ELECTROSTATICS - GRADE SUTHERLAND HIGH SCHOOL

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Basil Porter
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1 ELECTROSTATICS - GRADE SUTHERLAND HIGH SCHOOL

GRADE 10 REVISION CHARGE CAN BE POSITIVE

OBJECTS ARE CHARGED BY Friction, contact or

2 GRADE 10 REVISION CHARGE CAN BE POSITIVE Object has a shortage of electrons. CHARGE CAN BE NEGATIVE Object has a surplus of electrons. OBJECTS ARE CHARGED BY Friction, contact or induction. ELECTROSTATIC FORCE Attraction unlike charges. Repulsion like charges.

3 UNIT OF CHARGE COULOMB C C mc μc nc pc C C

4 FORCES BETWEEN CHARGES Charged objects exert electrostatic or Columbic forces on each other. All charges are considered point charges. (A very small charged particle.) This means that the volume and surface area (size) of the charged particle is negligible. We assumed that the charge is concentrated at the midpoint of the sphere and we treat that point as a little or point charge.

5 FORCES BETWEEN CHARGES

6 The strength of the force Is determined by the: A B

7 MAGNITUDE OF THE CHARGES ON THE CHARGED PARTICLES The relationship between the electrostatic force between the objects and the charge on each object is DIRECTLY PROPORTIONAL. If the charge on one of the objects increases, the electrostatic force will increase. If the charge on one of the objects decreases, the electrostatic force will decrease. F Q 1 Q 2

THE DISTANCE BETWEEN THE MAGNITUDE OF THE CHARGES ON MIDPOINTS THE CHARGED OF THE CHARGED PARTICLES PARTICLES The relationship between the electrostatic force between the objects and the distance

8 THE DISTANCE BETWEEN THE MAGNITUDE OF THE CHARGES ON MIDPOINTS THE CHARGED OF THE CHARGED PARTICLES PARTICLES The relationship between the electrostatic force between the objects and the distance between the charges is INVERSELY PROPORTIONAL. If the distance between the objects increases, the electrostatic force will decrease. If the distance between the objects decreases, the electrostatic force will increase. F 1 r 2

9 NICE TO KNOW Charge of an electron: C Charge of a proton: C Charge of an alpha particle / helium nucleus: C

10 Mrs Harrison being extra What are alpha rays? How are they produced? Alpha "rays" are actually high speed particles. Early researchers tended to refer to any form of energetic radiation as rays, and the term is still used. An alpha particle is made up of two protons and two neutrons, all held together by the same strong nuclear force that binds the nucleus of any atom. In fact, an alpha particle really is a nucleus - it's the same as the nucleus of a common atom of helium - but it doesn't have any electrons around it, and it's traveling very fast. Alpha particles are a type of ionizing radiation. To describe the production of alpha particles, we have to define radioactive decay. This process can be thought of as follows. Certain combinations of neutrons and protons in a nucleus are stable. For example, in a stable bismuth atom there are 83 protons and 126 neutrons. This is called bismuth-209 ( = 209). It will always be bismuth-209 *. But if we were to add one more neutron to this atom, and make it bismuth-210, it would now be unstable, or radioactive. The atom will eventually spontaneously change or "decay", to become more stable. There are only certain ways it can do this. One way is to emit an alpha particle. In this transition, it spits out a piece of itself (the alpha particle), and becomes more stable. The alpha particle is the radiation given off during the process of "alpha decay". Since it lost two protons and two neutrons, the old bismuth atom is now an atom of thallium Now, this thallium is more stable, but is also radioactive. It will decay again (but not by alpha decay), this time becoming a completely stable atom of lead. Only relatively "heavy" atoms - like bismuth - can go through alpha decay. Lighter radioactive elements go through other types of transitions to become stable. There are plenty of these radioactive materials naturally present on the Earth, which is how these radiations were discovered.

11 Mrs Harrison being extra What is the Alpha Particle? Rutherford No one contributed more than Ernest Rutherford ( ; see photo in Chemical Achievers at Chemical Heritage Foundation) to an understanding of radioactivity and its domain, the atomic nucleus, in the early years of the 20 th century. Working in J. J. Thomson's laboratory, Rutherford distinguished between two different forms of radioactivity, alpha (α) and beta (β) [Rutherford 1899]. He worked out the timedependence of radioactive decay and introduced the term half-life [Rutherford 1900]. With Frederick Soddy, he realized that radioactive decay actually tranforms an atom of one element into an atom of a different element [Rutherford & Soddy 1902].[1]This work was sufficient to earn Rutherford a Nobel Prize in 1908.[2] But some of his most important work was still ahead. Large-angle scattering of α particles was first reported in his laboratory [Geiger & Marsden 1909]. His correct interpretation of that scattering led to the realization that most of the mass of an atom is concentrated in a tiny core or nucleus [Rutherford 1911]; thus it is to Rutherford that we owe the nuclear atom and nuclear physics. The experiments which found a physically measurable quantity associated with atomic number were also carried out in his laboratory [Moseley 1913, 1914]. During the World War, Rutherford discovered that some atoms could be induced to fall apart in a process of artificial transmutation [Rutherford 1919; view photo of apparatus at Science Museum, London]. Rutherford characterized the α particle in work extending over several years with a variety of co-workers. The selection reproduced below represents the final step in the identification of the α particle as a positively-charged helium atom. (I would say "helium nucleus," but Rutherford had not yet discovered the nucleus.) In importance to the development of understanding about the atom, this paper does not rank with those cited above. It does, however, illustrate the simple and careful experimental methodology of Rutherford.

