Chapter 20 Electricity

20.1: Electric Charge and Static Electricity

Electric Charge Question: What do static cling and lightening have in common? Answer: They both have a similar cause – the movement of charged particles Electric charge: a property that causes subatomic particles such as protons and electrons to attract or repel each other. 

Types of Electric Charge There are two types of electric charge: Positive (protons) Negative (electrons)  Remember that in an atom, the charges are balanced…  An excess or shortage of electrons produces a net electric charge.  The SI unit of electric charge is the coulomb (C) 

Coulomb It takes about 6.2 x 1018 electrons to produce one coulomb A lightning bolt is about 20 coulombs of charge. A flash camera uses the energy from 0.025 coulombs of charge for each flash. 

Electric Charges Why do two balloons repel off each other after being rubbed on your hair?  Like charges repel, opposite charges attract.  Electric force- the attraction or repulsion between electrically charged objects

Electric Force Electric force depends on two main factors: Charge Distance How does changing the size of the charge or the distance between charges affect the electric force? (Refer to figure 3, p. 601) Doubling the net charge on one object doubles the electric force However, doubling the distance between the objects will reduce the electric force to one fourth as strong! 

Reviewing the Atomic Structure Why are negative charges (electrons) more mobile than positive charges (protons)? Think about where the subatomic particles are located… Protons concentrated in the nucleus Electrons found outside the nucleus

Electric Fields Electric field: the effect an electric charge has on other charges in the space around it   The strength of an electric field depends on the amount of charge that produces the field and on the distance from the charge.

Note the direction of the field of a positive versus negative charge. The direction of each field line shows the direction of the force on a positive charge (repels or attracts) The lines representing the field are close near the charge, where the field is strongest! The more net charge an object has, the greater the force is on it 

Electric Fields

Static Electricity Static electricity: the study of the behavior of electric charges, including how charge is transferred between objects  What are some ways that charges can build up on an object or move from one object to another? 

Transferring Charge Charge can be transferred three ways: Friction Contact Induction

Conserving Charge Guess what happens to the total amount of charge before and after the transfer of charge occurs (regardless of the way the charge is transferred)?  Law of Conservation of Charge: the total charge in an isolated system is constant. 

Charging by Friction A charge builds up when one material has a greater attraction for electrons than the other and a build up of charge is created when they are rubbed together (friction).  

Charging by Friction Because electrons transfer from one material to the other, an overall positive charge is produced on the material that loses electrons.   An electric charge is produced when the opposite charges come close together! 

Transfer of Charge (friction)

Charging by Contact A Van de Graaff generator is an example of how an electric charge can be obtained by contact.  When someone touches the metal sphere, enough charge is acquired to make hair stand on end. 

Charging by Induction Induction- the transfer of charge without contact between materials 

Why do I get shocked by the doorknob?

An Example of Charging by Induction This is what happens when you walk across carpet and pick up extra electrons (so your feet/hands are negatively charged).   The net negative charge in your hand repels the electrons in the doorknob.   The electrons in the doorknob move to the base and leave a net positive charge on the part closest to your hand.   The attraction between positive and negative charges creates a spark!  The name for this spark is called static discharge

Static Discharge Static discharge occurs when a pathway through which charges can move forms suddenly.  Lightning is a dramatic example of static discharge

Lightening & Static Discharge Friction between moving air masses causes a charge to build up in the clouds The lower part of the cloud has an overall negative charge, which induces an overall positive charge on the ground below it. As the amount of charge on the cloud increases, the force of attraction increases between the charges in the ground and the cloud Eventually, the air becomes charged, forming a pathway for the electrons to travel to the ground. 

