SM123 Topic 2 An introduction to energy

- energy is conserved
- if the temperature of an object goes up, its energy increases
- there must be a compensating decrease somewhere else within the system
- the energy 'account' of a system must always balance 

- energy can be quantified
- an electricity bill shows that a utility company charges you according to how many units of energy you take from them
- the unit of energy used by such companies is the kWh
- using SI units, joules (J) are used 
- the watt is the SI unit of power
- 1 watt = 1 joule of energy being used for something per second 
- for example, 2.2 kW means the kettle uses 2200 J every second 
- 1 kWh is defined as the amount of energy that something with a power rating of 1kW would use if it ran continuously for an hour 
- 1 kWh is equal to 3.6 x 10⁶ J 

- if an appliance required less electricity, fewer MJ of energy would be consumed 
- electric current is measured in amps
- electrical potential is delivered in volts 
- electric current is how fast electric charges pass into your home
- the potential difference is related to the forces acting on the electric current 

- energy in always equals energy out 
CONSERVATION OF ENERGY 
- discovered by James Joule and Robert Mayer in the 1840s
- idea of energy conservation arose from the observation that 'perpetual motion' machines cannot be made (devices that could perform mechanical tasks without a supply of fuel)

- an electric motor that can produce mechanical motion when supplied with current 
- a dyname that can produce electric current when its input shaft is turned 
- if you set up an experiemtn such that the output shaft of an electric motor is able to turn the input shaft of a dynamo, the dynamo could supply electric current to the motor and the system would be self-sustaining after starting one of the devices 
- the conservation of energy suggests that the experiment will not work
- there is always friction that will cause a rotating shaft to slow down 
- friction causes the moving parts to warm up by redistributing the energy associated with motion
- the result of this heating is that less energy than is input into the dynamo is available for turning the shaft that powers the motor, so the machine will eventually slow to a halt 
- for perpetual motion machine to be possible, something would have to overcome the losses of friction and allow the system to power itself
- this means that either of the two devices would need to be gaining energy from somewhere 

- in all experiments, the values of energy obtained must obey the conservation of energy
- electricity is the energy associated with the movement of charged particles

- the watt is a unit measuring the rate at which energy is supplied to an appliance and used to do…