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NF0607: Physics for Engineers 2 > Chapter 22 Electric Fields > Flashcards

Chapter 22 Electric Fields Flashcards

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1
Q

What is the relationship between charges?

A

Like charges repel and unlik charges attract.

Where the saying opposites attract come from.

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2
Q

What is the charge of electrons neutrons and protons?

A

Electrons - negative

protons - positive

Neutrons - neutral

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3
Q

Describe how to change the charge of an uncharged solid

A

Uncharged solids contain an equal mount of protons and electrons.

To make it negatively charged, electrons must be added.

To make it positively charged, electrons must be removed

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4
Q

What is the relationship between free electrons and conductors or insultators?

A

Electrical conductors contain lots of free electrons while electrically insulating materials do not contain free elctrons

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5
Q

What is the equation for average current round a circuit ?

A

I = Qf or I =Q/T

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6
Q

What is the path a free positive test charge follows called?

A

Line of force or field lines.

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7
Q

What is an electric field?

A

When two objects exert equal and opposing forces (charges) on each other without being in direct contact

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8
Q

Describe the electric field’s behaviour in oppositely charged objects.

A

-oppositely charged objects create a fiel where the field lines become concentrated at the points. A positive test charge released will follow a curved path to a negative charge point

  • between an object and a plate, the field lines are concentrated at the object but are at right angles to the plate when they meet. The field is strongest where the lines are most concentrated
  • In two oppositely charged plates, the field lines run parallel from one plate to another and meet the plates at right angles except near the edges of the field. The field is uniform between the plates since the field lines are parallel to each other.
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9
Q

What is electric field strenght? Its equation?

A

The force per unit charge on a positive test charge placed at that point in (NC^-1)

E = F/Q

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10
Q

Describe the field lines between two parallel plates which are oppositely charged.

A

Field lines are:

  • parallel to each other
    -right angle to the plate
    -run from the positive plate to the negative plate
    -uniform field since electric field streght is the same everywhere
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11
Q

What is the equation to calculate electric field strength in relation to potential difference and separation?

A

E = V/d

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12
Q

Prove E = V/d

A

If F = QE
W = Fd
d =QEd

sisnce potential dif is work done per unit charge when a small charge is moved

V= W/Q = QEd/Q = Ed

Rearrange for E = V/d

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13
Q

What is the relationship between the strength of the electric field and the strength of the charge?

A

The higher the charge on the body, the greater the electric field. The more concentrated the charge is on the surface , the greater the strength of the electric field above the surface.

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14
Q

Describe the charge on oppositely charge parallel plates

A

charge is spread evenly across the surface of the plate facing the other plate, measurements show that electric field strength between plates is proportional to charge per unit area

therefore, electric field strength is proportional to charge per unit area on the facing surfaces

E = Q/A

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15
Q

What is the constant of proportionality?

A

ε0 epsilon nought it is the permittivity of free space

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16
Q

What is electric potential?

A

The electric potential at aa certain position in any electric field is defined as the work done per unit positive charge on a positive test charge when it is moved from infinity to that position?

V = Ep/Q

the position of zero potential energy is infinity

17
Q

How can work done be calculated in relation to electric potential?

A

ΔW = Q(V2 - V1)

18
Q

Why is no work done by an electric field by a test charge in equipotentials?

A

The lines of force of the electric field cross the equipotential lines at right angles

19
Q

What is the potential gradient?

A

The change of potential per unit change of distance in a given direction?

20
Q

Explain the effect of non-uniform and uniform fields on the potential gradient/

A

If field is non uniform, potential gradient varies according to position and direction. The closer equipotentials are, the greater the potential gradient is at right angles to each other.

If the field is uniform, the equipotential the potential relative to the negative plate changes with perpendicular distance x from the negative plate.

21
Q

Describe the attributes of potential gradient

A
  • constant
    -potential increases in the opposite direction to the electric field
  • equal to V/d
22
Q

What is electric field strength equal to?

A

The negative of the potential gradient

= v/d directed from + to - plate

E = -ΔV/Δx

23
Q

What is coloumb’s law? It’s equation?

A

the force of attraction or repulsion between two charged bodies is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.

F = Q1Q2/r^2

24
Q

What is the constant of proportionality? k

A

The constant of proportionality, k can be shown to be equal to 1/4 π ε0

Which is equal to = 9 x 10^9 mF^-1

25
Q

What is a point charge?

A

An expression for a charged object in a situation where the distances under consideration is greater than the size of the object

26
Q

What is a test charge?

A

A point charge that does not alter the electric field in which it’s placed

27
Q

How do you calculate the force and electric field strength of a test charge q at distance r from Q?

A

F = I/4πε0 x Qq/r^2

E = Q/4πε0r^2

28
Q

Is electric field strength a scalar or vector?

A

Vector

It is the vector sum of the individual electric fields strength.

29
Q

How can resultant electric field strength be calculated for forces acting in the same direction

A

E = F/q = (qE1 + qE2)/q = E1 + E2

30
Q

How can resultant electric field strength be calculated for forces acting in opposite direction

A

E = F/q = (qE1 - qE2)/q = E1 - E2

31
Q

How can resultant electric field strength be calculated for forces acting at right angles to each other

A

E = F/q then

E^2 = E1^2 + E2^2

32
Q

For a radial electric field how can electric field strength be calculated?

A

E = Q/4πε0r^2

33
Q

For a radial electric field how can electric potential be calculated?

A

V = E = Q/4πε0r^2

Electric potential V is inversely proportional to distance r

34
Q

What is the relationship between elctric field strangth and electric potential?

A

Electric field strength is equal to the negative gradient of a potential against distance graph

Change of potential = area under an electric field strength against a distance graph

NF0607: Physics for Engineers 2 (13 decks)

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