12 You have 6 seconds to answer each question using: F 1 r 2 F Q 1 Q 2 Two objects, 1 and 2, with charges Q 1 and Q 2 at a distance r apart, exert a force F on one another. The charge on one of the objects doubles. What happens to the electrostatic force?

13 You have 6 seconds to answer each question using: F 1 r 2 F Q 1 Q 2 The charge on object 1 is now three times Q 1, and on object 2 is now four times Q 2. What would the electrostatic force be now?

14 You have 6 seconds to answer each question using: F 1 r 2 F Q 1 Q 2 When two point charges, Q 1 and Q 2, are a distance r apart, the electrostatic force between them is F. The distance between them is increased by a factor of 5. By what factor does the magnitude of F change?

COULOMB S LAW NOTES FROM LAST YEAR!!! Direction: The Repulsion electrostatic same force charges. between two charged particles is directly Attraction proportional opposite to the charges.

15 COULOMB S LAW NOTES FROM LAST YEAR!!! Direction: The Repulsion electrostatic same force charges. between two charged particles is directly Attraction proportional opposite to the charges. product of the charges on the particles Sign of charges and inversely (+ or -) proportional can be left to out the when square of the substituting distance but used between when the determining charges. direction. Electrostatic force (N) F = kq 1Q 2 r 2 Coulomb s constant ( N. m 2. C 2 ) Point charges(c) Distance between centres of charges (m)

16 EXAMPLE Calculate the electrostatic force that two charges of +3μC and 8μC exert on each other when they are a distance of 30mm apart.

17 EXERCISE 15 Pg

18 When 2 or more point charges exert forces on each other simultaneously, the net force can be calculated using the vector sum of the forces. In one dimension: Calculating the net force on K. J K L r 1 r 2 F net = F J on K + (F L on K )

19 EXAMPLE Determine the net electrostatic force on K if: +6μC J r K L 1 = 30mm r 2 = 60mm 4μC 8μC What is the electrostatic force that K exerts on J? (1) Explain. Draw and label a free body diagram for each of the forces that act on K.

20 When 2 or more point charges exert forces on each other simultaneously, the net force can be calculated using the vector sum of the forces. In two dimensions: Calculating the net force on S. (F net ) 2 = (F R on S ) 2 + (F P on S ) 2 tanθ = F P on S F R on S R r 1 P S r 2

21 EXAMPLE Determine the net electrostatic force on S if: 4μC P r 1 = 30mm 6μC R r 2 = 60mm S +8μC

22 EXERCISE 16 Pg

23 ELECTRIC FIELDS An electric field is an area where a charged object will experience an electrostatic force. The direction of the field line at a point, is the direction in which a positive point charge would accelerate, when placed at that point.

24 PROPERTIES OF ELECTRIC FIELD LINES. NEVER CROSS OR TOUCH EACH OTHER ARE DIRECTED AWAY FROM A POSITIVE CHARGE AND TOWARDS A NEGATIVE CHARGE ARE IMAGINARY STRONG LINES CLOSE TOGETHER WEAK LINES FAR APART EXISTS 3D AROUND A POINT CHARGE OR OBJECT PERPINDICULAR TO THE SURFACE OF THE CONDUCTOR

25 ELECTRIC FIELDS PATTERNS

26 ELECTRIC FIELD STRENGTH (E) The electric field strength (E) at a point is the force experienced per unit charge at that point in the field. Force (N) Electric field (V. m 1 ) Voltage (V) Constant E = F Q E = V d E = kq r 2 charge(c) Electric field (N. C 1 ) Charge (C) Distance between charges (m) Distance (m) Electric field (N. C 1 )

27 E = kq r 2 E Q E 1 r 2

28 EXAMPLE Calculate the electric field strength at a distance 5cm from a 5pC charge.

29 EXAMPLE Point P is between two charges of + 7μC and 3μCand as shown in the sketch. Calculate the resultant electric field at point P. Choose left as positive. +7μC +3μC mm 20mm

30 EXERCISE 17 Pg

Physics Electrostatics

Physics Electrostatics Homework Procedure: Read pages specified in Honors Physics Essentials by Dan Fullerton. Questions labeled TQ will be questions about the text you read. These TQ s can be answered in one word, one phrase,