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Physics AQA AS Level > Electrical Fields > Flashcards

Electrical Fields Flashcards

1
Q

Like charges….

A

….repel.

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2
Q

Opposite charges….

A

….attract.

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3
Q

What is an electrical conductor?

A

An electrical conductor is a material that allows the flow of charged particles through it. They do contain free electrons e.g. a metal

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4
Q

What is an electrical insulator?

A

An electrical insulator is a material that does not allow the flow of charged particles through it. They do not contain free electrons.

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5
Q

When is an electrical insualtor easy to charge?

A

Some insulators are easy to charge because their surface atoms lose or gain electrons easily.

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6
Q

What do all charged objects have?

A

All charged objects have an electrical field around them.

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7
Q

Any two charged objects…

A

…exert an equal and opposite force on each other.

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8
Q

What does the direction of the electrical field lines surrounding a charge show?

A

The direction of the electrical field lines surrounding a charge is the direction a free positive test charge would move along.

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9
Q

What is the electrical field strength at a point in the field?

A

The electrical field strength at a point in the field is the force per unit charge on a positive test charge placed at that point.

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10
Q

What is the units for electrical field strength?

A

NC⁻¹ or Vm⁻¹

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11
Q

If a positive test charge, Q at a certain point in an electrical field is acted on by force F due to the electrical field, the electric field strength E at that point is given by?

A

E = F/Q

E = Electrical field strength (NC⁻¹)
F = Force due to the electrical field strength on charge Q (N)
Q = Positive test charge in the electric field strength (C)
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12
Q

The field lines between two oppositely charged flat conductors are:

A
  • Parallel to each other
  • At right angles to the plates
  • From the positive plate to the negative plate.
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13
Q

The field between two oppositely charged flat conductors is? Why?

A

The field between two oppositely charged flat conductors is uniform because the electrical field strength has the same magnitude and direction everywhere between the plates.
This is show by the parallel field lines that are equally spaced.

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14
Q

What equation can we use to calculate the field strength between the oppositely charged conductor plates? (Is this only for uniform fields)

A

E = V/d

E = Electric field strength (NC⁻¹)
V = The potential difference between the plates (V)
d = The seperation distance between the plates (m)
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15
Q

What does a greater charge on an object tell you about the strength of the field surrounding it?

A

The greater charge of the object, the stronger the electric field is.

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16
Q

For a charged metal conductor, the charge on it spread across its surface. The more concentrated the charge on the surface…

A

…the greater the strength of the electrical field above the surface.

Look ar page364, to see an image of this and what this means.

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17
Q

When a charged object X is moved towards another object Y of the same charge (both positive or negative), describe what is happenining in terms of energy transfer?

A

The electric potential energy store of object X increases. This is because work is done to overcome the resistive force due to Y’s electric field.

This is the same when a charged object X is moved away from another object Y of the opposite charge as the object is doing work against the attractive force.

In both cases, the charged objects moves against the electric (field lines) so work is done by the electric field on the object. When a charged object moves in the direction of the electric field, its potential energy decreases.

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18
Q

What is electric potential?

A

The electric potential at a 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.

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19
Q

Where is the position of zero potential energy?

A

The position of zero potential energy is at infinity.

20
Q

What is the unit of electric potential?

A

JC⁻¹

21
Q

What is the equation to calculate the electric potential at a specific position in an electric field?

A

V = Ep/Q

V = Electric potential (JC⁻¹) at the position in the electric field.
Ep = Electrical Potential Energy at that position in the electric field(J)
Q = A positive test charge placed at that position in the electric field (C)
22
Q

What are equipotentials?

A

Surfaces of constant potential.

23
Q

What are potential gradients?

A

The potential gradient at any position in an electric field is the change of potential per unit change of distance in a given direction.

24
Q

If the field is non-uniform,how does the potential gradient vary?

A
  • The potential gradient varies according to position and direction. The closer the equipotentials, the greater the potential gradient is at right angles to the equipotentials.
25
Q

If the field is uniform, how does the potential gradient vary?

A

The potential gradient is constant through the uniform field such that the potential increases in the opposite direction to the electric field. This is because the equipotentials are equally spaced in the field.

An example of a uniform field is that between two oppositely charged plates.

26
Q

In a uniform and radial field what is the potential gradient?

A

The potential gradient is equal to -V/d. THIS IS BECAUSE THE NEGATIVE OF THE ELECTRIC FIELD STRENGTH IS EQUAL TO THE POTENTIAL GRADIENT.

27
Q

The force between two charged objects depend on?

A
  • The charge of both objects

- The distance between the two charged objects.

28
Q

The force between two objects can be?

A
  • Repulsive or attractive
29
Q

Which equation is used to calculate the force of attraction or repulsion between two charged objects?

A

F = kQ₁Q₂/r²

where k is = 1/4πε₀

Therefore F = Q₁Q₂/4πε₀r²

This is known as Coulomb's law:
F = Force of repulsion or attraction
Q₁ = Charge of object 1
Q₂ = Charge of object 2
r = the distance between object 1 and 2
ε₀ = permittivity of free space (8.85x10⁻¹² Fm⁻¹)
30
Q

How is force and distance related according to the equation?

