5: Current and Resistance
Magnetic resonance imaging (MRI) uses superconducting magnets and produces high-resolution images without the danger of radiation. The image on the left shows the spacing of vertebrae along a human spinal column, with the circle indicating where the vertebrae are too close due to a ruptured disc. On the right is a picture of the MRI instrument, which surrounds the patient on all sides. A large amount of electrical current is required to operate the electromagnets (credit right: modification of work by “digital cat”/Flickr).
In this module, we study the electrical current through a material, where the electrical current is the rate of flow of charge. We also examine a characteristic of materials known as the resistance. Resistance is a measure of how much a material impedes the flow of charge, and it will be shown that the resistance depends on temperature. In general, a good conductor, such as copper, gold, or silver, has very low resistance. Some materials, called superconductors, have zero resistance at very low temperatures.
High currents are required for the operation of electromagnets. Superconductors can be used to make electromagnets that are 10 times stronger than the strongest conventional electromagnets. These superconducting magnets are used in the construction of magnetic resonance imaging (MRI) devices that can be used to make high-resolution images of the human body. The chapter-opening picture shows an MRI image of the vertebrae of a human subject and the MRI device itself. Superconducting magnets have many other uses. For example, superconducting magnets are used in the Large Hadron Collider (LHC) to curve the path of protons in the ring.
So far we have talked about electric force, electric fields, and electric potential, but all in the context of stationary charges. Now we are going to begin discussing what happens when charges are put in an electric field or potential difference and they are allowed to move.
When charges move from one place to another, this is called electric current, and will be the focus of the second part of this module. We will investigate current from different perspectives like the current in an electric circuit (which is called conventional current) to the flow of Na+ and K+ ions through cell membranes. When these charges flow through materials like copper wires or cell membranes, the charges will bump into things along the way. This bumping results in some friction, which we call resistance. In this module, we will discuss what causes a current to flow (voltage) and what keeps that current flowing at a steady rate (resistance). The relationship between voltage, resistance, and current is called Ohm’s Law.
I have an electric car and I love it! Whenever I get the chance to connect this class to my car, I will do it. Like in this video…
5.1 Electrical Current
- Describe an electrical current
- Define the unit of electrical current
- Explain the direction of current flow
5.2 Model of Conduction in Metals
- Define the drift velocity of charges moving through a metal
- Define the vector current density
- Describe the operation of an incandescent lamp
5.3 Resistivity and Resistance
- Differentiate between resistance and resistivity
- Define the term conductivity
- Describe the electrical component known as a resistor
- State the relationship between resistance of a resistor and its length, cross-sectional area, and resistivity
- State the relationship between resistivity and temperature
5.4 Ohm’s Law
- Describe Ohm’s law
- Recognize when Ohm’s law applies and when it does not
5.5 Electrical Energy and Power
- Express electrical power in terms of the voltage and the current
- Describe the power dissipated by a resistor in an electric circuit
- Calculate the energy efficiency and cost effectiveness of appliances and equipment
- Describe the phenomenon of superconductivity
- List applications of superconductivity