Transcript The idiots guide to Physics!x

13.1 How can we describe the way things move?
• Speed (m/s) = distance (m) /time (s)
• Distance- time graph
•Steeper the line – greater the speed
• Velocity = speed in a stated direction
• Velocity-time graph
Acceleration(m/s2) = change in velocity(m/s)/time(s)
The steeper the slope the greater the acceleration.
A horizontal line indicates a constant speed
The area underneath a velocity-time graph represents the
distance travelled
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13.2 How do we make things speed up or slow down?
• Force = push or pull (N)
• Forces are balanced – object stationary or constant speed.
Resultant force = zero
•Forces are unbalanced – object will move, speed up,
slow down or change direction. Resultant force =
difference in direction of bigger one
• Force (N) = mass (kg) x acceleration (m/s2)
• Stopping distance = braking + thinking distance
• Braking affected by road, weather & vehicle conditions
• Thinking affected by tiredness, drugs & alcohol
• Falling objects – air resistance (up) & weight (down)
• weight (N) = mass (kg) x G.F.S (N/kg) [10n/kg]
• Terminal velocity – maximum speed of falling object
• 13.3 What happens to the movement energy when things speed
up or slow down?
• Work done [energy] (J) = force (N) x distance in direction (m)
• If there is friction, energy converted into heat energy
• Kinetic energy(J) = ½ x mass(kg) x speed2 ((m/s)2)
13.4 What is momentum?
• Momentum(kg m/s) = mass(kg) x velocity(m/s)
• has both magnitude and direction
• Momentum is conserved in any collision or explosion.
• Force(N) = change in momentum (kg m/s)/ time(s)
• Vehicles use momentum for safety features
• Seat belts increases the time taken for the body’s
momentum to reach zero, and so reduces the forces on it.
• Air bags increase the time taken for the head’s
momentum to reach zero, and so reduces the forces on it.
13.5 What is static electricity, how can it be used and what is
the connection between static electricity & electric currents?
• Rub insulating materials together they become charged
• Gain electrons = negatively charged
• Lose electrons = positively charged
• Like charges = repel
• Opposite charges = attract
• Charges move through conductors, not through insulators
• Charges are used in photocopiers and smoke precipitators
• Photocopiers – image copied to + charged plate. – charged toner
attracted to image, fixed in place by heat.
• Smoke precipitators – smoke particles pass through charged
grid become + charged. Particles attracted to –
• flow of electricity = current
• Charged conductor discharged by connecting to Earth
• Static can be dangerous
- petrol stations. Turn off the engine, don’t use mobile phones.
- refuelling planes. Tanker & plane linked by copper conductor
• 13.6 What does the current through an electrical circuit
depend on?
• Current (A) – ammeter connected in series
• Potential difference (V) – voltmeter connected in parallel
• Current in series circuit always the same, p.d splits up,
total resistance = resistance of components
• Current in parallel circuit splits up, p.d always the same
• Resistance = how hard it is for current to flow, measured
in ohms (Ω)
• Resistance(Ω) = potential difference (V) / current (A)
• LDR Ω decreases as light intensity increases
• Thermistor Ω decreases as the temperature increases
13.7 What is mains electricity and how can it be used safely?
• d.c flows in the same direction. Cells and batteries use d.c
• a.c changes direction continuously. Mains supply is a.c
• frequency = number of complete cycles of reversal per seconds
• UK frequency = 50Hz, p.d = 230-240V
• Neutral carries the current away
• Live carries the current to appliance
• Earth wire & fuse – safety
• Fuse breaks circuit is current too big
• Fuse used should be slightly higher
than current device needs:
• Wires are copper = a good conductor, surrounded by flexible plastic
which is a good insulator
• Pins made from brass - good conductor
• Oscilloscope trace - Frequency (Hz) = 1 / time period (s)
• Time period = horizontal distance between peaks - time period of the
signal in divisions. Multiply the number of divisions by the timebase per
division
13.8 Why do we need to know the power of electrical
appliances?
• Current = flow of charge (A)
• Charge flows through resistor electrical energy -> heat
energy
• Power = rate of energy transformed
• Power (W) = energy transferred (J) / time (s)
• Power (W) = current (A) x voltage (V)
• Amount of charge in a circuit is measured in coulombs (C)
• Charge (C) = current (A) x time (s)
• Greater the p.d the more energy transformed per
coulomb
• Energy transformed (J) = potential difference (V) x
charge (C)
13.9 What happens to radioactive substances when they decay?
Particle
Mass
Charge
Proton
1
+1
Neutron
1
0
Electron
Almost 0
-1
• Mass number = protons + neutrons, atomic number = protons
• Isotope = same # of protons, different # of neutrons
• Atoms that gain or lose e- are ions
• Radioactive isotopes = atoms with unstable nuclei that decay
• Alpha decay – alpha particle emitted. Mass number decrease by 4, atomic
number decreases by 2
• Beta decay – neutron -> proton + electron emitted. Mass number stays same,
atomic number increases by 1
• Ionisation - radioactive particles collide with neutral atoms -> become
charged as electrons knocked out
• Background radiation = naturally occurring. No threat, comes from
atmospheric gases, medical, nuclear industry, cosmic rays, gamma rays from
rocks, soil & building materials and from food
• Rutherford & Marsden fired alpha particles at gold foil. Some went through,
some deflected, some rebounded. Model changed from “plum pudding” to
nuclear atomic model.
13.10 What are nuclear fission & nuclear fusion?
• Nuclear fusion = joining of atomic nuclei. Forms a larger atomic
nucleus & releases more energy than needed - self sustaining.
• How stars release energy.
• Nuclear fission = splitting of atomic nuclei. Used in nuclear
reactors to produce energy to make electricity.
• Uranium 235 & Plutonium 239 - fissionable substances
• Nucleus absorbs a neutron -> nucleus unstable. Splits into two
smaller nuclei and additional neutrons. Energy is also released.
• Neutrons can go on to start a chain reaction.