# Physics - Science.

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## Distance Time Graphs.

For an object moving at a constant speed, we can calulate its speed using the equation:

speed (metre/second, m/s) = distance travelled (metre, m)

time taken (seconds,s)

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## Question

''A vehical on the motorway travels 1800m in 60 seconds. Calculate:

a) The speed of the vehical in m/s

b) how far would it travel at this speed in 300 seconds."

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## Velocity and acceleration

Velocity is speed in a given direction.

The Acceleration of an object is its change in velocity.

Acceleration = change in velocity (m/s)

Time taken for the change (s)

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## Resultant Force

When the resultant force on an object is at zero, the object:

• Remains stationary if it was at rest, or
• continues to move at the same speed and in the same direction it was already moving.
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## Resultant force

Resultant force (N)  = Mass (kg) x acceletation (m/s squared)

"Calculate the resultant force on an object of mass 6.0kg when it has an acceleration of 3.0m/s squared."

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## Equations...

Acceleration = force / mass

Mass = Force / acceleration.

"Calculate the acceleration of acar of mass 800kg acted on by a resultant force of 3200N."

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## Summary

"Complete a) to c) using the words below:

Acceleration    Resultant Force   Mass   Velocity"

a) A moving object decelerates when a ......... acts on it in the opposite directionto its ..........

b) The greater the ....... of an object, the less acceleration when a ......acts on it.

c) The..... of a moving object increases when a ...... acts on it in the same direction as its moving in.

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## Falling objects

Weight (n) = mass (kg) x gravitational field strength.

"calculate the weight of a steel bar of mass 20kg"

The weight of an object of mass:

• of mass 1kg is 10newtons
• of mass 5kg is 50newtons.
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## Energy and work

Work done (joules) = force (newtons) x distance moved in the direction of the force(m)

"A 20N weight is raised through a height of 0.4m. Calculate i) the work done, ii) the gain of the gravitational potential energy of the object."

"A weightlifter raises a 200N metal bar through a height of 1.5m. Calulate the gain of gravitational potential energy."

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## Kinectic Energy

Kinetic energy = 1/2 x mass(kg) x speed squared (m/s)2

"Calculate the kinetic energy of a vehicle of mass 500kg moving at a speed of 12m/s"

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## Momentum

The momentum of a moving object = its mass x its velocity.

"calculate the momentum of a 40kg person running at 6m/s"

"A 0.5kg trolley A is pushed at a velocity of 1.2m/s into a stationary trolley B of mass 1.5kg. The two trolleys stick to eachother after the impact.

Calculate:

a) the momentum of the 0.5kg trolley before the collistion,

b) the velocity of the two trolleys straight after the impact.

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## Resistance

Resistance = Potential difference (volts)

Current (Amps)

"The current through a wire is 2.0 when the potential difference across it is 12V.

Calculate the resistance of the wire."

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## Equations

I = Current.

= Potential difference (volts)

R = Resistance (ohms)

R =      gives  I = V

I                      R

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## Series Circuits

The same current passes through components in series with eachother.

"a) If the current through the lamp is 0.12A, what is the current through the cell?"

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## Electrical power and potential difference

Power = energy transformed (joules)

Time (s)

"A lamp bulb transforms 30'000J of electrical energy when it is on for 300s.

Calulate its power."

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## Calculating power

Power supplied = Current x Potential difference (V)

"Calculate the power supplied to a 5A, 230V electric heater."

Rearranging the formula gives:

Current, I = P/V

"I) Caluclate the normal current through a 500W, 230V heater.

ii) Which fusem a 3A, 5A or a 13A, would you use for this appliance?"

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## Electrical energy and charge

Calculating charge..

Charge Flow (coulombs) = current (amps) x  time (s)

For example..

• when the current is 2A for 5s, the charge flow is 10X (=2A x 5s)

"calculate the charge flow when the flow in the current is 8A for 80s."

"calculate the charge flowing in 50s when the current is 3a."

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## Energy and potential difference

When charge flows through a resistor, electrical energy is transformed into heat energy.