A

Force is inversely proprtional to r².

31
Q

What is the equation calculate the electric field strength near a point charge?

A

We know that the force on a test charge can be worked out by F = Q₁Q₂/4πε₀r², where Q₂ is the test charge.
We can write this as F = Qq/4πε₀r² to make the fact that Q is the point charge and q is the test charge near the point charge more clear.
We also know that the electric field strength on a test charge at a point in the electric field strength of Q is given by E = F/q
If we combine these two equations we get
E = Q/4πε₀r², which tells us the electric field strength near a point charge at distance r. This is used for radial fields only

32
Q

E = Q/4πε₀r². E can be positive or negative. What does this tell you?

A

E is positive if Q is positive - therefore a positive value of E indicates that the field lines point outwards. E is negative if Q is negative - therefore a negative value of E indicates that that the field lines point inwards.

33
Q

Is electric field strength a scalar or a vector?

A

Electric field strength is a vector.

34
Q

What is the resultant electrical field strength on an test charge that is in the electrical field of several point charges?

A

For a test charge in the electrical fields of several point charges, the resultant electrical field strength is given by the resultant force per unit charge on the test charge.

35
Q

How to calculate the electric field strength for test charges where the forces on it act in the same direction to each other?

A

If a positive test charge is in the same plane as a negative point charge and a positive point charge, the force per unit charge from both point charges will act towards the negative point charge. So we calculate the resultant force per unit charge (electrical field strength) from both point charges and add them together:

E = F1 + F2/q = qE1 + qE2/q = E1 + E2

36
Q

How to calculate the electric field strength for test charges where forces on it act in the opposite direction to each other?

A

If a positive test charge is in the point chargesarge, the force per unit charge from both point charges will act in oppositve directions to each other. So we calculate the force per unit charge from both point charges and substract them from each other.

E = F1-F2/q = qE1 - qE2/q = E1 - E2

37
Q

How to calculate the electric field strength for test charges where the forces on it act at right angles to each other?

A

If a positive test charge is in the electric field strength of two point charges whose force acts at right angles to each other, we can calculate the resultant force per unit charge (electric field strength) from both point charges using pythagorus’ theorem:

E = square root of (F1² + F2²)/q = square roots of qE1² - qE2²/q = square root of E1² - E2²

38
Q

For a radial field, the equipotentials are at ______ ______ to the field lines

A

right angles

39
Q

What is the relationship for electrical field strength and distance (from point charge). What will the graph look like? (Draw it

A

Electric field strength is inversely proportional to the distance squared. The curve on a graph is an inverse-square law curve, so as distance increases, electrical field strength decreases (never touches zero). On page373.

40
Q

What is the other equation to calculates electric potential?

A

We know that E = V/d aka E = V/r, We can rearrange this to get, V = Er
We also know that E = Q/4πε₀r²
If we combine these two equations we get V = Q/4πε₀r

41
Q

How is electric potential related to distance? What would this graph look like? What would this graph look liek in comparison to the electric field strength vs distance graph?

A

Electric potential is inversely proportional to the distance. So as distance increases electric potential decreases. In comparion to the electric field strength graph, it would always remain above it, so electric potential will not decrease as much with increasing distance. pg373

42
Q

What is the value of electric potential at infinity?

Is electric potential always negative like gravitational potential?

A

At infinity electric potential is zero. However, electric potential can be negative or positive because the force on a positive test charge can be attractive or repulsive, whereas with gravtiational potential the force is always attractive. When the point charge is negative, and +ve charge is moved away, work is done on the object by the electric field to increase its potential energy. If the potential at infinity is zero, this means the change in potential has to be negative in the first place, to increase its potential to zero. If the point charge is positive charge, and a +ve charge is moved away, work is done on the electric field by the test charge, so its potential energy decreases. If the potential at infinity is zero, and the potential decreases, then the potential has to be positive in the first place, to decrease its potential energy to zero.

At infinity, both electric potential and electric potential energy is negative, however depending on the charge, at other distances it can be positve or negative.

Another way this is explained:
So imagine bringing a positive charge from infinity (zero potential) in towards another positive charge. You are going to have to push it (do work on it) so it gains Ep. That’s easy! Maximum potential close to the object, zero potential at infinity.

Now think about bringing a negative charge from infinity towards the positive charge. They attract! So at infinity they have zero potential (as always) but, as they get closer together the Ep reduces (i.e. becomes less than zero). Right next to each other, they have very little Ep - if you let go of them, they just stay there! So, as with gravitational fields, you can have negative values of potential.

43
Q

How can you work out change in potential from an electric field strength against distance graph?

A

Area under an electrical field strength against distance graph.

44
Q

How can you work out how the force varies on a electric field strength against distance graph?

A

Electric field strength is the force per unit charge, so for the same charge, if electric field strength decreases the force on teh charge decreases.

45
Q

Is electric potential and electrical potential energy scalar or vector?

A

They are both scalar. This can confuse people because it they can both have a postive or negative sign but this does not refer to the direction. Its a property of the field.
Electrical ield strength is vector tho

Physics AQA AS Level (16 decks)

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