Physics Electrostatics: Coulomb s Law

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Elaine Horn
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1 A C U L T Y O E D U C A T I O N Depatment of Cuiculum and Pedagogy Physics Electostatics: Coulomb s Law Science and Mathematics Education Reseach Goup Suppoted by UBC Teaching and Leaning Enhancement und 0-03

2 Coulomb s Law e k +

3 Coulomb s Law I Conside two opposite chages, and a distance apat. Accoding to Coulomb s law, the magnitude of the foce between the two chages is: e k If is doubled, the magnitude of the foce will: A. Decease by a facto of 4 B. Decease by a facto of C. Remain the same D. Incease by a facto of E. Incease by a facto of 4 +

4 Solution Answe: D Justification: The electic foce one chage exets on anothe is diectly popotional to the poduct of the two chages, and thus is also popotional to each individual chage. If the magnitude of one chage is doubled then the magnitude of the foce is also doubled. The foce is not popotional to the suae of the chage, so the magnitude of the foce will not change by a facto of fou. k k k

5 Coulomb s Law II Conside two opposite chages, and a distance apat. Accoding to Coulomb s law, the magnitude of the foce between the two chages is: e k If is doubled, the magnitude of the foce will: A. Decease by a facto of 4 B. Decease by a facto of C. Remain the same D. Incease by a facto of E. Incease by a facto of 4 +

6 Solution Answe: A Justification: The electic foce is popotional to the invese suae of the distance between the chages. If the distance between the chages double, the magnitude of the foce deceases by a facto of 4. As the distance inceases, the suae inceases. Dividing by a lage numbe gives a smalle answe. Similaly, if the distance deceases, the suae deceases. Dividing by a smalle answe gives a lage answe. k k () k 4 4

7 Coulomb s Law III Conside two opposite chages, and a distance apat. Accoding to Coulomb s law, the magnitude of the foce between the two chages is: e k If,, and ae all doubled, the magnitude of the foce will: A. Decease by a facto of 4 B. Decease by a facto of C. Remain the same D. Incease by a facto of E. Incease by a facto of 4 +

8 Solution Answe: C Justification: Doubling one chage inceases the foce by a facto of, so doubling both chages inceases the foce by a facto of 4. Doubling distance deceases the foce by a facto of 4. The incease caused by the doubled chage is negated by the decease caused by the doubled distance. 4 4 ) ( k k k

9 Coulomb s Law IV Conside two unifomly chaged sphees a small distance apat. Sphee has a +3 chage while sphee has a - chage. Which of the following diagams coectly shows the magnitude and diection of the electostatic foces? A. B. on 3 - on 3 - on on C. D. on on on on

10 Solution Answe: C Justification: Opposite chages attact, like chages epel. This discounts D, whee the chages push each othe apat. Newton s Thid Law states that fo evey action thee must be an eual and opposite eaction. This means that the foce that chage exets on chage has the same magnitude as the foce that chage exets on chage, but acts in the opposite diection. Altenatively, the euation fo Coulomb s Law does not change when we conside the foce of chage on chage o vice vesa. 3 - on on

11 Coulomb s Law V Thee chages with eual magnitudes ae aanged hoizontally and spaced evenly apat as shown: Q Q Which of the following fee-body diagams best epesents the electic foces acting on Q 3? Q A. on 3 on 3 B. C. on 3 on 3 on 3 on 3 D. E. on 3 on 3 on 3 on 3

12 Solution Answe: E Justification: Both Q and Q ae positive, while Q 3 is negative. Opposite chages attact, so Q 3 is attacted to the othe two chages and expeiences a net foce to the left. Both Q and Q have the same chage,, so the magnitude of the foces they each exet on Q 3 will depend on the distance of each chage fom Q 3. The distance between Q and Q 3 is twice the distance between Q and Q 3. The foce one chage exets on anothe is invesely popotional to the suae of the distance between the two chages. Q is twice as fa as Q fom Q 3, and theefoe, on 3 must be fou times smalle than on 3. on 3 on 3

13 Coulomb s Law VI Thee chages with eual magnitudes ae aanged hoizontally and spaced evenly apat as shown: Q Q Which of the following fee-body diagams best epesents the electic foces acting on Q? A. B. C. on on on Q 3 on 3 on 3 on D. E. on on 3 on 3 on

14 Solution Answe: A Justification: Chages Q and Q have the same sign, so they epel each othe. Q is pushing Q away. Q and Q 3 ae of opposite sign, and will attact each othe. Q 3 is pulling Q close. The distance between Q and Q 3 is twice the distance between Q and Q 3. The foce is popotional to the invese of the distance, so 3 on must be smalle than on by a facto of 4. A. on 3 on

Electricity Revision ELECTRICITY REVISION KEY CONCEPTS TERMINOLOGY & DEFINITION. Physical Sciences X-Sheets

Electricity Revision ELECTRICITY REVISION KEY CONCEPTS TERMINOLOGY & DEFINITION. Physical Sciences X-Sheets Electicity Revision KEY CONCEPTS In this session we will focus on the following: Stating and apply Coulomb s Law. Defining electical field stength and applying the deived equations. Dawing electical field