Energy transformed (J)  = potential difference (V) x charge flow. (C)

"Calculate the energy transformed in a component when the charge flow is 30C and the potential difference is 20 V"

"Calculate the energy transformed when the charge flow is 30C and the p.d. is 4V."

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## Nuclear reactions

The uranium isotope is 238

U    .

92

It contains 92 protons and 146 neutrons (=238 - 92) in each nucleus. So its relative mass is 238 and its relative charge is 92.

"How many protons and how many neutrons are in the nucleus of the uranium isotope  235

U

92"

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An unstable nucleus becomes more stabe by emitting an a(alpha) or a B(beta) particle or by emitting y (gamma) radiation.

When an unstable nucleus emits an a paricle, its atomic number goes down by two and its mass number goes down by 4.

For example, the thorium isotope  228

TH

90

decays by emmiting an alpha particle. So it forms the radium isotope 224

RA

88

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b) How many protons and how many neutrons are in 228

TH

90

and in   224

RA

88

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## Beta emission

When an unstable nucleus emits B particle, it atomic number goes up by 1 but its mass number stays the same (because the neutron changes into a proton.)

Potassium..

40                                                                                                                    40

K      decays by emitting a beta particle. So it forms a calcium isotope.         CA

19                                                                                                                    20

How many protons and neutrons are in the potassium isotope and the calcium isotope?

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## Overview.

Change in the nucleus                      Particle emitted

a decay            Nucleus loses 2 protons                    2 protons and 2 neutons

and 2 neutrons                             emitted as a a particle.

b decay             A neutron in the nucleus                An electron is created in the

changes into a proton                     nucleus and instantly emitted.

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## Nuclear fission

Energy is released in a nuclear reactor as a result of a process called nuclear fission. In this process, the nuclear of an atom of a fissionable substance splits into two smaller 'fragment' nuclei. This event can cause other fissionable nuclei to split, so producing a chain reaction of fission events.

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## Fission Neutrons

When a nuclues undergoes fission, it releases

• two or three neutrons ('fission neutrons') at high speeds,
• energy in the form of radiation and kinetic energy of the fission neutrons and the fragment nuclei.

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## Inside a nuclear reactor

A nuclear reactor consists of uranium fuel rods spacced evening in the reactor core.

• The fission neutrons are slowed down by the collisions with the atoms in the water molecules. This is necessary as fast neutrons do not cause futher fission of Uranium
• The water acts like a moderator because it slows the fission neutrons down. It also acts as a coolant.
• Control rods absorb surplus neutrons.This controls the chain reaction.
• The reactor core is a thick steel vessel, designed to withstand the very high temperatue and pressure in the core. The core is enclosed by thick concrete walls which absorb radiation that escapes.
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## Summary of fission..

"Complete a) and b) using the list below:

Nucleus   Uranium-235   Uranium-238   Plutonium-239

a) Nuclear fission happens when a ...... of ..... or.... splits.

b) A nucleus of..... in a nuclear reactor changes without fission into a nucleus of......

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## Nuclear Fusion

Fusion reactions.

Two small nuclei release energy when they are fused together to form a single larger nucleus. The process releases energy only if the relative mass of the product nucleus is no more than about 55. Energy must be supplied to create a bigger nuclei.

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## Nuclear fusion

• When two protons (i.e hydrogen nuclei) fuse, they form a 'heavy hydrogen' nucleus, 2

H.    A positron, the antimatter counterpart of the electron, is

1          created and emitted at the same time.

• Two more protons collide separately with the two hydrogen nuclei and term them into heavier nuclei.
• The two heavier nuclei collide to form the helium nucleus 4

He

2

• The energy released at each stage is carried away as kinetic energy of the product nucleus and other particles emitted.
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## Fusion reactors

In a fusion reactor:

• The plasma is heated by passing a very large electric current through it,
• The plasma is contained by a magnetic field so it doesnt touch the reactor walls. If it did, it would go old and fusion woul stop.

Summary..

"Complete a) and b) using the words below:

Larger   Small   Stable. "

a) When two.....nuclei moving at high speed collide, they form a.... nucleus.

b) Energy is released in nuclear fusion if the product nucleus is not as ......as an iron nucleus.

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