Physics 2

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• Created by: jenni
• Created on: 16-05-14 11:57

Speed & Velocity

Speed and Velocity both say how fast you're going, and they're both measured m/s. But there's a subtle different between them which you need to know.

Speed is how you're going with no regard to the direction. Velocity however must also have the direction specified, e.g. 30 mph north.

VELOCITY IS SPEED IN A GIVEN DIRECTION

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Acceleration

Acceleration is definitely not the same as velocity or speed.

• Acceleration is how quickly the velocity is changing
• This change in velocity can be a change in speed or a change in direction or both.

You can calculate the acceleration of an object using this formula;

a = (v-u)/t

the 'v-u' represents a change in velocity i.e. the difference between the final velocity of an object and its initial velocity.

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Resultant Forces

In most real situations there are at least two forces acting on an object along any direction.

If you have a number of forces acting at a single point, you can replace them with a single force (as long as the single force has the same effect on the motion as the original forces acting all together). The overall force you get is called the resultant force.

The resultant force will decide the motion of the object- whether it will accelerate, deccelerate or stay at a steady speed. If there is a non-zero resultant force acting on an object, then the object will change its state of rest or motion. To sum this up;

A non-zero resultant force acting on an object causes a

change in its velocity. This means the object will accelerate.

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Forces and Acceleration

Resultant forces in different situations

The effect of a resultant force acting on an object depends on whether or not the object is moving. On a similar note, an object with zero resultant force acting on it will behave in a way that depends on its current motion. There are four different situations;

• Zero resultant force acting on a stationary object
• Non-zero resultant force acting on a stationary object
• Zero resultant force acting on a moving object
• Non-zero resultant force acting on a moving object
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Forces and Acceleration; Resultant Force & Station

Zero resultant force acting on a stationary object;

Objects don't just start moving on their own - if there is no resultant force acting on a stationary object, there's no acceleration and it will just stay put.

If the resultant force on a stantionary

object is zero, the object will remain stationary.

A non-zero resultant force acting on a stationary object;

A non-zero resultant force will always produce acceleration (or deceleration). So, if an object is stationary, its velocity will change and it will start moving.

If the resultant force on a stationary object is non-zero,

The object will accelerate in the direction of the resultant force.

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Forces and Acceleration; Resultants forces & movin

If there's no resultant force on an object, it won't change velocity. That means if it's already moving, it will just keep moving at the same velocity.

If there is no resulant force on a moving object

it'll just carry on moving at the same velocity.

If any object is moving at a constant velocity then the forces on it must all be balanced. THINGS DO NOT NEED A CONSTANT RESULTANT FORCE.

To keep going at a steady speed, there must be zero resultant force. This doesn't mean there must be no driving force, it means the driving force is balanced by other forces, like friction and air resistance.

On the other hand:

If there is a non-zero resultant force on a moving object

it will accelerate in the direction of the force.

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Resultant forces and acceleration

When a resultant force acts on an object, the acceleration it experiences can take five different forms:

• Starting
• Stopping
• Changing direction
• speeding up
• slowing down

To calculate the acceleration produced by a resultant force, or the size of the resultant foce produced, use the formula;

A=F/M

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Gravitational force

Gravity is the force that causes all masses to attract each other, but you only notice it when one or more of the masses are really big, e.g. a planet.

This has two important effects:

1. On the surface of a planet, it makes all things accelerate towards the ground.

2. It gives everything a weight.

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Static Electricity

Insulators and Conductors

Electrical charges can move easily through some materials, and less easily through others.

• If electrical charges can easily move through a material, it is called an electrical conductor. Metals are known to be good conductors.
• If electrical charges cannot easily move through a material, it is called an electrical insulator. Plastics and rubbers are usually good insulators.

Static Charge

A static charge is an electric charge which cannot move. They're often (but not always) found in electrical insulators where charge cannot flow freely. They can be positive (+ve) or negative (-ve).

When certain insulating materials are rubbed together, negatively charged electrons will be scrapped off one and dumped on the other.

This will leave a postive static charge on the one that loses electrons and a negative static charge on the one that gains electrons. Which way the electrons are transferred depends on the two materials involved. (See next card)

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Static Electricity continued

Both positive and negative static charges are only ever produced by the movement of electrons. The positive charges definitely do not move.

A positive static charge is always caused by electrons moving away elsewhere. The material that loses the electrons loses some negative charge, and is left with an equal positive charge.

Attracting and Repeling

When two electrically charged objects are brought close together, they exert a force on one another. These forces get weaker the further they are apart.

• Two things with opposite electric charges are attracted to each other.
• Two things with the same electric charge will repel each other.
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Circuit Diagrams

The circuit must be fully complete as shown below.

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Circuit Diagrams; Voltmeters & Ammeters

Voltmeters and Ammeters always have to be connected in a circuit in a certain way, otherwise they won't do what they are meant to.