Lightening  Static Discharge

20.2: Electric Current and Ohm’s Law

Electric Current For a flashlight to work properly, it must form a continuous path through which charges can flow, in other words, make an electric current.  Electric current- a continuous flow of electric charge The SI unit of electric current is the ampere (A), or amp 1 amp = 1 coulomb/second 

Types of Current There are two types of current: Direct Current (DC) Alternating Current (AC)

Direct Current (DC) Direct current: charge flows in ONE direction (DC) Example: battery-operated devices like a flashlight

Alternating Current (AC) Alternating current- a flow of electric charge that regularly reverses its direction (AC) Example: most homes and schools use this type of current

Important to Notice Notice Figure 7 on p. 604— In direct currents, like a flashlight, the electrons flow from the negative terminal of one battery to the positive terminal of another battery

Important to Notice However, look at the direction of the flow of the current… Why does the current seem to be moving in the opposite direction? Current moves in the opposite direction because scientists define current as the direction in which positive charges would flow. 

Conductors & Insulators What is the material required for running electrical currents usually made of and why?  Metals such as copper and silver are good electrical conductors.   Wood, plastic, rubber, and air are good electrical insulators. 

Resistance What do you think of when you hear the term resistance? Resistance: is opposition to the flow of charges in a material The SI unit of resistance is the ohm. 

Resistance As electrons move through the wire, they bump into each other and convert some kinetic energy into thermal energy.  Because of this, less energy is available to move the electrons through the wire, so the current is reduced. 

Various Ways to Increase Resistance There are various ways to increase resistance in a current…  A material’s thickness, length, and temperature affect its resistance. 

Thickness & Resistance Using a thicker wire is like drinking a milkshake through a thicker straw, it becomes easier!

Length & Resistance If you have a longer wire, the resistance is greater because it has to travel further—so length increases it! 

Temperature & Resistance As temperature increases, collisions increase. When there are more collisions, there is more energy therefore more resistance

Superconductors Could the temperature ever be lowered enough to achieve a resistance of zero???  This is the idea behind superconductors. 

Voltage In order for charge to flow in a conducting wire, the wire must be connected in a complete loop that includes a source of electrical energy  What would the electrical energy source be in a flashlight? 

Ohm’s Law German scientist Georg Ohm found a mathematical formula that relates voltage, resistance, and current.  He said that as resistance increases, voltage decreases. 

Ohm’s Law Ohm’s law is stated in the following formula: V = I x R or I = V/R V = voltage R = resistance I = current Increasing the voltage increases the current. Keeping the same voltage and increasing the resistance decreases the current.   

20.3: Electric Circuits

Electric Circuits Electric circuit: a complete path through which charges can flow  There are circuit diagrams designed to read the path of a current.  REMEMBER!!! The direction of current is defined as the direction in which positive charges will flow. Electrons flow in the opposite direction. 

Types of Circuits There are two types of circuits: Series Circuit Parallel Circuit

Series Circuit Series circuits: charge has ONE path through which it can flow.  There are some problems to this system that keeps people from wiring series circuits in their homes…  If one element stops functioning in a series circuit, none of the elements can operate!  Basically, if a light bulb were to burn out, the TV would shut off! 

Series Circuit Also, adding bulbs to a series circuit increases the resistance, therefore, decreasing the current and brightness of the bulb!

Parallel Circuit Parallel circuit: an electrical circuit with two or more paths through which charges can flow.  If one bulb burns out, the charge can still flow so the others stay lit.   If one element stops functioning in a parallel circuit, the rest of the elements still can operate.

Electrical Power To find the electrical power of the system, use the following equation:  P (watts) = I (amps) x V (volts) Electric power can be calculated by multiplying voltage by current. 

Power Ratings An appliance’s power rating tells how much power it uses under normal conditions.  To find the electrical energy used by an appliance, use the following equation:  E = P x T E = energy P = power T = time

Safety Measures for Circuits Fuses: Prevents current overload- wire melts if too much current and stops flow  “Blowing a fuse”

Safety Measures for Circuits Circuit breakers A switch that opens when current in the circuit is too high Must be reset before using again

Safety Measures for Circuits Insulation  Don’t touch bare wires!

Safety Measures for Circuits Grounded plugs (Ground Fault Circuit Interrupter  GFCI) Transfers excess charge through a conductor to the Earth Example: the third prong on electrical plugs