P7.1 Flashcards
What is energy?
Energy is a quantity (measured in joules) that tells you what is possible, not what will happen
Energy is store is the same thing (thermal energy = thermal store)
Give an example of a chemical store
- a pile of coal with oxygen
- glucose in your muscles with oxygen
Give an example of thermal store and what’s the equation for it?
- a hot bath
Change in energy = mass x SCH x change in temperature
Give an example of kinetic store and what’s the equation for it?
- a moving car
Energy = 0.5 x mass x speed^2
Give an example of gravitational store or gravity and what’s the equation for it?
- skier at the top of a hill
- energy = mass x gravitational field strength x height
Give an example of elastic store and what’s the equation for it?
- a stretched elastic band
- energy = 0.5 x spring constant x extension^2
Give an example of nuclear store and what’s the equation for it?
- hydrogen in the Sun (fusion)
- a lump of uranium - 245 (fission)
- energy = mass x velocity of light^2
Give an example of electrostatic store?
- two opposite charges held apart
Give an example of magnetic store?
- the opposite poles of a magnet held apart
What are the 4 ways energy can be transferred?
- mechanically (with forced)
- electrically (with electric current)
- heating by particles
- heating by radiation
How you calculate the work done mechanically?
Work done = force x distance
How you calculate the energy transferred electrically?
Energy transferred= power x time
How you calculate the change in thermal energy store?
Change in thermal energy store = mass x specific heat capacity x change in temperature
What is the law of conservation of energy?
Energy can not be created or destroyed. Energy can only be stored, dissipated or transferred between stores.
- after any energy transfer, the total energy is always the same
What happens to energy in a closed system?
If you have a closed system then there is no net change in energy. No external forces act on a closed system, and the system itself is not heated or cooled, although components of the system can be heated or cooled as long long there is not transfer of energy in or out of the system.
How can you analysis a situation using energy?
- choose 2 points in a process
- identify which stores have more or less energy in them at those points
- work out the type of transfer has happened between the 2 stores
—> first think about the physical situation
What type of force does a drag racer’s engine exert on the car over the distance of the race?
It exerts a constant force
What happens when a racer exerts a constant force on the car bed the distance of the race?
Before = high chemical energy(more fuel and oxygen) and no kinetic energy (stationary car)
Energy is transferred mechanically
After = low chemical store( less fuel and oxygen) and medium kinetic energy (moving car at the end of track)
In the closed system the decrease in energy in the chemical store is equal to the increase in energy of the kinetic store because of the mechanical work done
Why in reality is not all the energy transferred to the chemical store?
In reality some energy is transferred to the thermal store of the surroundings because of friction, sound, or air resistance
( before is low thermal store, but high thermal store after)
What is 1J in Nm
1 J = 1Nm
What is 1N in kgm/s^2
1N = 1 kgm/s^2
What happens when you brake?
Brake = force exerted on the car
Work done by brakes = slow down a car
Transfers energy from kinetic store ( of moving car) to the thermal store of brake pads and surroundings, and eventually the bale pads heat up the surroundings
What is the energy analysis when braking?
Kinetic store decreases
Thermal store increases
Energy transferred mechanically, heating by particles and heating by radiation
What happens when a gymnast jumps lands and compresses a springboard before a vault?
- first the spring is uncompressed and the gymnast is running and jumping
- gymnast exerts a changing force over a direction
- compresses spring and stationary gymnast at a lower height
What is the energy analysis for a gymnast jumping landing and compressing a springboard before a vault?
- no elastic store, high kinetic and gravitational store
- energy transferred mechanically
- high elastic store, no kinetic store, low gravitational store
( In reality some energy is transferred to the thermal store of the surroundings due to sound and air resistance)
What happens when a tennis player throws a ball upwards?
- tennis ball is moving and is close to earth
- Earth exerts a force over a distance
- stationary tennis ball (for a second in the air) and further from earth
What is the energy analysis for a tennis player throwing a ball upwards?
- high energy in kinetic store and low in gravitational store
- energy transfers mechanically
- no energy in kinetic store and higher energy in gravitational store
What are the ways of calculating work done when a tennis player throws a ball upwards?
Work done =
- force x distance
- weight of ball x change in height
- mass x gravitational field strength x change in height
What are the ways of calculating change in energy when a tennis player throws a ball upwards?
Change in energy in a gravitational store =
- mass x gravitational field strength x change in height
What is the change in every equal to?
Work done —> they both mean the same thing
Why doesn’t the ball in reality reach the height it should have?
Some energy is transferred to the thermal store due to air resistance (thermal store has medium energy before and high after)
How can you analyse an object rolling up a slope?
It’s the same energy analysis as a tennis player throwing a ball upwards, however you need to use the vertical height not the distance up a slope
What are the two things that can happen when two things collide?
- bounce
- stops
What happens when you drop your sunglasses on the beach?
- moving sunglasses and cooler sand and air
- Earth exerts a force over a distance
- stationary sunglasses and warmer sand and air
What is the energy analysis when you drop your sunglasses in the sun?
- high kinetic energy and low thermal store
- energy transferred mechanically and heating by radiation
- no kinetic store and high thermal store
Should all stores be at 0 at the start or the end, and what is the exception?
No, as for most stores, The energy associated with the store is not at zero at the start or at the end. The exception is the kinetic store as there is no energy in the store if the object is stationary
What is the internal energy of an object?
- it’s equal to the total amount of kinetic and potential energy of the particles that it contains
- can be increased by heating, work is done by the action of a force (e.g friction), or flow of electrical current through a resistor
What energy transfers are involved when Soja rides a bike down a hill?
- energy is transferred mechanically from the gpe store of Sonja to the kinetic energy store of the bike
- gpe decreases and ke increases