• A voltmeter is always connected 'across' a component - this is known as 'in parallel'.
• An ammeter is always connected 'in line' with a component - this is known as 'in series'.
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Electric Current & Potential Difference

Electric Current

Electric current is a flow of electric charge. The size of the electric current is the rate of flow of electric charge. The size of the current is given by the equation:

I= Q/T

• I is the current in amps (A)
• Q is the charge in coulombs (C)
• T is the time in seconds (s)

Potential Difference

The potential difference (voltage) between two points in an electric circuit is the work done (energy transferred) per coulomb of charge that passes between the points.

V=W/Q

• V is the potential difference in volts (V)
• W is the work done in joules (J)
• Q is the charge in coulombs (C)
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Resistance

Resistance is anything in the circuit which reduces the flow of current. It is measured in Ohms (Ω). The greater the resistance of a component, the smaller the current that flows.

• The resistance of a component can be found by measuring the current through, and potential difference across, the component.
• The current through a resistor is directly proportional to the potential difference across the resistor.

To calculate the current, potential difference or resistance use this equation;

V= I x R

• V is the potential difference in volts (V)
• I is the current in amps (A)
• R is the resistance in ohms (Ω)
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Current & Potential Difference

The current through a component depends on its resistance. The greater the resistance the smaller the current for a given potential difference across the component.

The potential difference provided by cells connected in series is the sum of the potential difference of each cell (Depending on the direction in which they are connected).

For components connected in series;

• The total resistance is the sum of the resistance of each component
• There is the same current through each component
• The total potnetial difference of the supply is shared between the components

For components connected in parallel;

• The potential difference across each component is the same
• The total current through the whole circuit is the sum of the currents through the seperate components
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Direct Current and Alternating Current

Electric current is the movement of charge carries. To transfer energy, it doesn't matter which way the charge carriers are going.

Cells and batteries supply direct current (d.c.). This just means that the current always keeps flowing in the same direction.

However, the UK mains supply is an alternating current (a.c.) supply- the current is constantly changing direction. The frequencing of the a.c. is 50 cycles per second or 50 Hz (Hertz) and the voltage is aproxim

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Parallel & series circuits

Parallel shares the Amps

Series shares the volts

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Atoms & Properties

In the early 1900s the model of the atom was called the plum pudding model. It was believed that the atom was a positively charged fluid (the pudding) with electrons dotted inside it (the plums). This model was later disproved by Rutherford and Marsden's scattering experiment.

The way they disproved this was by firing alpha particles (positively charged particles) at a gold leaf and observing that angles at which they got reflected. What they should have seen was the alpha particles passing practically straight through. However, what they discovered was that a number of the particles got deflected at different angles; with some coming straight back on themselves. What they concluded was that most of the atom was empty space with a small postively charged nucleus in the centre with electrons orbiting the outside.

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Atoms

Atoms normally have no overall charge, due to having equal numbers of electrons and protons. However, atoms can gain or lose electrons and form charged particles called ions. Some forms of radiation can create ions and this radiation is called ionising radiation.

Isotopes which have an unstable nucleus (radio-isotopes) emit radiation or decay. There are 3 forms of radiation they can give out, beta particles, alpha particles and gamma rays.

Alpha decay (α) is where an alpha particle (a positively charged particle consisting of 2 neutrons and 2 protons i.e a helium nucleus) is emitted from the nucleus of an atom. Alpha is the most ionising type of radiation.

Beta decay (β) is when a beta particle (a fast moving electron) is emitted from the nucleus of an atom.

Gamma decay (γ) is where a gamma ray (part of the electromagnetic spectrum) is emitted from the atom. Gamma rays have no charge or no mass. Gamma is the least ionising form of radiation.

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There are different sources that can give out radiation and radiation has been measured by geiger counters even when there was no known source of radiation around. This is called background radiation and some sources are natural and others are man made.

We can tell what radiation is emitted depending on how it gets deflected in a magnetic and electric field.

As a beta particle has a negative charge it will be repelled by the negatively charged plate and attracted to the positively charged plate. As a gamma ray is part of the electromagnetic spectrum and has no charge it will pass straight through. As an alpha particle has a positive charge it will be repelled by the positively charged plate and attracted to the negatively charged plate.

The different types of radiation emitted from isotopes can be stopped by different substances. It depends on how penetrating the radiation is. Alpha particles can be stopped by your skin, paper or even a few centimetres of air. Beta is more penetrating and is stopped by a few centimetres of aluminium. Gamma is the most penetrating as it is stopped by lead.

Alpha can be the most dangerous to humans as it is more likely to be absorbed by the cells. Beta and Gamma are more likely to pass through your cells.

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In order to measure how much radiation is given off by a substance we can use a Geiger counter. A Geiger counter measures the count rate which is the amount of radiation emitted. The higher the count rate the more radiation given off.

Radioactive decay is a random process but there is a pattern to it. The pattern is called the half-life. Half-life is the amount of time it takes for the radiation count rate to fall by half.

