Physics for Electricity

Copyright © 2014 Delmar, Cengage Learning

Physics for Electricity

Instructor Name: (Your Name)

3CHAPTER


Learning Objectives

• Use Watt’s law to solve for electric power• Discuss the concepts of electrical fields

and magnetic lines of force• Explain how an electron magnet is formed• Put in your own words how a voltage is

induced in a conductor by movement through a magnetic field


Learning Objectives (continued)

• Describe how an inductor and capacitor store energy

• Discuss how a negative voltage spike is generated when current flow through an inductor is ceased

• Explain the similarities between a capacitor and an accumulator used in a hydraulic system


Electric Power

• Work is force that results in movement in the direction the force is applied

• The amount of force exerted times distance traveled (lb-ft) yields the amount of work performed

• Power is the rate at which work is performed, in other words the amount of work over a period of time

• Electrical power is measured in watts


Watt’s Law

• Electrical power is calculated by multiplying current by voltage

• Watts=Amps x Volts (P=IE or PIE)• Volts=Watts ÷ Amps• Amps=Watts ÷ Volts• P=Watts (Power)• I=Current• E=Voltage


Magnetism

• Current flow produces magnetism• Magnetism can produce current flow• Every magnet has two distinct poles, north

pole and south pole• Magnetic lines of force (magnetic flux) are

present between the north and south poles of all magnets


Magnetic Lines of Force

Figure 3-1 Magnetic lines of force illustrated by iron filings.


Arrows Indicate Direction the North End of a Compass Will Point

Figure 3-2 Directional arrows on magnetic lines of force indicate direction the North end of a compass needle would point when placed in the magnetic field.


Magnetism (continued)

• Opposite poles of a magnet attract each other

• The attraction of unlike poles increases as they get closer to each other

• Like poles of a magnet repel each other• The repulsion of the like poles increases

as they get closer to each other


Reaction of Unlike and Like Poles of a Magnet

Figure 3-3 Unlike magnetic poles attract; like magnetic poles repel.


Current Flow and Magnetism

Figure 3-4 Lines of force around a current-carrying conductor

Figure 3-5 Right hand rule of thumb


The Strength of the Magnetic Field Surrounding a Conductor is Directly Proportional to the

Current Flow

Figure 3-8 Magnetic field around conductor with 1 amp of current flow and 3 amps of current flow.


Magnetic Fields Cancel Out Space Between Conductors Causing the Conductors

to Move Closer

Figure 3-10 Magnetic field cancels out in the space between conductors with current flow in same direction. This causes two conductors to move toward each other.


Two Conductors With 10A of Current Combine to Produce 20A of Current

Figure 3-11 Two conductors with 10A of current flowing through each conductor has the same combined magnetic field strength as 20A flowing through a single conductor.


When Current Flows Through a Loop of Wire the Magnetic Fields Combine to Form

a Single Stronger Magnetic Field

Figure 3-12 Magnetic field around one loop of a conductor.


Electromagnet

• Reluctance is the opposition to the “flow” of magnetic lines of force

• Some materials such as nickel, iron and steel, provide a path of less reluctance for magnets

• When current flows through a wire wrapped around an iron core the magnetic lines of force will concentrate on the core and create a strong magnet, an electromagnet


Electromagnet

Figure 3-14 Adding an iron core to the coil to form an electromagnet.


Creating Current Flow With a Magnet

• If a conductor is passed through a magnetic field so the field “cuts” through the conductor a voltage is induced in the conductor

• Only movement that is 90° to the magnetic lines causes a voltage to be induced into the conductor

• It does not matter if the magnet or the conductor is moved, a voltage will be induced


Cutting Lines of Force Induces a Voltage in a Conductor

Figure 3-16 Cutting magnetic lines of force to induce a voltage in the conductor: conductor is moving from the right to the left perpendicular to the magnetic lines of force.


Movement Parallel to Lines of Force Result in No Induced Voltage in the Conductor

Figure 3-17 Up and down movement of conductor parallel to magnetic lines of force resulting in no induced voltage because no magnetic lines of force are being cut by the conductor.


Inductors• An inductor is a wire that is wound in a series

of coils• Inductors have a core of ferrous metal• Inductors are rated in henries• Inductors store energy in the form of a

magnetic field• An inductor will inhibit the initial flow of current,

known as counter electromotive force (CEMF)• Once current flow is interrupted an inductor will

try to maintain the flow


Current Flow Through an Inductor

Figure 3-22 Magnetic field surrounding an inductor with no change in current flow.

Figure 3-23 Decreasing current flow through inductor


Lenz’s Law

Lenz’s Law: The polarity of an induced voltage is such to oppose the change in current that produced it.

