topic 14 + 15

• What is the formula for density?

  Density = mass/volume
• How can you find the density of a liquid?

  - Place a measuring cylinder on a mass balance and set the mass back to 0

  - pour 10ml of the liquid into the measuring cylinder and record the liquids mass

  - pour another 10ml into the measuring cylinder, repeating the process until the cylinder is full

  - use the volume and mass to find the density at each step, then take an average to find an accurate estimate of the density
• How can you find the density of a solid object?

  - find the mass by using a mass balance

  - either take measurements of width, height, length etc if the shape's volume can be calculated width

  - or for a more irregular object, submerge it in a eureka can filled with water (the volume of displaced water - collected in a measuring tube - is the same as the volume of the object

  - put the mass and volume into the formula to find the density
• What is absolute zero?

  the lowest possible temperature (0 Kelvin)
• Why is there no limit on how hot an object can get?

  Because as an object cools down, the particles slow down. When the temperature meets 0K, they stop, so the substance can no longer cool down. However, particles accelerate when the substance increases in temperature, and they can always keep accelerating, so there is no limit to how hot the temperature can get.
• What is the formula for change in thermal energy?

  Change in thermal energy = mass x specific heat capacity x temperature change
• What is specific heat capacity?

  The amount of energy needed to increase the temperature of 1kg of a substance by 1 degree.
• What is the formula for thermal energy?

  mass x specific latent heat
• What is specific latent heat?

  The energy needed to change the state of a 1kg mass without changing its temperature
• What are the units of specific latent heat?

  J/kg
• What are the units of specific heat capacity?

  Joules per kilogramme per degree Celcius [J/kg/°C]
• How can you find the specific heat capacity of water?

  - find the mass of some water in a container

  - there should be a joulemeter connected to an electric immersion heater, which should be in the container, along with a thermometer

  - make sure the joulemeter is on zero, and place a lid on the container if you have one

  - find the temperature of the water, then turn on the power. Watch the thermometer - when the temperature has increased by (eg) 10°C, stop the experiment and record the energy on the joulemeter, as well as the change in temperature

  - put into the equation, then repeat the experiment 3 times to find an average
• How can you get more accurate results when finding the specific heat capacity of water?

  Use thermal insulation - eg wrap it in a thermally insulating material, like cotton wool, and place the container on a thermally insulating surface, like a cork mat. This reduces the rate at which energy is transferred by heating, which means less energy is transferred to the thermal stores of the surroundings.
• How can you investigate what happens to water during a change in state?

  1) fill a beaker with crushed ice and place a thermometer in the beaker to record the temp

  2) start a stopwatch and gradually heat the beaker using a bunsen burner

  3) every 20s, record the temperature and the current state

  4) continue until the water starts to boil

  5) plot a graph of temperature against time
• Why does the temperature of a substance not change when it is changing state?

  Because all the energy is being put into the potential energy stores of the particle, causing the state change. If the temperature were increasing, it would be being put into the kinetic stores of the particles.
• How can you change the external pressure on a gas?

  - for a gas in an air-tight syringe, pushing hard on the plunger of the syringe will increase the internal pressure on the gas, so that it is larger than the outward pressure, causing the gas inside the syringe to be compressed.

  - atmospheric pressure decreases as altitude decreases , so the gas expands as altitude increases.
• How can you change the pressure of a gas inside a container like a balloon?

  - if you heat the container, the gas particles in it gain energy, so move more quickly and increase the pressure of the gas on the inside of the container.

  - the outward pressure is now greater than the inward pressure, so the balloon (or other container) would expand until the pressures are equal.
• How can you increase the temperature of a gas?

  Do work on it:

  - heat it

  - or mechanically - e.g. the use of a bike pump
• how does a bike pump do work on a gas? (and therefore increase its temperature)

  the gas applies pressure to the plunger of the pump, and so exerts a force on it.

  Work has to be done against this force to push down the plunger. This transfers energy to the kinetic energy stores of the gas particles, increasing the temperature. If the pump is connected to a tyre, you should feel it getting warmer.
• What is the formula for pressure (general)?

  Pressure = force/area
• What direction does fluid pressure always exert a force at?

  Right angles to the surface
• Why does fluid pressure depend on density?

  If there is a higher density, more particles are able to collide with the surface, so pressure increases.
• Why does fluid pressure depend on depth?

  As the depth of a fluid increases, the number of particles above that point increases. The weight of these particles adds to the pressure felt at that depth, so fluid pressure increases with depth.
• What is the formula for Pressure due to a column of liquid?


  P = h (of the column) x p (density) x g (gravitational field strength)
• What are the units for pressure?

  Pascals (Pa), 1 Pa = 1 N/m^2
• Why does atmospheric pressure decrease with altitude?

  The weight of the atmosphere above decreases as altitude increases, and the density of the atmosphere increases further from the earth.
• Why can an object denser than the fluid it is in never float in it?

  It will never be able to displace a volume of water equal with a weight equal to its weight.
• Why do objects float?

  The upthrust of the object is equal to the object's weight, thus the force balances. For this to be possible, it must have a density lower than the fluid it is in.
• What is elastic distortion?

  A material will return to its original shape after the deforming force is removed. All the energy used to distort the object can be converted back into a useful form.
• What is inelastic distortion?


  A material will not return to its orbital shape when the deforming force is removed. All the energy used to distort the object cannot be returned to a useful form.
• What is a spring constant?

  the force per unit extension needed to stretch it - how hard it is to stretch a spring.

  The greater the spring constant, the harder it is to stretch.
• What are the units for spring constant?

  N/m
• What is the elastic limit?


  The maximum amount a material can be stretched by a force without permanently deforming (and still return to its original length when the load is removed)
• How does the relationship between force and extension change after the elastic limit is passed?

  Linear -> non-linear
• What is the formula for a spring constant?

  k = F/x (extension)
• How can you investigate the link between extension and work done?

  1) measure the natural length of the spring with a millimetre rule clamped to the stand with the spring

  2) add a mass to the spring (eg start at 100g, or 1N), and record the new length, then use that to calculate extension

  3) repeat this process until you have enough measurements (no less than 6)

  4) plot a graph of extension against force (the greater the force, the more work done)
• What is the formula for the energy transferred in stretching? (or work done to stretch it)


  E = 1/2 x k x extension^2
• What is up thrust?

  The upward force that a liquid or gas exerts on a body floating in it.
• What is 0K in celcius?

  -273