Current Electricity refers to

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1 Unit 3 Fields Graviational, Electrical, Magnetic Review of Grade 11 Electrostatics Review of Matter 1. All matter is composed of or Our ideas about the nature of atoms have progressed over the last two centuries (and continue to develop today). John Dalton introduced a new form of the ancient Greek idea of atoms at the beginning of the nineteenth century. In 1897, J.J. Thomson discovered the and suggested the model of the atom. In 1911, Rutherford suggested that. In 1914, Bohr modified Rutherford's model by. 2. Atoms contain, and. Of these three subatomic particles, only the (positively charged) and the (negatively charged) are considered charged particles. A neutral object has an number of protons. A charged object has an number of electrons and protons. (see hyperlink for more information) < Electrostatics Static means Electrostatics: is the study of the phenomena arising from what seems to be electric charges. Current Electricity refers to Objects become charged by friction Objects are considered neutral because When two objects rub against each other and each object has a different level of attraction for their electrons, electrons from The electron affinity series helps predict which object will gains and becomes and which object loses and becomes. 1

2 Charge is distributed uniformly in a conductor As an object becomes charged because the number of does not equal the number of then the excess charges will distribute themselves uniformly in the object. This means... Electric potential Electric potential refers to the build up of charge. The greater the charge differences, the greater the potential for danger. Damage, however, will only happen when Induced Charge Separation Induced charge separation refers to the influence another object can have on a conductor. As a charged object is brought near a conductor, the charges will Medium Definition the surrounding environment, substance. Examples Conductors vs. Insulators The bonding nature (structure) of a material affects its ability to be influenced by a charge. A conductor a charge (electrons) to move within the material rather easily where an insulator restricts any internal movement of charge (electrons). An insulator the movement of particles including electrons. < astr.gsu.edu/hbase/electric/conins.html> Humidity Humidity is a measure of the amount of in the air. Water is a better then air and Lightning is more likely to happen when the humidity is high or raining. Static electricity is a greater concern in weather because the water in the air is likely to grab onto free electrons. Since protons are fixed in the of an atom, electrons are the charge which become influenced and. But, because charges obey the law of opposites, we can analyze charges through the movement of either positive or negative charges, ignoring their connection to the atom. 2

3 The choice of which type of electricity is called "positive" and which "negative" was made around 1750 by Ben Franklin, early American scientist and man of many talents (the stamp on the left commemorates his role as first US postmaster and colonial postmaster before that). Franklin studied static electricity, produced by rubbing glass, amber, sulfur etc. with fur or dry cloth. Among his many discoveries was proof that lightning was a discharge of electricity, by the foolhardy experiment (he claimed) of flying a kite in a thunderstorm. The kite string produced large sparks but luckily no lightning, which could have killed Franklin. Franklin knew of two types of electric charge, depending on the material one rubbed. He thought that one kind signified a little excess of the "electric fluid" over the usual amount, and he called that "positive" electricity (marked by +), while the other kind was "negative" (marked ), signifying a slight deficiency. It is not known whether he tossed a coin before deciding to call the kind produced by rubbing glass "positive" and the other "resinous" type "negative" (rather than the other way around), but he might just as well have. Later, when electric batteries were discovered, scientists naturally assigned the direction of the flow of current to be from (+) to ( ). A century after that, electrons were discovered and it was suddenly realized that in metal wires the electrons were the ones that carried the current, moving in exactly the opposite direction. Also, it was an excess of electrons which produced a negative electric charge. However, it was much too late to change Franklin's naming convention Charging an object 1. Friction Whenever two different objects are rubbed together, will occur due to their electron affinity. Electron affinity refers to how much hold a substance has on its electrons. More information can be found at < including the triboelectric series 2. Arcing/Contact If a charged object is brought near another charged object or a conductor, electrons might through space due to their attraction. This is called arcing. is an example of an arc a large transfer of charge. 3. Induced Charge Separation / Grounding / Charging by grounding As the diagram indicates the following explains another method of charging an object In the first picture, the charges are distributed. When a negative rod is brought near, the charges realign called If the neutrally charged ball (same number of positive and negatively charged objects despite the new alignment) is then connected to or the, the charge will flow to even out the local excess of charge. If the ground is removed, the once neutrally charged ball is charged. Note: the Earth is considered a super large neutral with the ability to absorb or give millions and millions of. 3

