Chapter 17 : Electromagnetism - LCHS Physical Science LCHS Main SiteSD271 Main siteUof I New Century Classroom  
Objectives: The learner will...
...understand and apply concepts of electricity and magnetism in various problems including: resistance, voltage, current, capacitance, inductance and power
Vocabulary:
resistance : a measure of opposition to current flow
ampere : the electrical flow of 1 Coulomb (6.25 x 1018 electrons) per second
volt : potential difference (electrical pressure) between two points on a conductor carrying a current of 1 ampere and a power of 1 watt
series : resistances wired in a single chain
parallel : resistances wired with like ends together
 
ELECTRICITY
The smallest unit of electrical charge is contained in a single electron or proton. Electrons have a negative charge and protons have a positive charge. Opposite charges have an attractive force on each other and like charges have a repulsive force. It is this electric force that holds electrons in orbit around the protons in the nucleus of atoms. When a charge moves it generates a magnetic field. A standard unit of charge (Q) is the coulomb. (1 coulomb = 6.25 x 1018 electrons )
RESISTANCE
When electrons (charges) are pushed by a Voltage source along a conductor, the Resistance of the circuit regulates the flow ( I - amps) of charge. This is stated in Ohm's Law: V = I x R
(1 Amp of current equals 1 Coulomb per second. Ohms are the unit of resistance). Computing the total resistance of a circuit (RT) composed of multiple resistances ( R1, R2, R3 ) is done by determining the arrangement (series, parallel or combination) and using the associated equation below.

Series RT = R1 + R2 + R3
Parallel RT = 1/(1/R1 + 1/R2 + 1/R3)
Combination RT = R1 + (1/(1/R2 + 1/R3)

Also, the square of the Voltage of a circuit over it's total Resistance determines its Power requirements in watts: P=V²/R

CAPACITANCE
A capacitor stores electrical charge similar to a battery. 2 conductive plates separated by a non-conductive layer hold a collection of opposite charges (fig 1). If a 1 Volt battery completely charges a 1 Farad capacitor, it holds 1 Coulomb of charge. The capacitor is charged through a resistor that controls the rate of current.



INDUCTANCE
A cylindrical coil (induction coil) of wire generates a magnetic field when current passses through it (fig 2). The strength of the magnetic field is measured in Henries (L) and can be calculated with the equation: L = oN² A / l
( o = 4 x 10-7 or 1.26 x 10-6 , universal magnetic permiability constant)

An induction coil will store magnetic potential energy within the magnetic field according to this equation: PEm = ½ L I²
(L=inductance in Henries, I = current in amps)

Since current in a wire creates a magnetic field, 2 such wires carrying current (I1 I2) can attract or repel (Force in Newtons) each other through their magnetic fields (fig 3) according to this equation: F=oI1I2l/(2d)

A charged capacitor can oscillate through an induction coil at a fixed frequency (fig 4). This frequency can be adjusted (tuned) to resonate with other oscillators by varying the value of the capacitor (C) or the coil (L). Radio station transmitters and receivers work according to this principle.

 
Practice: Help:
PB1
How many volts is required to push 5.7 amps of current through a resistance of 45 ohms?
 V=IR
V=5.7x45
PB2
How much current flows through a resistance of 41 ohms under the pressure of 65 volts?
 V=IR
65=Ix41
PB3
How many ohms of resistance will restrict a current to 3 amps at 64 volts?
 V=IR
64=3xR
PB4
3 resistors in series, 86 , 35 , and 42 ohms have what total resistance?
 RT = R1 + R2 + R3
PB5
3 resistors in parallel, 55 , 48 , and 14 ohms have what total resistance?
 RT = 1/(1/R1 + 1/R2 + 1/R3)
PB6
1 69 ohm resistor in series with 2 in parallel, ( 28 and 28 ) ohms have what total resistance?
 RT = R1 + (1/(1/R2 + 1/R3)
PB7
What is the inductance (henries) of a cylindrical coil of wire of 260 turns, a length of .055 m and an area of .24 m^2?

L = oN² A / l
o = 1.26 x 10-6

PB8
How many Joules of magnetic potential energy does a 85 henry coil have with .17 amps?
 PEm = ½ L I²
PB9
What is the force (N) between 2 parallel conductors, each .76 m long, carrying 29 amps each and .027 meters apart?
 F = o I1 I2 l / (2d)
o = 1.26 x 10-6
PB10
How many volts does it take to push 38 watts of power through a resistance of 80 ohms?
 P=V2/R
38=V2/80
answer bank:
.3716771
163
1.585366
55.1362
 83
9.054385
1.22825		 256.5
.004747189
21.33333

Scientist Spotlight:
Alessandro Volta 1745-1827 Italian physicist who constructed a device for accumulating a large charge by a series of inductions and groundings.

Resources (Study Links/Study Tips/Reading Lists)
CircuitMaker Virtual Lab
Chapter 17 : Electromagnetism - LCHS Physical Science LCHS Main SiteSD271 Main siteUof I New Century Classroom  
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