Download 34-3 Voltage Sources 34-4 Electric Resistance 34

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34-1
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34-2
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Chapter 34 Electric Current
In chapter 33 we learned about electric potential (voltage), now we will learn about what can happen
when two areas have different levels of voltage.
Flow of Charge
Heat flows when there is a difference in temperature between two areas. It always flows from the area
of high temperature to the area of low temperature.
Water flows when there is a difference in pressure between two areas. It always flows from the area of
high pressure to the area of low pressure.
The same is true for electrical charges. When there is a difference in voltage between two areas, charge
will flow from the area of high voltage to the area of low voltage.
o We call this concept potential difference, and it refers to one area having more energy than the
other, this difference creates the potential for movement and work.
o In order to maintain a flow of charge over along period of time an arrangement must be provided
to keep one end of the conductor at a different potential or voltage than the other.
Electric Current
The flow of charge spoken of in 34.1 is called electric current.
o In solids the flow of charge consists of electrons that are loose and moving. In some liquids
though there are loose ions that move also.
We measure electric current in the SI unit of amperes which is abbreviated A and represents the flow of
1 coulomb of charge per second.
o Important note: a current-carrying wire does not have a net electric charge, electrons are moving
through the wire, but they are leaving as often as they are entering.
34-3 Voltage Sources
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Something that provides the potential difference necessary for an electric current to flow is called a
voltage source. Some good voltage sources are dry cells, wet cell, and generators. The voltage in
these sources is what provides the ‘electric pressure’ that is needed to get the current flowing.
Power utilities use generators to provide the 120 volts that come out of our outlets. This means that each
electron that comes out of the wall has 120 joules of energy.
o Current flows through a circuit and voltage is established across a circuit.
34-4 Electric Resistance
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The amount of charge that flows in a circuit depends on the voltage that comes from the source – it also
depends on the resistance that the conductor offers.
o The resistance that the conductor gives to the flow of charge is called the electrical resistance.
This resistance depends on a few things:
• First the conductivity of the material the wire is made of. If electrons can move through
the material better, then there will be less resistance.
• Second it depends on the thickness of the wire. The thicker the wire, the more paths it
offers for flow of charge; therefore, the less resistance it gives to the movement of charge.
• Third it depends on the length of the wire. If the wire is longer it will have greater
resistance to the flow of charge.
• Finally, it can depend on temperature. Generally as the temperature increases so does the
resistance, however there are some exceptions to this rule.
The unit for resistance is the Ohm, which we abbreviate Ω.
34-5 Ohm’s Law
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Ohm discovered that current is directly proportional to the voltage impressed across a circuit and is
inversely proportional to the resistance of the circuit.
V
o Current = Voltage / Resistance or I =
R
volt
• The units for this equation are ampere =
Ohm
o This means that if you increase the voltage you increase the current, but if you increase the
resistance you decrease the current.
o We call electrical devices that provide resistance in a circuit resistors, some common resistors
are a light bulb, a toaster, a radio or a television.
34-6 Ohm’s Law and Electric Shock
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The damaging results of electric shocks are the result of current passing through the body – this current
of course depends on the voltage applied and the resistance of the material.
o The resistance of the body can range from 100 ohms when wet to 500,000 ohms when totally dry.
• This is why electrical appliances (like curling irons) have tags on them warning you not
to use them near water, because your wet skin does not provide enough resistance to
protect you from a high current shock.
You can only receive a damaging shock if two parts of your body are at different potentials (otherwise
no current will flow). This is why birds can sit on power lines – as long as they don’t touch anything
else, their entire body is at the same incredibly high potential.
34-7 Direct Current and Alternating Current
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Direct Current is charge that always flows in one direction. All batteries are DC
Alternating current is a current where the electrons switch back and forth in the direction of their
movement. Most generators and outlets provide an AC current. In America this AC current has 110 or
120 volts (tradition says that this was adopted because of problems with early light bulbs burning out
with higher voltages). In Europe they use 220 volts in their standard socket.
o It is possible to run an appliance through an American outlet at a voltage higher than 120, if you
hook the device up to the circuit so that it is making use of both directions of the alternating
current at once, it doubles the potential difference giving a total voltage of 240 instead of 120
(devices like electric stoves, air conditioners, and clothes dryers use this process).
34-8 Converting AC to DC
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Because our wall sockets run on AC and many of electrical appliances we use run on battery which is a
DC current, we need to convert between the two current types in order to plug these devices in.
o To convert we use a diode which is an electronic device that acts as a one way valve for electron
flow. This way the AC current is made into a pulsed DC current that flows in just one direction
that has been let through the valve. To smooth out this rough DC current we use a capacitor.
34-9 The Speed of Electrons in a Circuit
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When we talk about the speed of electrons in a circuit it is important to distinguish between the speed of
any one individual electron and the overall speed of electron movement. Individual electrons can move
at average speeds of a few million kilometers per hour, but this motion is random and so it not the
movement of electrons that we talk about when we talk about the flow of charge in an electric current.
The actual movement of charges that creates the current is caused by acceleration by the electric field
and is in a direction parallel to the electric field lines (see figure 34.13 on page 541). The actual net
speed of electron movement through a wire is very low. In a DC circuit it is less than one centimeter per
second, and in an AC circuit it is 0.
34-10 The Source of Electrons in a Circuit
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The source of electrons in a conducting circuit is the conducting material itself. It is true that electrons
from the walls of our home are providing the energy into that circuit, but when you plug something into
the wall an electron from that socket transfers its energy to an electron in the device that you have
plugged in, and from then on all of the charges involved in the current come from the device itself.
34-11 Electric Power
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In general charges moving through circuits are using energy. This energy can be given off as heat, or as
mechanical work (like turning a motor). The rate at which electrical energy is converted into another
form of energy (such as heat, mechanical work or light) is called electric power.
o Electric power = current x voltage or E. Power = I x V
• The units for this equation are 1 watt = (1 ampere) x (1 volt)