Electric Fields

Question 1

what is static electricity?

Answer 1

an electric charge within a material or on its surface which remains stationary until it is discharged

Question 2

how can the force between two charged spheres be measured?

Answer 2

• negatively charged rod brought near sphere, electrons move through metal away from rod, sphere momentarily earthed, electrons flow to earth leaving a net positive charge on sphere
• after charging both spheres, force between spheres determined at different separations by converting mass reading to weight
• charge on each sphere measured withs coulomb meter
• F = Q(1)Q(2) /4piE0r^2

Question 3

define coulomb’s law

Answer 3

force between two POINT charges is directly proportional to the product of the two charges + inversely proportional to the square of their separation

Question 4

define permittivity of free space

Answer 4

charge per unit area on oppositely charges parallel plates in a vacuum when electric field strength between plates is 1 V/m

Question 5

define electric field

Answer 5

region around a charged object or particle within which a force would be exerted on other charged particles or objects

Question 6

define electric field strength

Answer 6

force per unit charge on a positive test charge placed at that point

Question 7

define electric potential difference

Answer 7

work done per unit charge in moving a charge between two points

Question 8

describe and explain the path of a moving charge in an electric field

Answer 8

• vertical force on electron F=Ee
• vertical acc. on electron a=Ee/m
• if electron travels distance x at speed v, t=x/v
• vertical displacement y=ut+1/2at^2, y=1/2at^2
• substitute for a and t: y=Eex^2 / 2mv^2
• all constant except x and y therefore y=kx^2
• therefore direction = vertical parabola

Question 9

define electric potential

Answer 9

work done per unit charge in bringing a small positive charge from infinity to that point

Question 10

how would the total potential at a point be calculated if two charged spheres close to point?

Answer 10

electric potential is scalar, therefore add potential of both

Question 11

why are metal objects equipotential surfaces?

Answer 11

charge on the surface will always redistribute as free electrons in conducting material will flow go cancel imbalance, therefore potential same over whole surface