forces and energy Flashcards
When a system changes what happens
Energy is transferred into or away from the system
Energy is dissipated and stored in less useful ways
When there are energy transfers in a closed system what happens
There is no overall change to the total energy in that system
How can energy of a system be changed
- work done by forces
- electrical equipment
- in heating
What is energy transferred to equal to
Work done
How to calculate work done
E =FxD
work done (joule, J) = force (newton, N) × distance moved in
the direction of the force (metre, m)
What is the equation for change in gravitational potential energy
∆
GPE ∆= m×gxh
change in gravitational potential energy (joule, J) = mass
(kilogram, kg) × gravitational field strength (newton per
kilogram, N/kg) × change in vertical height (metre, m)
How to work out kinetic energy
0.5 x mass x speed squared
kinetic energy (joule, J) = 0.5 x mass (kilogram, kg) × (speed)2
What does 1 Nm (newton metre) equal to
One joule
Equation for efficiency
Useful energy divided by total energy
What are some examples of how in all system changes energy is dissipated so that it’s stored in less useful ways
An electric toothbrush is a system
It’s transfers energy electrically from the chemical store of its battery to the kinetic energy store of its bristles
Some of this every is transferred out of the system to the surroundings by sounds and by heating
A hairdryer transfers energy into the system electrically from mains supply to the kinetic energy store of the heating element and some energy is transferred away by sound
What is power and examples
The rate at which energy is transferred
An electric heater has a power of 600W this means it transfers 600J of energy per second. A 1200W would transfer twice as much energy and so would heat a room quicker
What is the equation for power
power (watt, W) = work done (joule, J) ÷ time taken
(second, s)
P = E/t
What is one watt equal to
One joule per second (J/s)
Why do mechanical processes become wasteful
Because their releasing dissipated energy into the atmosphere
E.g a bubble releases light energy which is useful however heat is a unusefuk source of energy and so it heats surrounds unneeded
