocity per second

A body travelling at a steady speed is accelerating if its direction is changing.

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Forces• Force is measured in newton's (N)• The force of friction always acts in the opposite direction of

the movement.• Friction occurs when an object moves through air or water,

when solid objects slide (they heat up and wear away).• Weight= mass x gravitational field strength

(newton, N) (kilogram, kg) (newton/kilogram, N/kg) Usually 10N/Kg, unless the question states otherwise.

Stopping DistanceStopping distance of a car= braking + thinking distance

Depends on the weather conditions (wet/icy roads) and the vehicle, (worn brakes/tyres).

Depends on the driver’s reaction time: tiredness, drugs, alcohol, and the speed.

Remember: You cannot SLOW down time, you can just INCREASE collision time, INCREASE reaction time etc.

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• A stationary object remains still because the resultant force is zero. This is because both forces acting on it are balanced.

• E.g. a book is sat on in your hand, the weight of the book is equivalent to the force your hand is exerting to keep it in place.

Resultant force= mass x acceleration (newton, N) (kg) (m/s²)

Terminal velocity= constant speedWhen an object falls through a fluid, the faster it moves, the greater the force of friction.When a body falls:1) At the start, it accelerates (due to the force

of gravity-weight),2) Frictional forces (e.g. air resistance)

increase …until they balance the gravitational force,

3) Then the resultant force is zero, and the body falls at its ‘terminal velocity’.

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Work and Energy• A moving object also has movement energy, also called kinetic energy.• When a force moves an object, energy is transferred and work is done: Work done (joules , J) = energy transferred (joules , J) Work done (J) = force applied (N) x distance moved in the direction of the force (m)

Kinetic energy= ½ x mass x speed²Kinetic energy can be transformed into other forms of energy as shown in the table:Example: K.Energy is

transformed into:A car braking Heat in brakes +

tyresA wind turbine Electricity, heat,

soundRollercoaster car, going up a ramp

Gravitational potential energy (g.p.e), heat

An object has more kinetic energy:-If it has a bigger mass-If it travels at a higher speed

VideoPotential & Kinetic Energy

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Kinetic Energy= Work Done

Momentum

• Momentum has both MAGNITUDE and DIRECTION.

• ‘Momentum is conserved’ means that the momentum before=momentum after. i.e. the total momentum remains the same.

• In a collision, momentum isn’t always conserved because there are external forces acting on the colliding objects.

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Momentum=Mass x Velocity

Force = change in momentum ÷ time taken for change Newtons kgm/s seconds

In a car crash…• Crumple zones are created to increase collision time,

therefore decreasing the force.• Seatbelts stretch slightly, increases the time,

decreasing the force acting on the driver.• Airbags also slow you down more gradually.Remember always refer to, ‘force= change in momentum ÷ time taken for change’.

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Static Electricity

• Electrostatic precipitators and photocopiers make use of electrostatic charges.

• When certain materials rub against each other they become charged.

• The material that gains electrons becomes negatively charged.

• The material that loses electrons becomes positively charged.

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Electrostatic Precipitators

• Smoke particles pick up a negative charge.

• Smoke particles are attracted to the collecting plates.

• Collecting plates are knocked to remove the smoke particles.

DangersThe more charge put on an object, the higher the voltage (p.d) between the object and earth. If the p.d is very high, then a spark can jump between the object and any earthed conductor.

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Preventing fires and explosions

• A liquid flowing through a pipe can become charged by rubbing. This can be dangerous because it could cause a spark if the substance is inflammable.

• Whenever an aeroplane is being refuelled by a tanker, they are always connected by a copper wire.

• Friction between the fuel and pipe gains electrons, charging the fuel and pipe, earthing the object prevents a spark and explosion.

