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Module 6- Particles and medical physics > 6.2 Electric fields > Flashcards

6.2 Electric fields Flashcards

1
Q

What is an electric field?

A

An electric field is the reigon around a body in which other charges bodies will feel a force due to the electric charge on the body.

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2
Q

What are electric field lines?

A

They show the shape of the field and the direction of the field line at a point is the direction in which a small positive charge would move when placed on that spot.

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3
Q

what is the electric field strength?

A

The electric field strength is defined as the force per unit positive charge. Units are newtons per coulmb (NC-1). It is the force that a charge of +1C would experiece if placed at that point in the field.

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4
Q

What are electric fields caused by?

A

Electric charge

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5
Q

What force does an electric field exert on a charge object?

A

An electric field exerts an electrical force on any charged object.

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6
Q

Which way do electric field lines act in regards to it’s source?

A

Electric field lines can axct towards or away from the charged sourse as the electric feild can be attractive or repusive in nature.

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7
Q

Define electric field strength?

A

The force acting on unit charge placed at that point in the field.

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8
Q

What sort of quantity is electric field strength?

A

A vector quantity

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9
Q

What is the equation for electric field strength?

A

E= F/Q

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10
Q

E=F/C
What does each letter represent?

A

E= electric field strength
F= force
Q= size of positive charge in coulmbs

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11
Q

What are the units for electric field strength?

A

NC^-1

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12
Q

What does the direction of the electric field lines show?

A

It shows the direction that unit positive charge would move if placed at that point in the fields.

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13
Q

What do close electric field lines indicate?

A

A strong electric field.

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14
Q

What is a neutral point between 2 charged objects?

A

The neutral point is the pace between 2 charged objects where the feilds exactly cancel and the net force is zero.

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15
Q

What is Coulmb’s law?

A

Coulmb’s law states that the electrical force, F between two point charges is directly proportional to the product of their charges and inversley proportional to the square of their separation, F=Qq/4πε0r^2

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16
Q

What is permittivity of free space?

A

ε0, is. the physical constant related to the size of the firce between electric charges in free space (a vacuum). It has a value of 8.85x10^-12C^2m^-2N^-1/

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17
Q

What did the french physicist Charles-Augustin de Coulmb discover with his investigations relating to the behavior of charged bodies in the vicinity of one another.

A
  • The magnitude of the force between 2 charged bodies is directly proportional ti the product of their charges.
  • The magnitude of the force is inversely proportional to the square of their separation.
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18
Q

What equation provides an explanation od Coulmb’s law?

A

F=kQq/r^2

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19
Q

In F=kQq/r^2, what does k stand for?

A

k is a constant of proportionality. The physicist James Cleak Maxwell was able to relate te value of k to a physical constant known as the permitttivity of free space ε0.

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20
Q

What is the relationship between k and ε0?

A

k=1/(4πε0)

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21
Q

How can you write coulmbs law from F=kqQ/r^2 and k=1/4πε0?

A

F=Qq/4πε0r^2

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22
Q

When there is more than one force on a charge, how do you find the resultant force?

A

The resultant force is equal to the vector sum of all the forces.

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23
Q

Using F=qQ/4πε0r^2 abd E=F/q, what new equation can be used to find the electric field strength for a point charge?

A

E=Q/4πε0r^2

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24
Q

What does the graph of electric field strength ,E, agaist distance from the point charge, r look like?

A
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25
Q

What does this graph show us?

A

Electric field strength is an inverse square law with distance, written as E∝1/r^2

26
Q

How do you find the resultant field of 2 or moe charges?

A

Find the vector sum of the electric fields of individual charged.

27
Q

What is the similarity between an electric field and gravitational field and how their strength is defined?

A
  • Feild strength in an electric field is defined as the force per unit positive charge.
  • Feild strength of gravity is defined as force per unnit mass.
28
Q

What is the similarity between an electric field and gravitational field and the equations that related force and field strength?

A
  • In electric fields, both force ans field strength follow an inverse square law: F∝q/r^2 and E∝Q/^r
  • In gravitational fields the force and field strength follow an inverse square law F∝Mm/r^2 and g∝M/r^2.
29
Q

What is the similarity between an electric field and gravitational field and the lines that represent the field?

A
  • Electric field for a radial source act radially outwards from a positive charge and inwards for a negative charge.
  • Gravitational field lines always act radially inwards.
30
Q

What is the similarity between an electric field and gravitational field and if they are attractive or repulsive forces?

A
  • The electrostatic force can be attractive or repulsive.
  • The gravitational force is always attractive- it attracts masses towards one another, so both F and g have a negative sign in this equation.
31
Q

What is permittivity?

A

ε is a constant releated to the size of the capacitance of a capacitor. For a parallel plate capacitor of plate induction d and area, A, C=εA/d. Permittivity has the units C^2m^-2N^-1.

32
Q

What is relative permittivity?

A

εr of a material is the factor by which the capacitance of a capacitor with that material between its plates is inreased relative to the capacitance of a capacitor with air or vacuum between its plates.Relative permittivity is a pure umber and has no units, and is related to the permittivoty of free space, ε0 by εr=C/C0= εA/d/ε0A/d= ε/ε0

33
Q

What happens the the field lines of an electric field as we move further away from the source?