4 More on the electron the Elementary Charge In 1896 Joseph John Thomson discovered the electron. However he didn't manage to define its mass neither its charge. He only calculated the proportion of the charge to the mass of the electron (q/m). He believed that the newly discovered particle was a part of an atom. But he didn't give enough evidence for that hypothesis. Defining the charge and the mass could be the final proof. Many scientists tried to solve this problem. The first who managed to measure what was the elementary charge was one of Thomson's students J.S. Townsend. Unfortunately the final result he got had a big measurement error. The first scientist who quite precisely defined that charge was Robert Andrews Millikana. It must be said that he didn't measure the charge of the electron. He only showed that there is some indivisible amount of charge in nature the elementary charge. All other charges are exact multiples of that elementary one. Later it emerged that the elementary charge was the the charge of the electron. What does the word elementary charge mean to you? Millikan and the Oil Drop Experiment Robert Millikan hypothesized that all charges were based on some multiple of an elementary charge the electron. His claim was verified by his famous oil drop experiment in August For more reading see < Q stands for the charge units are Coulombs N stands for a the number of electrons in excess or deficit e stands for the charge of the elementary charge the electron 4

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6 Summary of Equations from grade 11 Formula Measures Concept Q=Ne I = Q/Δt V = E/Q W = ΔE P = VI Q amount of excess(deficit) charge in Coulombs C N the number of excess (deficit) electrons no unit e the charge of a single electron 1.6 x C I current in Amperes A Q charge in Coulombs C Δt change in time in seconds s V potential difference, electric potential or more commonly known as voltage in volts V E energy in Joules J Q charge in Coulombs C W work in Joules J ΔE change in energy in Joules J P power in Watts W V Voltage in Volts V I Current in Amps A All charges, as argued by Millikan are multiples of the elementary charge. All electrons have the same charge and the number in excess or deficit result in the total charge on an object. Current measures how much charge is moving in a circuit. Specifically, it measures how many electrons move past a given point in a second. Voltage is a measure of how much energy each unit of charge possess, energy that can do work or be transferred to another object When work is done on an object, the energy of the object has been changed. Power the rate at which work is done. It is one thing to do work but the speed at which it is done has an implication on many devices which use this energy. 6

7 Practice Problems on Electricity equations 1. What is the potential difference of a battery if it does 7.50 x 10 2 J of work when it moves 3.75 x 10 3 C of charge onto the anode? 2. A 9.00 V battery causes a charge of 4.20 x 10 2 C to move through a circuit. Calculate the work done on the charge. 3. A 12 V battery does 0.75 J of work on a quantity of charge it moved through a circuit. Calculate the amount of charge that was moved 4. A battery sends a 2.25 A current through a circuit for 1.50 min. If a total of 8.10 x 10 2 J of work was done by the current, what was the potential difference of the battery? 5. How long would it take a 17 V battery, sending a 5.0 A current through a circuit, to do 680 J of work? 6. How much work is done by a 25.0 V battery when it drives a 4.70 A current through a circuit for 36.0 s? 7. If a 160 V battery did 9.6 x 10 5 J of work in 2 min, what was the current? 8. A light draws a current of 0.48 A. How long must it be left on for charge of 36 C to pass through it? 9. An electric circuit draws 20 A. If the electric potential drop over the entire circuit is 120V, calculate the total charge passing through the circuit in 1h. 10. A cellular phone battery is recharged in 0.25 h after receiving 2.5 x 10 3 C of charge. Calculate the amount of electric current that the battery draws during recharging? 11. A physics student wishes to determine the amount of electric energy consumed in one day at his school as a result of classroom and hallway lighting. A quick survey revealed that there were approximately 200, 40W fluorescent lights operating under a potential difference of 240 V for 16 hours each day. 12. Calculate the current if 2.85 x elementary charges pass a point in a circuit in 5.70 min. 13. A 16.0 V battery does 5.40 x 10 4 J of work in s. a.calculate the current through the battery. b.calculate the number of elementary charges that pass through the battery. 14. Calculate the number of elementary charges that pass a point in a circuit when a current of 3.50 A flows for 24.0 s. 15. In transferring 2.5 x elementary charges in 12 s, a battery does 68 J of work. a.calculate the current through the battery. b.calculate the potential difference of the battery V J x 10 2 C V s x10 3 J 7. 50A 8. 75s 9. 7 x 10 4 C A x 10 8 J A 13.(a) 9.38 A (b) 2.11 x elementary charges x elementary charges 15. (a) 3.3 A (b) 1.7 V 7