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Circuits NextBack

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Resistors in series Resistors in parallel

The same current goes through The current is shared all the components The p.d is equal for bothThe p.d across the cell is shared Resistance= R1 + R2

Voltmeters are placed parallel

Ammeters are placed in series

Light Dependent Resistor (LDR)

Thermistor

Potential difference (V) = current (I) x resistance (R) amps

Mains Electricity• Frequency of mains supply in UK = 50 Hertz

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Alternating Current (a.c) Direct current (d.c)

-Is constantly - Flows in only one direction changing direction-As the frequency is 50Hz, each cycle lasts for 1/50th second

The mains supply in the UK is 230VThe live terminal alternates between a positive and negative voltage with respect to the

neutral terminal.The neutral terminal stays at a voltage close to 0 with respect to earth.

The 3 pin plug

The cable has a plastic cover because plastic is a good insulator.Copper wires inside the cable are good conductors.

The Earth Wire- for safetyAppliances with metal cases need to be earthed.The case is connected to the earth pin (by the green/yellow wire).If a fault connects the case to the live wire, then a large current flows to earth and melts the fuse.The fuse:Must be in the live wire so the appliance becomes disconnected,Should have a value (rating) higher than (but as close as possible to) the normal working current,Can be replaced by a circuit breaker.

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Power in electrical appliances

• Electrical appliances are used to transform electrical energy to some other useful form of energy.

• This may be light, sound, heat, kinetic energy and many more. Power(W)= energy transformed(J) ÷ time taken(s)

Power(W)= current (A) x p.d (V)Energy Transferred(J)= p.d (V) x charge (Coulomb)

Charge(C)= Current(A) x Time (S)

An electric current is a flow of chargeFrequency = 1 ÷ time

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Radioactive decay• Atomic StructureAt one time, scientists believed in a ‘plum pudding’ model of the atom. They believed that the negative electrons were stuck in a blob of matter.

Then Rutherford and Marsden fired alpha particles at gold foil (gold leaf) which scattered them, as shown:

Science Daily

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Rutherford had shown that the atom had a tiny, heavy, positively charged nucleus.

Protons +Neutrons = Mass NumberProtons = Atomic numberNumber of Electrons =Number of Protons

The nucleus is made up of protons AND neutrons, with electrons orbiting.In a neutral atom, the number of electrons equals the number of protons.Atoms can gain or lose electrons to become charged particles called ions.

Different elements have a different number of protons (atomic number).Atoms of the same element can have different number of neutrons but to remain the same element they have to have the same number of protons.These are called isotopes.

Lithium can have several numbers of neutrons for one isotope and less/more in another.

Background radiation This comes from natural and artificial sources

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Radioactive decay…• Radioactivity occurs as a result of changes in the nuclei of atoms.• A radioactive isotope is an atom with an unstable nucleus.

• When it splits up (decays):• It emits alpha, beta or gamma radiation,• A different atom is formed, with a different number of protons.

The new atom has 2 protons (and neutrons) less than the original atom= different element.

In Beta you lose 0 in the mass and gain 1 proton. So 6 protons turns to 7 protons, 8 neutrons goes to 7 neutrons. This is why the mass remains the same number. The proton has gained one, neutron lost one.

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• In a nuclear power station energy is released from the nuclei of uranium atoms in a process called, ‘nuclear fission’. This energy is transformed into electrical energy.

• When an atom is bombarded with a neutron: the nucleus may split, ‘fission’ into two smaller nuclei.

• This can lead to a chain reaction.

Fission and FusionHome

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Uranium 235 and Plutonium 239 are used in nuclear fission.

Fission and Fusion• Nuclear fusion is the joining of two nuclei, forming a

larger one, e.g. hydrogen -> helium• The sun is an example of nuclear fusion. • Nuclear fusion reactors are not used to produce energy

in a nuclear power station because the reaction does not last long enough and they use more energy than they produce.

• However, scientists continue to try and develop a practical fusion reactor because it will give another source of energy, would not produce any radioactive waste and want to show that it can be done.

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