Download Calculating Resistance and Other Goodies You

Calculating Resistance and Other Goodies You
Were Afraid to Ask About
Resistance hinders or slows down the flow of electrons in a current and is a
natural quality of all materials, (with the exception of superconductors that
are cooled to such low temperatures, -250 C that resistance is almost nonexistent). There are four qualities of a conductor that affect the amount of
resistance it will offer to an electric current, they are; temperature of the
conductor, (the hotter the material gets the more energy its particles have
and the more they move creating more resistance to the flow of the
electrons in the electric current), the diameter of the conductor, (the
larger the wire is the less resistance it offers. Imagine water flowing
through a garden hose with a width of 3 cm and then being forced into a
hose with a width of 1 cm. The resistance would increase tremendously with
the result that the hose would probably burst. Well that happens with the
wires in a conductor when they are forced to carry too large a current.) the
material that the conductor is made of, (some materials are better
conductors than others, for example silver is a better conductor than
copper), the length of the conductor, (the longer the conductor the more
resistance it offers).
The late 18th and early 19th centuries were a very exciting time in the world
of scientific discovery and innovation. Volta had invented the voltaic pile and
Galvani had proven that nerves in the body carried and were stimulated by
electric currents. Hans Christian Oersted and Michael Faraday had
independently shown that a flowing electric current generated a magnetic
field and wire moving within a magnetic field could generate an electric
current, this is the basis upon which modern generators work and produce
the electricity we use everyday. Georg Ohm, a German scientist was also
working with electric fields trying to determine the relationship, if any,
between resistance, voltage and current. Ohm eventually determined the
relationship and formulated Ohm’s Law. Ohm’s Law basically states that as
long as temperature stays the same the resistance in a conductor will remain
constant and the size of the current is directly proportional to the voltage
applied. This physical law can be reduced to a simple mathematical formula:
R = V
I
R= resistance in Ohm’s ()
V = voltage in volts (V)
I = current in amperes (A)
Complete the following calculations.
1. What is the resistance of a light bulb if a 12 V battery sends a current of
2.4 A through it?
2. A toaster with a resistance of 145  is connected to a 120 V source. What
current will flow through the toaster?
3. What is the potential difference, (voltage) across a 1500  resistor
carrying a current of 0.075 A?
4. An extension cord rated at 15 A is connected to a 120 V power supply.
What is the smallest load resistance with which the cord can safely be
used?
5. What is the voltage across an electric water heater element that has a
resistance of 32  when the current through it is 6.8 A?
6. What current flows in a 20-ohm electric toaster connected to a 120 V
toaster?
7. Calculate the resistance of an electric kettle carrying a current of 9.5 A
from a 120 V supply.
8. A 15 A current flowing through a bulb that provides 3 will have a voltage
of?
9. If the resistance of a conductor is 98  and 2.6 V are used, what is the
current?
10. A voltmeter connected to an electric bell reads 3.0 V and an ammeter in
series with the bell reads 0.75 A, what is the resistance in the bell?
11. A resistor connected to a 3.0 V battery produces a current of 0.040 A.
Calculate the resistance of the resistor.
13a. If two identical bulbs are connected in series to a 3V battery, how much
current will flow to each bulb?
b. If a third bulb is added to the series above, how much current would each
bulb receive?
14. A very large current has to be transmitted along a conducting wire. What
characteristics of the wire would be best suited to do this?
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15. An electric motor has a resistance of 185 . It is connected to a power
source that has a potential difference of 120 V. Calculate the current
that flows through the motor.
In physics, power is defined as energy per unit time. Electric power
describes the amount of electric energy that is converted into other forms
of energy, (heat, light, sound or motion) every second. As well, electric
power can also describe the amount of electric energy that is transferred
from one place to another in a certain period of time.
Power (in watts) = Energy in Joules
Time (in seconds)
P = E
t
The units of power are Joules per second. One Joule per second is also
called one watt, (W). A 100 W light bulb for example converts 100 W of
electric energy into heat and light every second.
Electric power is usually calculated by measuring voltage and current in a
circuit.
Power = current X voltage
P = IV
Complete the following calculations
1. What is the power in watts and kilowatts of a hair dryer that requires
10 A of current to operate on a 120 V circuit?
2. The maximum current that 54” TV can withstand is 2A. If the TV is
connected to a 120 V circuit, how much power is the TV using?
3. A 900 W microwave requires 7.5 A of current to operate. What is the
voltage of the circuit to which the microwave is connected?
4. A flashlight using two 1.5 V D-cells contains a bulb that can withstand up
to 0.5 A of current. What would be the maximum power of the bulb?
5. Bob has a stereo that operates at 120 V using 2.5 A of current. How much
power does Bob’s stereo need to operate?