Figure 3-24 Current flow through an inductor is interrupted causing a reverse-polarity voltage to be induced across the inductor as the magnetic field surrounding the inductor collapses.


Inductor Suppressed by Parallel Resistor

Figure 3-26 Inductor suppressed by parallel resistor limits the negative voltage spike to a low amplitude.


Transformer Used to Step Up a Changing Voltage

Figure 3-27 Transformer steps up a changing voltage: a transformer cannot increase a DC voltage.


Spark Ignition System Used on Gasoline Engines

Figure 3-28 Spark-ignition system utilizes a negative voltage spike and a type of transformer to produce thousand of volts.


Capacitors

• Capacitors are electrical devices used to store energy in the form of an electric field

• Capacitors consist of two metal plates spaced close together

• A thin non conductive material, a dielectric material, separates the plates

• A dielectric has a very high resistance value• The metal plates of the capacitor are connected to

the electrical circuit• Capacitors are rated in farads


Capacitors Store Energy in the Form of An Electric Field

Figure 3-33 Capacitor stores energy in the form of an electric field.


Capacitor in a Circuit

Figure 3-34 Capacitor charging.

Figure 3-35 Capacitor maintaining its charge.

Figure 3-36 Capacitor discharging.


Work

• Work is an activity that involves force and movement in the direction of that force, as described by the equation:

Work = Force x Distance• Work is measured in pounds-feet• If a 500 pound weight is lifted one foot that

equals 500 lb-ft of work


Energy

• Energy is the capacity or ability to perform work

• Energy is measured in pounds-feet• Kinetic energy is the energy of motion• Heat is a form of energy• Potential energy is energy that is stored

and ready to perform some amount of work


Power

• Power is the rate at which work is done• Power is often measured in watts or

horsepower• One horsepower is defined as the amount of

power necessary to lift 550 lbs at a rate of one foot per second

• One horsepower is equal to 746 watts• The equation to calculate horse power is:

Horsepower =(Torque x RPM) ÷ 5252


Kinetic Energy

• Kinetic energy is the energy of motion• The amount of kinetic energy depends on

the velocity and mass of the object• The formula to determine kinetic energy is:

KE= ½ x Mass x Velocity²


Forms of Potential Energy

• Gravitational Potential Energy (GPE)GPE = Mass x Acceleration Due to Gravity x

Height• Spring potential energy• Chemical potential energy; an example of

chemical potential energy is diesel fuel or gasoline


Summary

• Power describes the rate at which work is being performed and has units of watts.

• Electric power measured in watts is the product of volts and amps.

• Magnetic lines of force are means of graphically illustrating the strength and orientation of a magnetic field. Magnetic lines of force are drawn so that they appear to originate at the north pole of a magnet and end at the south pole of a magnet.


Summary (continued)

• Like magnet poles repel each other and unlike magnetic poles attract each other. This attraction or repulsion increases at a rapid rate as the distance between two magnets decreases.

• Current flow through a conductor causes a magnetic field to encircle the conductor. The direction of the arrows on the magnetic lines of force can be determined using the right hand rule.


Summary (continued)

• A conductor that is carrying current is compelled to move out of a strong magnetic field into a weaker magnetic field.

• A wire that is formed into a coil is called an inductor. Inductors store energy in the form of a magnetic field. Inductors are rated in units of henries.


Summary (continued)

• Current flow through an inductor causes a magnetic field to surround a coil much in the same way that a magnetic field surrounds a permanent magnet. Placing a piece of metal such as iron inside the inductor causes the metal to temporarily become a magnet. The temporary magnet is an electromagnet. Electromagnets are used in many truck electronic components.


Summary (continued)

• Passing a conductor through a magnetic field so that lines of force are interrupted or cut by the conductor causes a voltage to be induced in the conductor. This is the basis of generators.

• Changing the current flow (increasing or decreasing) through an inductor causes the expanding or contracting magnetic field to induce a voltage across the coil. This is known as self-inductance.


Summary (continued)

• The polarity of voltage induced across an inductor due to a decreasing magnetic field is opposite the polarity of the voltage that produced the initial voltage flow through the inductor. This is described by Lenz’s law.

• The negative voltage spike produced by an inductor when current flow is rapidly interrupted can damage a truck’s electronic components. Suppression is typically used across the inductor to reduce the negative voltage spike.


Summary (continued)

• Capacitors store energy in the form of electric fields. Capacitors are rated in units known as farads.

• The time it takes for a capacitor to charge depends on the series resistance in the circuit. The larger the value of resistance, the longer it takes the capacitor to charge.

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Physics for Electricity

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