Electric Fields

Question 1

How do charges interact in these situations?

Answer 1

1. Like charges repel
2. Opposite charges attract

Question 2

Which direction will these charges move?

Answer 2

Electric field lines shows direction of Force on +ve charges

(-ve charges are opposite)

Question 3

How are radial and uniform electric fields different?

Answer 3

1. Radial fields have a varying field strength (weaker when further apart)
2. Uniform fields have a constant field strength

Question 4

For electric fields, how are are equipotentials related to the field lines?

Answer 4

Equipotentials always perpendicular to field lines

Question 5

How can you change this situation to increase the force on the charge?

Answer 5

1. Increase field strength
2. Increase magnitude of charge

Question 6

What field lines are produced by…

a) +ve charge
b) -ve charge

Answer 6

Field lines always act…

• Away from +ve
• Towards -ve

Question 7

How do the field lines look for these two interacting oppositely charged particles?

Answer 7

Question 8

How do the field lines look for these two interacting like charged particles?

Answer 8

NOTE: Field lines never cross

Question 9

What is the electric field strength at the following points?

Answer 9

Field strength is constant between parallel plates (capacitor)

E₁ = E₂ = E₃

Question 10

What is the electric potential at the following points?

Answer 10

Electric potential linearly increases between parallel plates (capacitor)

Question 11

Define Coulomb’s Law

Answer 11

Question 12

Define electric field strength

Answer 12

Force per unit charge acting on a small positive charge

Question 13

What force would act on a 5C charge placed at 6NC^-1?

Answer 13

F=Eq → 5x6 = 30N

Question 14

What is wrong here?

Answer 14

Electric field strength ≠ acceleration

Question 15

How do you calculate electric field strength outside a conducting sphere?

Answer 15

Treat it as a point charge

Question 16

How do you calculate electric field strength inside a conducting sphere?

Answer 16

E=0 everywhere!!!

F_res= 0 , change in E_p = 0 , change in V = 0

Question 17

What is the graph of electric field strength for a conducting sphere?

Answer 17

Question 18

How is electric field strength calculated here?

Answer 18

For parallel plates calculate E first if possible

Question 19

How do you work out the resultant field strength between charges?

Answer 19

1. Work out field strength from each
2. Label vectors
3. Add or subtract field strengths

Question 20

Can you use SUVATs here?

Answer 20

Yes!

Field strength constant → Acceleration constant

Question 21

Why is potential energy here +ve?

Answer 21

E_p = 0 at ∞

Increases as charge moves closer

Question 22

Why is potential energy here +ve?

Answer 22

E_p = 0 at ∞

Increases as charge moves closer

Question 23

Why is potential energy here -ve?

Answer 23

E_p = 0 at ∞

Decreases as charge moves closer

Question 24

Why is potential energy here -ve?

Answer 24

E_p = 0 at ∞

Decreases as charge moves closer

Question 25

Define electric potential and what is its equation?

Answer 25

Work done per coulomb moving positive charge from infinity to that point

(¼πε₀) * (Q/r)

Question 26

Why does the moving charge’s potential energy increase?

Answer 26

Equipotentials show the change in potential of a +ve charge

If +ve charge → decreases

If -ve charge → increases

Question 27

How are these charges different?

Answer 27

Q is the charge creating the field

q is the charge moving in the field

Question 28

What is wrong here?

Answer 28

Electric potential is scalar

But they are opposite → must be subtracted

Question 29

How do you calculate the neutral electric field strength (or force) point between charges?

Answer 29

Question 30

How do you calculate the neutral electric potential point between charges?

Answer 30

Question 31

What is the graph of electric force for a conducting sphere?

Answer 31

Same as field strength graph

Question 32

What is the graph of electric potential for a positively charged conducting sphere?

Answer 32

The gradient of the graph at a point = |E|.

The E_p - r graph for like charges is the same and the gradient at a point is |F|

Question 33

What is the graph of electric potential for a negatively charged conducting sphere?

Answer 33

The gradient of the graph at a point = |E|.

The E_p - r graph for opposite charges is the same and the gradient at a point is |F|

Question 34

What two situations produce a uniform electric field?

Answer 34

1. Radial field over a short distance
2. Field between 2 parallel plates

Question 35

What is the equation for electric potential energy?

Answer 35

(¼πε₀) * (Qq/r)

V x q

Question 36

How do you convert E_P to V in electric fields?

Answer 36

Divide by q

Electric potential is independent of the charge being affected by the field (independent of small q)

Question 37

When must you consider negative values in electric fields?

Answer 37

Only when a question asks to ‘state the direction’ or if you are combining equations such as ‘potential’ equations