flashcard corrections

• What is the Earth's atmosphere?

  layer of air that surrounds the Earth
• Explain why a submerged object experiences upthrust in terms of fluid pressure

  • submerged object experiences a force on all submerged surfaces as a result of the pressure of the fluid
  • because pressure is greater with depth, the force experienced at the bottom of object is greater than at the top
  • there is a resultant force upwards, aka upthrust
• What causes atmospheric pressure on Earth?

  • air molecules of atmosphere colliding with a surface
• Ball A floats when placed in water, but Ball B sinks. What must be different about the balls in order for one of them to sink?

  • the density
  • Ball B sinks because denser than water
  • since upthrust = weight of displaced water, this means the ball's weight will always be greater than the upthrust it experiences, so resultant force downwards, causing it to sink
• Explain why atmospheric pressure is lower on top of mountain than by the sea

  • atmosphere = less dense at higher altitudes - there are fewer air molecules to collide with a surface, so pressure is lower
  • also fewer air molecules above that point so the weight of them, which contributes to atmospheric pressure, is smaller
• State what causes pressure in gases

  • particles in gas colliding with surface causes a force and so a pressure
• Absolute zero

  • the coldest anything can get
  • particles have as little energy in kinetic energy store as possible
  • so cannot cool substance further
• Why will gas pressure increase if heated?

  • heating gas = particles will move faster and collide with walls of container more often and with more force
  • increases the total force exerted on the walls
  • and so increases pressure
• The volume of a sealed gas container is increased. The pressure of the gas inside the container decreases. How?

  • increases in volume = gas particles are more spread out and collide with walls of container less often
  • reduces the total force exerted by the gas on the walls
  • and so reduces the pressure
• Cold water is denser than hot water.

  Explain why this is the case in terms of particles

  • When water is heated, its particles gain energy and move faster.
  • This causes them to spread out, increasing the volume.
  • Since the mass stays the same, the density decreases.
  • Cold water has slower, more closely packed particles,
  • so it has a higher density.
• What does a 'force normal to a surface' mean?

  a force that is exerted at right angles to a surface
• Explain why the pressure of the water in the well increases with depth

  • the greater the depth, the more water particles there are above that point
  • this means the total weight of particles above that point is greater
  • so the pressure is higher
• Cold water has a higher density than hot water. Explain in terms of particles why the pressure in cold water will be greater than the pressure in hot water

  • cold water has higher density
  • so more particles per unit volume than in hot water
  • this means there are more particles to collide
  • so the pressure is higher (and the total weight of the particles above that point is greater) than in hot water
• Balloon expanding - greater outward or inward pressure?

  • greater outwards pressure than inwards
  • expands until equal again
• If pressure of air outside a balloon increases, will the gas in the balloon expand or compress? Why?

  • gas will be compressed
  • because the net force exerted on the balloon by the pressure outside is initially greater than the net force exerted by the pressure inside the balloon
• Explain why the volume of a weather balloon increases as its altitude increases

  • as altitude increases, atmospheric pressure decreases
  • so the inward pressure on the weather balloon decreases
  • this means outward pressure of gas inside balloon is greater than inward pressure
  • causes the gas inside balloon to expand, increasing the volume of balloon until pressures are equal
• A force compresses a gas and the temperature of the gas increases, why?

  • force that compresses the gas does work against the pressure of the gas
  • this transfers energy to the kinetic energy stores of the gas particles
  • increases the internal energy of the gas, which increases the temperature of the gas
• What is the pd between the live wire and the earth wire?

  230 V
• Describe functions of live and neutral wires

  • live wire - carries alternating voltage
  • neutral wire - completes circuit to allow current to flow in the appliance
• Why should a switch be connected in the live wire of a circuit?

  • so that circuit can be broken
  • cutting off live supply from mains
  • and reducing risk of electric shock
• Is it dangerous to touch hairdryer's live wire when switched off?

  • yes
  • current is not flowing but there is still a potential difference across live wire
  • if made contact - body provides a link between supply and earth
  • so large current flows through you
  • cause electric shock, injury or kill you
• In which wire of appliance should a fuse be connected to?

  live wire - so when fuse melts it cuts off live supply
• Replacement of household fuse?

  circuit breaker
• What is purpose of earth wire?

  • stops appliance from becoming live
  • if fault develops which if live wire touches metal case
  • prevents electric shocks
• How does fuse work to minimise risks if surge in current?

  • surge in current
  • fuse melts
  • breaks circuit - cuts of live supply
  • prevent current surge flowing through appliance
  • so minimised risk of fire from heating effect of current
• Describe how a pd in wire can be induced using magnetic field

  • move wire relative to the magnetic field
  • or change the magnetic field around the wire
• Explain how applying an alternating pd across primary coil of transformer induces an alternating pd across secondary coil

  • alternating pd across primary coil produces alternating magnetic field
  • core is magnetic, so alternating magnetic field causes magnetisation of iron core between two coil to alternate too
  • this changing magnetic field induces a pd across secondary coil
• A greater magnetic flux density leads to what?

  a higher induced current
• How is current induced in a generator?

  • coil of wire rotated in magnetic field OR magnet rotated in coil
  • induces a pd across coil
  • coil is part of complete circuit so current also induced
• Function of split-ring commutator in dynamo

  • current in coil swaps direction every half turn
  • so this swaps the connections
  • to keep current in circuit flowing in same direction
• How does the design of alternator differ from a dynamo?

  • alternator - slip rings and brushes instead of split-ring commutator
  • means the coil is always attached in same way in circuit, instead of swapping every half turn
  • alternator produces AC rather than DC
• Why does static electricity typically build up on insulators?

  because in insulators electrons are not free to move
• Anna rubs a balloon against her hair and gains a negative static charge. She then holds the balloon against the wall. Why does it stick?

  • electrons are transferred to the balloon - balloon has negative charge
  • when she holds against wall, negative charge on the balloon repel the negative charges on surface of wall, leaving a positive charge on the surface
  • negatively charged balloon attracted to positively charged wall, so sticks
• Give two uses of static electricity

  • electrostatic paint sprayers
  • insecticide sprayers
• Explain how electrostatic paint sprayer produces a fine, even spray of paint

  • paint spray gun is charged
  • so paint drops passing through it becomes charged too
  • all paint drops receive same type of charge
  • so repels all the others
  • and spread evenly
• What happens in a storm cloud?

  • raindrops and ice collide with one another
  • causing top of cloud to become positively charged
  • and bottom negatively charged
• How does storm clouds lead to lightning strikes?

  • electric charge builds up in cloud - pd between bottom of cloud and earth (0V) increases
  • pd eventually becomes so huge that electrons can jump across gap between cloud and earth aka lightning strike
• What does it mean if a charged object is earthed? How does it reduce static charge?

  • if connected to the ground using conductor e.g. copper wire
  • provides an easy route for electrons to travel away into ground
• Explain why fuel tanker must be earthed when being filled

  • to prevent static charge building up as fuel flows into it from filler pipe
  • build of static charge could lead to spark
  • might cause explosion due to fumes in environment around tanker
• Why does a charged object feel a force when placed in electric field of another object?

  because field of charged objects interact and cause a force