Nuclear fission and Nuclear fusion,DENSITY

Nuclear fission and Nuclear fusion

• There are two fissionable substances in common use

 in nuclear reactors, uranium 235 and plutonium 239.

• Nuclear fission is the splitting of an atomic nucleus.

• For fission to occur the uranium 235 or plutonium 239

 nucleus must first absorb a neutron:









     neutron







• The nucleus undergoing fission splits into two smaller

 nuclei and 2 or 3 neutrons and energy is released.

• The neutrons may go on to start a chain reaction, AS ABOVE.

• Nuclear fusion is the joining of two nuclei to form a larger one.

• Nuclear fusion is the process by which energy is released in stars.

. DENSITY

•  Density of an object = its mass divided by its volume,     

 Density  =  mass  /  volume                                                                       ρ =  m  / V                                      

      (kg/m3)      (kg)         (m3)

• You should be able to describe an experiment to measure the density of an object

  involving measurements of its mass, and its volume.

RADIOACTIVITY

 Radioactivity

• Know the relative masses and relative electric charges

  of protons, neutrons and electrons.

• In an atom, number of electrons = number of protons

 in the nucleus. The atom has zero net electrical charge.










• Atoms may lose or gain electrons to form ions.

• Atoms of a particular element have the same number of protons.

• Atoms of different elements have different numbers of protons.

• Atoms of the same element which have different

 numbers of neutrons are called isotopes.

• The number of protons is called the atomic number.

• The number of protons and neutrons is called the mass number.

• Know the effect of a and ß decay on radioactive nuclei.

• Know the origins of background radiation

electric power

Electrical Power

• Electric current is the rate of flow of charge.

• When an electrical charge flows through a resistor,

 electrical energy is transformed into heat energy.

• power = energy transformed / time taken

• Power, voltage and current are related by the equation:

power = voltage x current

(watt, W) (ampere, A) (volt, V)

Energy, voltage and charge are related by equation:

energy transformed = voltage × charge

(joule, J) (volt, V) (coulomb, C)

• charge = current × time

(coulomb, C) (ampere, A) (second, s)

STATIC ELECTRICITY,ELECTRIC CURRENT&mains electricity

 Static electricity

• When materials are rubbed against each other they can

become electrically charged. Negatively charged electrons

are rubbed off one material onto the other.

• The material that gains electrons becomes negatively charged.

The material that loses electrons has an equal positive charge.

• Two charged bodies will exert a force on each other.

• Like charges repel, unlike charges attract.

• Electric charges move easily through metals.

• The rate of flow of electric charge is called the current.

   current I   =  charge / time

• A charged body can be discharged by connecting it to earth

with a conductor. Charge then flows through the conductor.

The greater the charge on an isolated body the greater the potential

difference between the body and earth. If the pd is high enough a

spark may jump to earth.

• Electrostatic charges can be useful, eg in photocopiers

and smoke precipitators

Electric Current

• Current-potential difference graphs are used to show how

current through a component varies with pd across it.

                                 A resistor               A filament lamp                  A diode

• The current through a resistor (at a constant temperature)

is proportional to the voltage across the resistor.

• Voltage = current × resistance

                      (volt, V) (ampere, A) (ohm, O)

• The resistance of a filament lamp increases as the

 temperature of the filament increases.

• The current through a diode flows in one direction only.

The diode has a very high resistance in the reverse direction.

• The resistance of a light-dependent resistor (LDR)

 decreases as light intensity increases.

• The resistance of a thermistor decreases as the temperature increases.

• The current through a component depends on its resistance,

 the greater the resistance the smaller the current.

• The voltage from cells in series is the sum of the voltage of each cell.

• For components connected in series:

- total resistance = sum of the resistance of each component

- there is the same current through each component

- the total voltage of the supply is shared between the components.

• For components connected in parallel:

- voltage across each component is the same

- the total current through the whole circuit is the sum

 of the currents through the separate components.

 Mains electricity

• Cells and batteries supply current which always passes in

 the same direction. This is called direct current (d.c.).

• An alternating current (a.c.) is one which is constantly

changing direction. Mains electricity is an a.c. supply.

In the UK it has a frequency of 50 cycles per second (50 Hz).

• UK mains supply is about 230 volts.

• Know the structure of electrical cable.

• Know the structure and wiring of a three-pin plug.

• If an electrical fault causes too great a current, the circuit

 should be switched off by a fuse or a circuit breaker.

• When the current in a fuse wire exceeds its rating the

 fuse will melt, breaking the circuit.

• Appliances with metal cases are usually earthed. The earth

wire and fuse together protect the appliance and the user

• The live terminal of the mains supply alternates between

positive and negative potential with respect to the neutral terminal.

• The neutral terminal stays at a potential close to zero

 with respect to earth.

WORK,ENERGY,POWER,MOMENTUME

Work, Energy, Power

• When a force causes a body to move through a distance,

 energy is transferred, and work is done.

• work done = force × distance moved in direction of force

     (joule, J) (newton, N) (metre, m)

• Work done against frictional forces is mainly changed into heat.

• Squashed materials have elastic potential energy stored in them.

• The kinetic energy of a body depends on its mass and its speed.

      kinetic energy = ½  x  mass  x  v2

(joule, J) (kilogram, kg) (metre/second)2 , (m/s)2 )

• Gravitational Potential Energy GPE depends on height and weight:

    GPE  =  weight  x  height  ,   GPE  =  m g h

   (Joule J,  Newtons N,  metres m)

•  Power  =  work done /  time taken

     P = Work / t   or  Energy / t ,  units are Watts

Momentum

• momentum = mass X velocity

(kilogram metre/second, kg m/s) (kilogram, kg) ( m/s)

• Momentum has both size and direction.

• When a force acts on a body a change in momentum occurs.

• Momentum is conserved in any collision/explosion,

provided no external forces act

• force = change in momentum / time taken for change

FORCE

 Forces

• can you sketch a velocity-time graph for a body at terminal velocity.

• The faster a body moves through a fluid the greater

the frictional force which acts on it.

• A body falling through a fluid will initially accelerate

due to gravity, eventually the resultant force on the body

will be zero, and it will fall at its terminal velocity.

• at terminal velocity Weight down = Friction up

• weight = mass × gravitational field strength

(newton, N) (kilogram, kg) (newton/kilogram, N/kg)

• Whenever two bodies interact, the forces they exert

on each other are equal & opposite.

• A number of forces acting on a body may be replaced by

a single force which has the same effect as the original set

of forces. The single force is called the resultant force (here in red):

• If the resultant force acting on a stationary body is zero,

it is either at rest, or moving at a steady speed.

• If the resultant force acting on a stationary body is not zero,

the body will accelerate in the direction of the resultant force.

• Resultant force = mass × acceleration

(newton, N) (kilogram, kg) (metre per second squared m / s2 )

• When a vehicle travels at a steady speed the frictional

forces balance the driving force (zero resultant force).

• Stopping distance = braking distance + thinking distaNCE


• A driver’s reaction time is affected by tiredness, age, drugs, or alcohol.

• A vehicle’s braking distance depends on the brakes, tyres, the road, and weathe

MOTION

 Motion

• The slope or gradient of a distance-time graph represents speed.

• The velocity of a body is its speed in a given direction.

• Average velocity = total distance / total time