A

The field lines diverge as the firld strength follows an inverse square relationship.

34
Q

What happens to the field strength as you move further away from the an electric source?

A

The strength of the field decreases

35
Q

Is the field stength uniform with a radial field?

A

No.
THe field strangth is not uniform within a radial field.

36
Q

What sort of electric field will 2 conducting plates that have been charged by connection to the terminals of a battery create?

A

They will create a uniform field between them.

37
Q

Which way do field lines showing an electric field point?

A

They will point from the positive charge to a negative charge.

38
Q

How could you set up a uniform electric field?

A

Put 2 parallel plates a small distance apart from eachother and are connected to a battery, so that the potential difference across the plates is V. The plates carry equal and opposite charges, so there is a uniform field between them except just at the edges.

39
Q

What is required to move a small positive charge to a positive plate in a uniform electric field?

A

It will require a force, beacuse the electric field acts downwards.
This force is constant because the electric fied is constant and does not vary with distance.

40
Q

What is the work done on a charge against the electric field equal to?

A

Work= force x distance
W=Fxd

41
Q

How can you derrive the equation E=V/d

A
  • W=Fxd
  • V=W/q, W=Vq
  • E=F/Q, F=EQ
  • Vq=EQ x d
  • V=Exd
  • E=V/d
42
Q

What are the units for electric field strength?

A

NC^-1

43
Q

What equation shiws the relationship between electric field strength, potential difference between the plates and the separation of the plates?

A

E=V/d

44
Q

What will happen to a charged particle when placed in an electric field?

A

It will cause the particle to accelerate.

45
Q

What are the 2 stituation when placing a charged particle in an electric field, causing it to accelerate?

A
  • Placing a stationary charged particle within a uniform electric field.
  • Injecting a moving charged particle into a uniform electric field, initially at right angles to the direction of the field.
46
Q

How do you work out the gain in kinetic energy on a charged particle places in a one-dimensional electric field?

A

The charged particle will either accelerate to one of the charged plates depending on the charge on the particle. We can applythe equation F=ma to describe it’s motion and also use the transfer of energy is equal to work done, to calculate the gain in kinetic energy.

47
Q

Why will charged particles move in a parabollic motion When thy enter sn electric field at a right angle to the direction of the field?

A

A charged particle hass a component of motion in the direction perpendicualr to the field as well as a component of the motion parallel to the field. Since the components of motion are independant from each other, there will be a constant velecity in the direction parallel to the field but am acceleration in the direction in the field- giving a parabolic motion.

48
Q

How can we decribe the motion of a charged particle travelling parallel to the plates?

A

We can use the equations of linear motion to describe the motion of the charge parallel to the plates, as well as F=ma to a=decribe it’s acceleration in the direction perpendicular to the plates.

49
Q

What equation gives the capacitance of a capacitor with the permittivity of the material between the plates, the area of the overlap of the plates and the separation of the plates?

A

C=ε0A/d

50
Q

How do you work out permittivity?

A

When a material of relative permittivity εr is used, the permittivity is found by multiplying the relative permittivity of the material by the permittivity of free space, ε=εrε0

51
Q

What is electric potential?

A

Electric potential is the work done per unit positive charge to move that charge from infinity to a point in an electric field. The unit is JC^-1 or V. One volt is 1 joule per coulmb.

52
Q

What is electric potential energy?

A

The electric potential energy of a body of charge is the work done to move that charge from infinity to a point in an electric field.

53
Q

To move a charge in a direction opposite to the electric field requires work done. What is this work done equal to?

A

The work done/ is equal to the stored electric potential energy.

54
Q

At infinity, what will the electric potential be?

A

0

55
Q

For point Q, what equation gived the magnitude of the electric potential?

A

V=Q/4πε0r

56
Q

V=Q/4πε0r. What will happen as a small positive charge,q gets closer to a positive charge, Q?

A

q will need to do work against the field, due to the repulsive force exerted by Q.

57
Q

What does the graph of electric potential against distance look like for a repulsive field? Why does it look like this?

A

In a repulsive feild, potential will inecrase as the sparation decreases. This is because more and more energy is requires to bring a unit of positive charge closer to a like charge since the force of repusion increases the closer it gets.

58
Q

What does the graph of electric potential against distance look like for an attractive field? Why does it look like this?

A

Electric potential will become more negtive as the separation decreases beacuse as a positive charge moves towards a negative charge they will be attactive, ‘negative work’ is done.

59
Q

Describe the electric field on the sphere?

A

The charge is uniformly distrubuted on it’s outer surface and so outside the sphere the charge can be treated as a point charge at the centre of the sphere. The electric field is radial and all points on the sphere have the same value of potential, V.

60
Q

What do you get is you substitiute C=Q/V into V=Q/4πε0r?

A

C=Q/(Q/4πε0r) or C=4πε0r

61
Q

What equations give the electric potential energy?

A
  • E=Vq
  • E=Qq/4πε0r
62
Q

For a charge being moved in a repulsive radial electric field, how do you find the work done to move that charge a cerain distance?

A

As work done= force x distance, the area beheath a force-distence graph is equal to the work done.

Module 6- Particles and medical physics (5 decks)

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