HESI A2 Exam: Magnetism and Electricity
• A simple circuit is composed of what three items?
•What is the equation for Ohm’s Law?
• How do you calculate power rating (power = _____ x _______)
• Provide three examples of good conductors
• In an electromagnet, how is the magnetic field produced?
Current
An electric current is a flow of electric charge. In electric circuits this charge is often carried by moving electrons in a wire.
Conductors allow electrons to move freely. Most metals, like iron and gold, are good conductors.
Current can be direct (also known as dc) where it moves only in one direction. It may also be alternating (also known as ac) in which it reverses direction at a certain frequency.
Direct current typically is used only for battery operated devices, such as a smoke alarm or a remote control for the television. However, most household appliances utilize alternating current.
Particles with different electric charges attract one another while particles with the same electric charges repel one another.
Circuits, Transistors and Transformers
A simple circuit is composed of a battery, a device such as a light bulb, and conducting wires as well as an on/off switch.
A transistor is not part of a simple circuit. A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power.
A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. A varying current in one coil of the transformer produces a varying magnetic field, which in turn induces a voltage in a second coil. Power can be transferred between the two coils through the magnetic field.
Transformers are essential for the transmission, distribution, and utilization of alternating current.
Volt, Watt and Ohm
The volt (symbol: V) is the derived unit for electric potential. One volt is defined as the difference in electric potential between two points of a conducting wire.
The watt is a unit of electrical power.
The ohm is a unit of electrical resistance.
Electromagnets and Magnetic Fields
An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. The magnetic field disappears when the current is turned off. Electromagnets usually consist of insulated wire wound into a coil. In a permanent magnet, the spinning of electrons creates a magnetic field.
Magnetic fields are widely used throughout modern technology, particularly in electrical engineering and electromechanics. Rotating magnetic fields are used in both electric motors and generators. A varying current in one coil of the transformer produces a varying magnetic field, which in turn induces a voltage in a second coil.
Ohm and Colomb
Ohm's Law states that I = v/r, with I representing current, v representing voltage, and r representing resistance.
Colomb's law states that F = k((q1 x q2)/d^2). The charges (q1 and q2) are directly proportional to the force. If we increase one we will increase the force by the same proportion.
Equations
P=VI, which means that power = voltage x current.
Force is inversely proportional to the square of the distance between the two charges. For example, If the distance is tripled, the inverse of 3 is 1/3. But remember that the answer will be the inverse proportion to the square.
(1/3)^2 = 1/9.
If we know that the current is 5 amperes and the resistance is 2 ohms, what is the voltage?
Ohm's Law states that I = v/r, with I representing current, v representing voltage, and r representing resistance.
5 = v/2
Multiply each side by two
v = 10
What is the power rating given two amperes of current and an 80 volt circuit?
P=VI, which means that power = voltage x current.
80(2) = 160
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