electric fields

Question 1

what is Coulomb’s Law

Answer 1

• states that the magnitude of the force between two point charges in a vacuum is directly proportional to the product of their charges
• inversely proportional to the square of the distance between the charges

Question 2

what is the equation for Coulomb’s Law

Answer 2

F = k Q1Q2/r^2

k = 1/4piEo

Eo - permittivity of free space

charge may be assumed to act at the centre of the sphere

Question 3

how do we know the nature of the force?

Answer 3

• if charges have same sign then repulsive force
• if charges have different signs then attractive force

Question 4

compare the magnitude of electrostatic forces between magnitude of gravitational forces

Answer 4

• the magnitude of e forces are larger than magnitude of g forces
• masses of the subatomic particles are incredibly small whereas their charges are much larger

Question 5

what is electric field strength?

Answer 5

• force per unit charge experienced by an object in an electric field

Question 6

how can we calculate the electric field strength?

Answer 6

E = F/Q

E = V/d

E = k Q / r^2

k = 1/4pi(8.85x10^-12)

Question 7

whats the main difference between uniform and radial fields?

Answer 7

• uniform foelds exerts the same electric force everywhere in the field
• radial fields, the magnitude of the electric force depends on the distance between the two charges

Question 8

how can we calculate the work done by moving a charged particle between parallel plates

Answer 8

wd = Fd
F = EQ
d = DELTA V / E

work done = Q DELTA V

Question 9

how can we use a uniform electric field to find out wether a particle is charged?

Answer 9

• fire the particle at right angles to the field and observe its path
• charged particle will experience an electric force, causing it to accelerate and follow a PARABOLIC SHAPE
• if the charge is negative : opposite direction to field
• if the charge is positive : follows direction of field

Question 10

what is absolute electric potential

Answer 10

• potential energy per unit charge of a positive point charge at that point in the field

Question 11

where is the absolute magnitude of the electric potential the greatest?

Answer 11

• at the surface of the charge

Question 12

why is electric potential at infinity zero?

Answer 12

• because the magnitude of the electric potential at the surface is the greatest
• as the distance from the charge increases, the potential decreases

Question 13

whats the equation for value of potential in a radial field?

Answer 13

V = k Q / r

where k is 1/4piEo

Question 14

whats the equation for value of potential in a radial field?

Answer 14

V = k Q / r

where k is 1/4piEo

Question 15

how do we know if the value of potential is negative or positive

Answer 15

• when the charge is positive, potential is positive and charge is repulsive
• when charge is negative, potential is negative and force is attractive

Question 16

what does the gradient of a tangent to a V - r graph give?

Answer 16

value of electric field strength at that point

Question 17

what is electric potential difference?

Answer 17

energy needed to move a unit charge between two points

Question 18

on a graph of electric field strength against distance what does the area give?

Answer 18

• electric potential difference

Question 19

in an electric field, where do the arrows go from?

Answer 19

positive charge to negative charge

Question 20

what direction will a positive test charge experience a force EQ in an electric field?

Answer 20

• direction of the field as positively charged

Question 21

what direction will a negative charge experience a force EQ in an electric field?

Answer 21

• opposite direction to the field lines, as negatively charges

Question 22

what does the amount of deflection depend on in an electric field?

Answer 22

mass - greater mass less deflection

charge - greater charge greater deflection

speed - greater speed less deflection

Question 23

explain why potential increaases as the seperation decreases for a positive point charge?

Answer 23

• energy must be supplied to a positive test charge to overcome the repulsive force

Question 24

explain why potential decreases as seperation increases for a negative point charge

Answer 24

-energy is released as a positive test charge moves in the direction of the attractive force

Question 25

how do we represent field lines around a positive point charge

Answer 25

- lines coming out at right angles to the charge - direction of lines are from surface outwards

Question 26

how do we find the resultant electric field strength?

Answer 26

same direction - E1 + E2 opposite direction = E1 - E2 90* angle = pythag

Question 27

what is electric potential

Answer 27

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