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Essential idea: Generation and transmission of alternating current (ac) electricity has transformed the world.
Nature of science:
Bias: In the late 19th century Edison was a proponent of direct current electrical energy transmission while Westinghouse and Tesla favoured alternating current transmission. The so called “battle of currents” had a significant impact on today’s society. (3.5)
Understandings:
Alternating current (ac) generators
Average power and root mean square (rms) values of current and voltage
Transformers
Diode bridges
Half-wave and full-wave rectification
Applications and skills:
Explaining the operation of a basic ac generator, including the effect of changing the generator frequency
Solving problems involving the average power in an ac circuit
Solving problems involving step-up and step-down transformers
Describing the use of transformers in ac electrical power distribution
Investigating a diode bridge rectification circuit experimentally
Qualitatively describing the effect of adding a capacitor to a diode bridge rectification circuit
Guidance:
Calculations will be restricted to ideal transformers but students should be aware of some of the reasons why real transformers are not ideal (for example: flux leakage, joule heating, eddy current heating, magnetic hysteresis)
Proof of the relationship between the peak and rms values will not be expected
Data booklet reference:
International-mindedness:
The ability to maintain a reliable power grid has been the aim of all governments since the widespread use of electricity started
Theory of knowledge:
There is continued debate of the effect of electromagnetic waves on the health of humans, especially children. Is it justifiable to make use of scientific advances even if we do not know what their long-term consequences may be?
Aims:
Aim 6: experiments could include (but are not limited to): construction of a basic ac generator; investigation of variation of input and output coils on a transformer; observing Wheatstone and Wien bridge circuits
Aim 7: construction and observation of the adjustments made in very large electricity distribution systems are best carried out using computer-modelling software and websites
Aim 9: power transmission is modelled using perfectly efficient systems but no such system truly exists. Although the model is imperfect, it renders the maximum power transmission. Recognition of, and accounting for, the differences between the “perfect” system and the practical system is one of the main functions of professional scientists
Consider an electric immersion heater used to heat a cup of water to make tea. Construct a problem in which you calculate the needed resistance of the heater so that it increases the temperature of the water and cup in a reasonable amount of time. Also calculate the cost of the electrical energy used in your process. Among the things to be considered are the voltage used, the masses and heat capacities involved, heat losses, and the time over which the heating takes place. Your instructor may wish for you to consider a thermal safety switch (perhaps bimetallic) that will halt the process before damaging temperatures are reached in the immersion unit.
(a) What current is needed to transmit 1.00 × 10^2 MW of power at 10.0 kV? (b) Find the resistance of 1.00 km of wire that would cause a 0.0100% power loss. (c) What is the diameter of a 1.00-km-long copper wire having this resistance? (d) What is unreasonable about these results? (e) Which assumptions are unreasonable, or which premises are inconsistent?
(a) What is the cost of heating a hot tub containing 1500 kg of water from 10.0ºC to 40.0ºC, assuming 75.0% efficiency to account for heat transfer to the surroundings? The cost of electricity is 9 cents/kW⋅h. (b) What current was used by the 220-V AC electric heater, if this took 4.00 h?
(a) An aluminum power transmission line has a resistance of 0.0580Ω/km. What is its mass per kilometer? (b) What is the mass per kilometer of a copper line having the same resistance? A lower resistance would shorten the heating time. Discuss the practical limits to speeding the heating by lowering the resistance.
Resources:
Resources:
In class Activities:
In class Activities:
AC/DC - Spinning a coil in a magnetic field
Transformers - Derivatives of Sine and Cosine waves.
Bridge Rectifiers - Smoothing the bumps
Concept Checks:
Concept Checks:
Problem Solving:
Problem Solving:
Transformers TSPs
Formative:
Formative:
Readings:
Readings:
All Sections:
AC Current:
OpenStax - College Physics - Electric Generators
Kognity - Power Generation and Transmission
Transformers:
Rectifiers:
How do diodes work? - video
Bridge Rectifiers - Article
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