Module 6: Chapter 22 - Electric Fields

Question 1

What is an electric field?

Answer 1

The area around an object in which a charge would experience a force

Question 2

What is the electric field strength?

Answer 2

The force per unit positive charge

Question 3

What is the equation for the force acting upon an object within an electric field?

Answer 3

F = EQ
Force = Electric Field Strength x Charge

Question 4

What are the rules for drawing electric field lines?

Answer 4

• The arrow on an electric field shows the direction of the field
• Electric field lines are always at right angles to the surface of a conductor
• Closer electeric field lines represent greater electric field strength

Question 5

What does a uniform electric field look like?

Answer 5

Equally spaced, parallel electric field lines

Question 6

In what direction do electric field lines go?

Answer 6

• Away from a positive charge
• Towards a negative charge

Question 7

Draw a diagram showing the electric field between 2 positive charges

Answer 7

Question 8

Draw a diagram showing the electric field between 2 negative charges

Answer 8

Question 9

Draw a diagram showing the electric field between a positive and a negative charge

Answer 9

Question 10

Draw a diagram showing the electric field around a positive bar

Answer 10

Question 11

Draw a diagram showing the electric field between a positive and a negative plate

Answer 11

Field in the centre is a uniform field

Question 12

Draw a diagram showing the electric field between a positive sphere and a negative bar

Answer 12

Question 13

A proton is accelerated from rest by a uniform electric field of field strength 2x10⁵ NC⁻¹. Mass of proton = 1.7x10⁻²⁷ kg. What is the time is takes to travel 5cm?

Answer 13

7.3x10⁻⁸s

Question 14

What is coulombs law?

Answer 14

The force between 2 electric charges Q and q is proportional to Qq and inversely proportional to the square of their separation r

Question 15

What are the 2 equations for the force between 2 charges?

(coulombs law)

Answer 15

F = kQq/r²

F = qQ/4πεₒr²

Question 16

What is the value of the constant of proportiobality (k) in coulombs law?

Answer 16

k = 8.99x10⁹

Question 17

What is the equation for the constant of proportionality (k) in coulombs law?

Answer 17

k = 1/4πεₒ

Question 18

What is εₒ?

Answer 18

The permittivity of free space

Question 19

What is the value of εₒ (permittivity of free space)?

Answer 19

8.85x10⁻¹²

Question 20

What is the equation for electric field strength with varying distance?

Answer 20

E = Q/4πεₒr²

Question 21

What is the major difference between electric and gravitational fields?

Answer 21

• Gravitational fields ALWAYS attract
• Electric fields can both attract and repel

Question 22

What is the property that creates gravitational fields?

Answer 22

Mass

Question 23

What is the property that creates electric fields?

Answer 23

Charge

Question 24

What is the equation for the uniform electric field between 2 parallel charged plates?

Answer 24

E = V/d

Question 25

Explain why the electric field between 2 parallel plates is uniform:

Answer 25

A charge will experience the same force wherever it is placed between the plates

Question 26

What are the units for electric field strength?

Answer 26

NC⁻¹ or Vm⁻¹

Question 27

What is the equation for the capacitance on a parallel plate capacitor?

Answer 27

C = εA/d d = separation between the plates | A = area of overlap of the plates ## Footnote ε = εₒεᵣ ε = permittivity of free space x relative permittivity

Question 28

How was it discovered that charge is quantized?

Answer 28

Millikan's oil drop experiment

Question 29

Explain Millikan's oil drop experiment

Answer 29

A mist of oil droplets are sprayed inside the chamber using an atomizer. The friction experienced by the oil droplets as they are sprayed caused some of them to become charged. The oil droplets then fell down due to gravity and some of them passed through the gap in the anode. A potential difference was then applied, causing the electric field to turn on. By carefully adjusting this potential difference they could cause an oil droplet to be suspended stationary in mid air. From this the charge on the oil droplet could be calculated: Force from electric field = Weight EQ = mg Q = mg/E Q = ρvg/E The volume could be determined as the drops were modelled as perfect spheres (and the radius could be determined by measuring the terminal velocity of the drop as it fell without an electric field. Therefore the drag is equal to the weight and can be used to determine r) **This was repeated and it was found they were all multiples of the lowest charge found on any droplet, this was the elementary charge**

Question 30

What is the equation for the work done on a charge within an **uniform** electric field?

Answer 30

W = Vq Work done = Potential Difference x Charge

Question 31

Calculate the velocity an electron is accelerated to by a potential difference of 1.5V

Answer 31

730 kms⁻¹

Question 32

What is the mass of a proton?

Answer 32

1.673x10⁻²⁷kg

Question 33

How do you predict the motion of a charged particle travelling at a right angle to a uniform electric field?

Answer 33

Analyse the vertical and horizontal components of the motion independently

Question 34

How far will the electron "fall" from its path while in-between the 2 plates?

Answer 34

4.11x10⁻³m

Question 35

Explain Electric potential energy

Answer 35

If 2 attracting charges were put in an electric field, side by side, and then a force was applied on the negative charge in order to separate them, the work done to do this separation would be the electric potential energy difference (measured in joules) between the 2 charges in their new location

Question 36

How can you calculate the electric potential energy from a force-separation graph?

Answer 36

area under the curve

Question 37

What is the equation for electric potential energy (difference)?

Answer 37

E = Qq/4πεₒr

Question 38

Find the work done to move a charge from position 1 to position 2

Answer 38

0.899J

Question 39

Calculate the electrical potential energy within a hydrogen atom if the electron is orbiting at a distance of 5.29x10⁻¹²m (in Electron Volts)

Answer 39

272eV

Question 40

What is electrical potential?

Answer 40

The work done to bring a unit **positive** charge from infinity to a point

Question 41

What is the equation for electrical potential?

Answer 41

V = Q/4πεₒr

Question 42

What is the electrical potential in the field of a 5nC point charge at a distance of 0.75m from the charge?

Answer 42

59.9V

Question 43

What is the equation for the capacitance of a charged sphere?

Answer 43

C = 4πεₒr

Question 44

What is the equation for capacitance?

Answer 44

C = Q/V

Question 45

What are equipotential lines?

Answer 45

Lines that link points of equal electric potential in an electric field

Question 46

What is the link between electric field lines and equipotential lines?

Answer 46

They are perpendicular to one another

Question 47

What are similarities between a graviational and an electric field?

Answer 47

* Both produce a radial field * Field strength ∝ 1/r²

Question 48

What happens to the angle of deflection if the distance between the moving charge and stationary charge increases?

Answer 48

The angle of deflection decreases