People who work with radioactive source often were special badges. These badges have a special photographic film in them which turns darker the bigger the exposure. Radioactive sources can be used as tracers. They can be added to plant fertiliser and you can then check if the plant has taken up the fertiliser. It is also used in the medical industry but doctors must ensure that it has a short half life so that it doesn't stay in the body very long and cause damage. Alpha sources are used in smoke detectors. The alphaq particles help to create an electric current in the smoke detector by ionising the air. When smoke particles enter the smoke detector the electric current drops, this causes the Alarm to go off. Beta particles are often used to measure the thickness of materials. A Geiger Counter measures the amount of radiation passing through the material. If the radiation is too high then the sheet is too thin. If the radiation is too low the material is too thick.

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Nuclear Fusion and Fission

Fusion is where two atomic nuclei join together to form a larger one. When this occurs, energy is realised. It is by this process that stars get their energy.

Fission is the opposite; it is the splitting of an atomic nucleus and it is the process that nucleur power plants use. The two most common fissionable materials are uranium 235 and plutonium 239.

In order for fission to occur the atomic nucleus must absorb a neutron. The neutron is fired at the nucleus and causes the nucleus to split, forming to smaller nuclei. When the splitting occurs energy is released along with 2 or 3 more neutrons. These neutrons are then absorbed by other nuclei causing the process to repeat. This is called a chain reaction. This reaction is controlled in a nuclear reaction by using a nuclear reactor by using control rods. These rods absorb neutrons if the reaction needs to be slowed down.

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Life cycle of stars

Planets form when lumps of rock get attracted to each other due to gravity.

Stars form when clouds of gas and dust from space gets pulled together due to gravitational attraction. The amount of gas build up gets more concentrated and forms a protostar. When the protostar gets denser and hotter, nuclear reactions (i.e. fusion) start which cause hydrogen and other lighter elements to fuse together. During fusion energy gets released which is what makes stars hot.

Protostars then become main sequence stars when the forces within the star are balanced (gravitational force and expansion/outward force). Our sun is a main sequence star. After the main sequence star their life cycle can take 2 possible routes depending on their mass.

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Life cycle of stars; continued

When the big bang occurred 13 billion years ago the only elements in existence was hydrogen. However, due to nuclear fusion in stars all the other elements were created and when stars explode (turn into supernovas) all of those elements are released into the universe. This means that the elements that make up your body, the oxygen that you breathe right now were formed inside stars.

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Forces and Braking

Friction is a force that occurs when

• An object moves through a medium e.g. air or water
• Surfaces slide past each other

Friction works against the object in the opposite direction to which it is moving i.e. it's a resistive force.

When a vehicle travels at a speady speed the resistive forces (mainly air resistance) balance the driving force. The resultant force is the difference between the driving and resisitive forces.

To increase a vehicle's top speed you need either a greater driving force or a reduction in the resistive force, e.g. by altering its shape (become more streamlined)

The greater the speed of the vehicle the greater the breaking force needed to stop it in a certain time or certain distance.

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Stopping Distance

The stopping distance of a vehicle depends on

•  the thinking distance (the distance travelled during the driver's reaction time)
•  the breaking distance (the distance travelled under the braking force)

Thinking distance + braking distance = stopping distance

The overall stopping distance is increased if

• the vehicle is travelling at greater speeds
• The driver is tired or under the influence of drugs or alcohol or is distracted (e.g. mobile phone) and cant react as quickly as normal
• the vehicle is in poor condition e.g. under-inflated tyres, worn brakes

Friction forces between the brakes and the wheel, and between the wheel and the road surface reduce the kinetic energy of the vehicle. This kinetic energy is transformed into heating the brakes resulting in an increase in brake temperature. If a vehicle's wheels lock when braking, a skid results. Overheating can result in brake failure.

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Forces and Weight

All falling objects experience two forces:

• A downward force, called weight (W)
• An upward frictional force e.g. air resistance or drag through a fluid

Although weight always remains the same, the faster an object moves through the air or fluid the greater the frictional force that acts on it.

The weight of an object is the force exerted on its mass by gravity (sometimes called gravitational field strength). Weight is measured in newtons.

To calculate the weight of an object the following equation is used:

W = m x g

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Terminal Velocity

An object falling through the air or a fluid will initially accelerate because of the force due to gravity. Eventually the resultant force will be zero as the weight and resistive forces balance. At this point the object will move at a steady speed, called the terminal velocity

If a Skydiver jumps out of an aeroplane, the speed of their descent can be considered in two seperate parts:

• Before the parachute opens
• After the parachute opens

Before the parachute opens:

•  The skydiver accelerates due to the force of gravity.
• The skydiver experiences frictional force due to air resistance in the opposite direction. At this point weight is greater than resistance, so the skydiver continues to accelerate.
•  Speed increases, so does resistance.
•  Resistance increases until it is the same as the weight. At this point the resultant force is zero. There is no more acceleration and the skydiver falls at a constant speed called the terminal velocity.
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Terminal Velocity; continued

After the Parachute opens:

• The resistive force is now greatly increased and is far bigger than the weight.
• The increase in resistive force decreases the skydiver's speed. As speed is reduced so is the value of resistive force.
• Resistive force decreases until it is the same as weight. The forces balance for a second time and the skydiver falls at a steady speed although slower than before. This is a new terminal velocity.
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