Module 6 Electric Fields

Question 1

State Coulombs law in words

Answer 1

The force between two charges is proportional to the product of the charges and inversely proportional to the square of their separation

Question 2

For Coulombs law, how do you know the sign of the force calculated?

Answer 2

Positive is repulsive
Negative is attractive
Put actual charge ± in the equations

Question 3

If a known mass has a charge and is on a string at an angle and attracted by a nearby charge, how can you calculate the tension if both charges are known? (mass at rest)

Answer 3

set up a triangle of forces
F is Coulomb force
W=mg
Use Pythagoras and trigonometry

Question 4

Equation for electrostatic or Coulomb force between two charges?

Answer 4

Question 5

In an electric field diagram, which direction do field lines point?

Answer 5

in the direction which a POSITIVE test charge will move

Question 6

What is an electric dipole?

Answer 6

diagram of electric field lines between two equal and opposite charged particles

Question 7

Draw a simple diagram for the electric field around a dipole

Answer 7

Question 8

Draw electric field around a point positive charge

Answer 8

Radial field
Electric fields line spherically symmetrical
Arrows away from charge as positive test charge would be repelled

Question 9

Draw a simple diagram for the electric field between a positive point charge and a metal plate

Answer 9

Modified radial field .
Field lines distorted by plate and meet the plate at right angles

Question 10

Describe and explain a diagram for a positive point charge and a metal plate

Answer 10

Metal plate has many free electrons which are attracted to the positive charge

Side closer to the charge becomes negatively charged

Field lines move from the charge and meet the plate at 90º

Question 11

Features of electric field diagrams

Answer 11

arrow to show the direction of the field = direction that a positive charge will move

The density of the field lines gives an indication of the field strength.

Question 12

Define electric field strength

Answer 12

Electrostatic force per unit positive charge

Question 13

Which direction is the electric field in?

Answer 13

same direction as the direction a positive charge will move

Question 14

When can the equation E=F/q be used?

Answer 14

For any electric field E
F is force on charge q placed in the electric field

Question 15

When two of the same point charges are placed near each other, how do you find the distances where the resultant electric field is 0?

Answer 15

Plug charges (or factor of charges) into E=Q/4πε0r^2 for each charge

Replace r^2 for x^2 and y^2
Cancel out everything but factors and x y
Square root both sides
This is the ratio from of x to y (make sure you get it the right way round

Question 16

Describe the similarities and differences between electric and gravitational fields around a point mass

Answer 16

Similarity: point mass or charge produces a radial field that has spherical symmetry and infinite range

Similarity: Both fields obey the inverse-square law

Difference: Gravitational fields affect all masses, electric fields only affect charged masses

Difference: gravitational fields always produce attractive forces, electric fields can produce attractive or repulsive forces

Difference: the strength of an electric field is generally much higher

Question 17

Which equations can you use for uniform electric fields?

Answer 17

E=F/q or E=V/d (parallel plates)

Question 18

How is a uniform electric field produced?

Answer 18

placing two oppositely charged parallel plates parallel to each other

Question 19

Which way do electric fields lines go in a parallel plate?

Answer 19

Same direction as a positive charge

Same direction as conventional current

From the more positive voltage to the more negative voltage

Question 20

Draw a simple uniform field

Answer 20

Field lines are straight, parallel and uniform separation

Question 21

Derive the equation for E=V/d for from W=Fd

Answer 21

Work done W=Fd and electric field strength E = F/q

Replace F so W = Eqd

W/q=Ed but W/q = V (from W=QV)

so V = E/d

Question 22

Draw a simple graph of variation of electric field strength E against separation d for a uniform field

Answer 22

Constant E

Question 23

Draw a simple graph showing the variation of potential difference V against distance x for a uniform field

Answer 23

Constant gradient . V/d = E

Question 24

Units for electric field strength?

Answer 24

Newtons per coulomb
or
Volts per metre

Question 25

What is an equation which can be used to find final velocity for a charge being accelerated through a uniform electric field starting from rest

Answer 25

QV = 1/2mv^2

Question 26

What do you do if you’re asked to compare the value of force due to an electric field on a charge when compared to force due to gravity?

Answer 26

Work out each force then

make a ratio of one to the other

Question 27

What is the name for the path of charged particle sent through a uniform field at right angles?

Answer 27

parabola
Constant horizontal velocity
Constant vertical acceleration

(think suvat)

Question 28

What does vertical deflection mean?

Answer 28

distance moved up or down due to force from uniform field

Question 29

Define electric potential?

Answer 29

work done to bring a unit charge from infinity to that point where potential is defined as zero at infinity

Question 30

Is electric potential scalar or vector?

Answer 30

scalar

Question 31

What is the more useful equation for electric potential?

Answer 31

V = PE/q = Q/4πε0r

(PE = Qq/4πε0r)

Question 32

Describe how the equation for capacitance for a metal sphere is calculated

Answer 32

A charged metal sphere behaves like a charged mass Q at the centre of the sphere

So at the surface V = Q/4πε0R

Rearrange for Q/V = 4πε0R
Q/V is capacitance
C=4πε0R