Yr 10 Physics iGCSE: Revision Notes 2

Formulae you need to know are in bold.

CONTENTS

1. MOTION GRAPHS
2. FORCES
3. WORK, ENERGY, AND POWER
4. MOMENTUM
5. STATIC ELECTRICITY
6. ELECTRIC CURRENT
7. MAINS ELECTRICITY
8. ELECTRICAL POWER
9. ATOMIC STRUCTURE BY RUTHERFORD SCATTERING
10.NUCLEAR FISSION AND NUCLEAR REACTORS
11. DENSITY

1 Motion Graphs

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

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

  NB other examples of vectors are force, acceleration, momentum.

• quantities with no specified direction are called scalars

  NB examples of scalars are energy, time, distance, temperature, etc

• average velocity = total distance / total time,   v  =  d/t

• acceleration = change in velocity / time taken,    a  =  v - u / t  

• The slope(gradient) of a velocity-time graph = acceleration.

• The area under a velocity-time graph = distance travelled.

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2 Forces

• Force is a vector (like velocity, it has size and direction)

• Distance or speed are called  scalars, they have size but not direction.

• The faster a body moves through a fluid the greater

the opposing 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 Χ gravitational field strength,  W  = mg

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

• Whenever two bodies interact, the forces they exert

on each other are equal & opposite (Newton's Third Law)

• 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 Χ acceleration,   OR,   F = ma

(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.

Typical Stopping Distances


30
MPH

The Highway Code by Select All

The Highway Code by Select All

 
 

9 metres     14 metres

= 23 metres
(75 feet)
or 6 car lengths


50
MPH

The Highway Code by Select All

The Highway Code by Select All

 
 

15 metres

38 metres

= 53 metres
(175 feet)
or 13 car lengths


70
MPH

The Highway Code by Select All

The Highway Code by Select All

The Highway Code by Select All

 

21 metres

75 metres

= 96 metres
(315 feet)
or 24 car lengths


 The Highway Code by Select AllThinking Distance

 The Highway Code by Select AllBraking Distance

 http://www.highway-code.com/images/wt.gifhttp://www.highway-code.com/images/wt.gifaverage car length = 4 metres

• 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 weather.

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3 Work, Energy, Power

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

 energy is transferred, and work is done.

• work done = force Χ distance moved in direction of force,   W  =  F x d

     (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 ,     KE =   ½  m 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  =  W / t

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

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4 Momentum

• momentum = mass Χ velocity ,   mom. = mv

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

• Momentum has both size and direction (another vector)

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

• Momentum is conserved in any collision/explosion,

provided no external forces act,

ie. momentum before collision = momentum after collision

 Top - two trolleys of same masses exploding apart, bottom - two trolleys of different masses exploding apart

• force = change in momentum / time taken for change,   F  =  mv - mu / t

we use this equation to explain why the force is large when the impact time is small

in a collision.

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5 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 (conductors), but not through insulators.

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

• current in a wire is a flow of negatively charged electrons

   current  =  charge / time,  OR,   Q  =  I t

     (Amps)       (Coulombs)   (seconds)

• 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 ink jet printers.

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6 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 Χ resistance,   V  =  I R

     (volt, V) (ampere, A) (ohm, Ω)

• 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.

• Rules for components connected in series like below:

 

 

 

 

− 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.

• Rules 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.

•  in series circuits if 1 item fails, all items turn off, in parallel circuits

  other items still work, ALSO all items can be switched independently so

  parallel wiring for lighting is preferred.

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7 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 and wiring colours 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.

• in a circuit breaker a magnetic force acts to break the circuit (this

  has the advantage of a quick re-set)

• Appliances with metal cases are usually earthed. The earth

wire and fuse together protect the appliance and the user

• Plastic cased appliances need no earth (they are said to be double insulated)

• 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.

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8 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,   P =  E / t

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

power = voltage x current,   P  =  VI

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

Energy = power x time, so:

energy = voltage x current x time,   E  =  VIt  = VQ.

• Voltage is the energy transferred per Coulomb of charge flowing

  1 VOLT  =  1 JOULE /  1 COULOMB 

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9 Atomic structure by Rutherford Scattering

Remember the structure of an atom from Year 9 -   Rutherford, an English physicist is credited

with doing the experimental work which established the so called 'nuclear model' :

In RUTHERFORD SCATTERING alpha particles are fired at a thin gold foil.

● Most alphas go straight through unaffected.

● A small number are reflected through large angles

● A tiny number bounce directly backwards.

From these observations we deduce:

● an atom is mostly empty space

● all of the positive charge and most of the mass is concentrated in a

    tiny dense nucleus

● negatively charged electrons are in orbits around the nucleus

10 Nuclear fission

• 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 (blue in the diagram below)

 nucleus must first absorb a neutron (red in the diagram):

• The nucleus undergoing fission splits into two smaller

 nuclei (yellow in the diagram) and 2 or 3 neutrons and energy is released.

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

• In a nuclear reactor (below), the uranium or plutonium is contained in fuel rods. Fissions occur

   inside these rods producing fast neutrons and causing heating.,

• the moderator (usually water) is used to slow down the fast neutrons to increase the chances

of further fissions occurring (to promote a chain reaction)

• the control rods (usually boron) are lowered  to absorb neutrons to reduce the reaction

  or to shut down the reactor. 

•  The fission fragments (the leftovers after a fission reaction) are highly radioactive with long half lives,

   this is the radioactive waste from a reactor and disposal is a problem.

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11. 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.