Electric Fields - Coulomb's Law, strength and types

Question 1

What is a field?

Answer 1

A field is a region of space where an object experiences a force.

Question 2

What is an electric field?

Answer 2

An electric field is a region of space where a charged object experiences a force.

Question 3

What are the three types of fields?

Answer 3

• Electric
• Gravitational
• Magnetic

Question 4

What does the ‘r’ stand for in Coulomb’s Law?

Answer 4

It is the radius from the centre of the field.

Question 5

What can we say about the force in Coulomb’s Law Equation?

Answer 5

• it is directly proportional to the charge of each particle
• it is inversely proportional to the square of the distance between the particles

Question 6

What is the charge of a proton?

Answer 6

1.60 x 10 ^-19 C

Question 7

What does the epsilon (Ɛ) in Coulomb’s Law Equation represent?

Answer 7

Ɛ = permittivity of free space

—– basically what is between the charges

Question 8

Two balloons are charged with an identical quantity and type of charge: -6.25 nC.
They are held apart at a separation distance of 61.7 cm.
Determine the magnitude of the electrical force of repulsion between them.

Answer 8

F = 9.22 x 10 ^-7 N

Question 9

Two balloons with charges of +3.37 µC and -8.21 µC attract each other with a force of 0.0626 N.
Determine the separation distance between the two balloons.

Answer 9

r = 1.99 m

Question 10

Which equation should you use to calculate the electric field strength of a RADIAL field (around a point charge)?

Answer 10

E = k Q/r^2

Question 11

Which equation should you use to calculate the electric field strength of a UNIFORM field?

Answer 11

E = V/d

Question 12

What two units are equivalent and can be used for Electric Field Strength?

Answer 12

NC^-1 = Vm^-1

Question 13

Electric Field strength is a …. .

Answer 13

Vector

Question 14

What direction would the arrows be if an electric field has a positive charge?

Answer 14

Outwards, as conventionally it is if a positive charge is placed in the field.

Question 15

What direction would the arrows be if an electric field has a negative charge?

Answer 15

Inwards, as conventionally it is if a positive charge is placed in the field.

Question 16

During a lightning strike, the Earth’s electric field might be 3000 NC^-1.
Calculate the p.d. between two points on the lightning strike that are 20 m apart.

Answer 16

V = 60,000 V

Question 17

In the electron beam of a cathode ray oscilloscope, electrons are accelerated through a potential difference of 3000 V, which is set up between parallel plate electrodes that are 3 cm apart.

a) Calculate the electric field strength between these electrodes, assuming it is a uniform field.
b) How much faster will the electrons be moving when they emerge from this field?
c) Explain how the field’s effect would be different if a proton were placed in it.

Answer 17

a) E = 100,000 V m^-1
b) F = -1.6 x 10^14 … a = 1.7563… x10^16 ms^-1 … v = 3.25 x 10^7 ms^-1
c) - Electric Field Strength will remain constant
- The magnitude of the force will remain constant too
- The acceleration will be lower though, due to the mass of the proton being larger

Question 18

Neutrally charged objects always …

Answer 18

… attract charged objects.

Question 19

Equipotential lines are …

Answer 19

… link points of the same potential.

Question 20

What does the density of field lines show?

Answer 20

Strength of field

Question 21

Uniform field has a …

Answer 21

… constant strength of field throughout

Question 22

Field lines are …

Answer 22

… lines showing the direction of the field in relation to a positive charge.

Question 23

As you go further from the centre of the charge …

Answer 23

… the field’s strength decreases (due to the inverse square law).

Question 24

4 things to remember when sketching Radial Fields;

Answer 24

• Use a ruler
• Make all lines equal in length (by eye)
• Make all lines equally spaced (by eye)
• Use arrows to show the direction a positive charge would move.

Question 25

a) What force will an electron feel when it is in an X-ray machine’s electron field which has a range of 4.5 x 10^5 Vm^-1?
b) How fast will the electron be travelling if this field accelerates it from rest and it is within the field for a distance of 10 cm?

Answer 25

a) F = -7.2 x 10^-14 N

b) v = 1.26 x 10^8 ms^-1