Resistors (Oxford AQA IGCSE Combined Science Double Award)
Revision Note
Resistors
The current through a resistor (at a constant temperature) is directly proportional to the potential difference across it
This means that the resistance remains constant as the current changes
A current-potential difference (I-V) graph can be used to represent this relationship:
The graph shows a straight line with a constant positive gradient that passes through the origin
The gradient is equal to the reciprocal of resistance ()
This comes from a rearrangement of Ohm's Law:
Gradient is the change in the y axis over the change in the x axis, so if current is on the y axis and potential difference is on the x axis:
Flipping the rearranged Ohm's law equation from above (the Δ symbols can be removed for simplicity):
Current-potential difference graph for a fixed resistor
Heating in Resistors
Extension Tier only
When an electrical charge flows through a resistor, the resistor gets hot
This is because of collisions between the moving charges (electrons) and stationary atoms (metal ions) in the wire
The ions get in the way of the electrons, resisting their flow
If the wire is longer, each electron will collide with more ions so there will be more resistance
The longer a wire, the greater its resistance
If the wire is thicker (greater diameter) there is more space for the electrons so more electrons can flow
The thicker a wire, the smaller its resistance
Inefficient energy transfer in a filament bulb
In a filament bulb, almost 100% of energy is transferred from the chemical store of a cell to the thermal store of the filament wires due to resistance
This energy is considered wasted, as the function of a bulb is to emit light, not heat
Only 5% of the energy in the thermal energy store is transferred to the light store due to the glow of the filament
A filament bulb is very inefficient
More efficient energy transfer in other lamps
An efficient system is one where most of the energy going into that system ends up in the form that is wanted
For a lightbulb this is to the light energy store and not the thermal energy store
An inefficient system is one where most of the energy ends up in forms that weren’t wanted
A compact fluorescent lamp heats up less and therefore wastes less energy than a filament bulb
Compact fluorescent lamps are more efficient than filament bulbs
reduces the electric power consumption for a given luminosity by 80% compared to a filament lamp
Hence, a more efficient lightbulb is cheaper to run
Assuming a constant usage (24 hours a day) and a tariff of 10p, or £0.10 per kWh the annual cost of running each lightbulb is:
An LED is £10.51
A CFL is £13.14
A halogen is £36.79
An incandescent is £52.56
Therefore, an LED lightbulb is about 5 times cheaper to run than a filament bulb
Efficiency of appliances
When purchasing new appliances it is possible to choose how efficiently they transfer energy for their intended purpose
More efficient appliances normally bear a higher purchase cost but cost less to run
On most appliances, manufacturers provide energy rating labels to allow customers to make comparisons
Energy rating labels describe:
The energy consumption of the appliance in kWh per year
The efficiency of the appliance by assigning a band value from A to G, where A is the most efficient and G is the least efficient
Energy efficiency rating label for a fridge
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