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A-Level Physics [Edexcel] > Electric and Magnetic Fields > Flashcards

Electric and Magnetic Fields Flashcards

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1
Q

What is a field?

A

A region of space where an object experiences a non-contact force.

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2
Q

What is an electric field?

A

A region of space where a charged object experiences a non-contact force.

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3
Q

What is the force between charged objects called?

A

Electrostatic force.

Non-contact

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4
Q

What is coulomb’s law? (quantitatively)

A

F = (kQ₁Q₂) / (r²)

where k = 1 / 4πε₀

F: force
Q: charges
r: (the radial) distance between two charges
k: coulomb's law constant
ε₀: permittivity of free space
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5
Q

When drawing a charged particle, which direction do arrows face if it is:

  • Positively charged?
  • Negatively charged?
A

Positively charged: radial arrows pointing OUT.

Negatively charged: radial arrows pointing IN.

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6
Q

What is the electric field strength?

What is it useful for?

A

The force acting per charge.

Useful in comparing the strength of fields.

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7
Q

What are the different equations used for?

E = F/Q

E = kQ/r²

E = V/d

A

E = F/Q : To calculate teh electric field strength acting on a charge to prodcue a force.

E = kQ/r² : To calculate an radial electric field strength around a point charge.

E = V/d : To calculate the electric field strength of a uniform field (usually between two plates).

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8
Q

What are the different values in the following equations:

E = F/Q

E = kQ/r²

E = V/d

A 
E = electric field strength.
F = force exerted by the field.
Q = charge of point exterting the field.
k = constant (Coulomb's Law).
r = radial distance of feild
V = potential difference
d = distacne between charged areas (plates)
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9
Q

What are the units for the following equations?

E = F/Q

E = kQ/r²

E = V/d

A

E = F/Q : NC⁻¹

E = kQ/r² : NC⁻¹

E = V/d : Vm⁻¹

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10
Q

Is electric field stregth scalar of vector?

A

Vector.

In the same direction as its force acts in.

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11
Q

What are the similarities and differences between gravitational and electric fields?

A

Similarities:
- Both obey inverse sqaure law.

  • Both act on objects without contact.

Differences:
- Gravitational forces acts on a mass whilst electrical act on a charged object.

  • Gravitational force are only attractive while electric field an be attractive and repulsive.
  • Electric fields are much stronger than gravitational fields.
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12
Q

What is the difference between equipotential lines ad field lines?

A

An equipotential line is a line along which the electric potential is constant and is perpendicular to the field lines.

Feild lines show the direction of an electric field within a region of space.

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13
Q

What are the properties of a radial field?

A
  • A radial electric field decreases in strength with increasing distance from the charge.
  • The field strength follows an inverse square law.
  • The field lines spread out from a point charge.
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14
Q

What must you do when drawing radial field lines?

A
  • Use a ruler.
  • Make all lines equal in length (by eye)
  • Make all lines equally spaced (by eye)
  • Use arrows to show the direction a positive charge would move.
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15
Q

What are the properties of a uniform field?

A
  • A uniform electric field has a constant strength at all points between the plates.
  • In a uniform field, like the one between two oppositely charged parallel plates, the field lines maintain a constant separation.
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16
Q

What is the difference between electric potential difference and electric potential energy?

A
  • When a charged particle moves between two points the difference between the electric potentials is the potential difference.
  • To get the empirical potential energy the electric potential difference must be multiplied by the charge of the moving object.
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17
Q

What is Milikan’s oil drop experiment?

A
  • T terminal velocity the forces on the drop are balanced OR weight = drag.
  • The p.d. creates an electrostatic force acting upwards on the drop.
  • The electrostatic force increases as p.d. increases.
  • The net upwards force causes the drop to have negative acceleration.
  • As speed decreases, so does drag.
  • The drop remains stationary when the forces are balanced OR until the drop remains stationary when weight = electrostatic force.
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18
Q

What do capacitors do?

A

Store energy as electric potential energy.

They consist of two parallel plates which gain an opposite charge when connected in a circuit.

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19
Q

How do capacitors work?

A

When a potential difference is applied across a capacitor electrons flow onto one plate - charging it negatively - and flow off the other - charging it positively.

This creates an electric field between the plates, asn a potential difference across them.

This acts as a store of energy.

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20
Q

What is capacitance?

A

The ratio of the electric charge on the plates to the potential difference between the plates.

21
Q

What is the unit for capacitance?

A

Farad F.

22
Q

Relate capcitance to:

  • the area of the plate.
  • The distance between the plate.
A

Capacitance ∝ area of plate:
- Larger surface area plate means there are more electrons on the plate.

Capacitance ∝ 1/distance:
- because a larger distance means there is a smaller potential difference.

23
Q

Decribe an explain the different charging graphs?

A

Q-t: Logarithmic growth.
- as more work is required to move a charge as the capacitor charges.

V-t: Logarithmic growth.
- as more work is required to move an electron as the capacitor charges, taking longer to build up a larger p.d.

I-t: Exponential growth.
- electrons become less attractive to the capacitor as more electrons accumulate on it.

24
Q

Decribe an explain the different discharging graphs?

A

Q-t: Exponential decay
- The capacitor starts with a high charge but as a result of discharging looses this. More work is done to do this as the cell/ charge acceptor gains more charge.

V-t: Exponential decay.
- An initially large pd, decreases as the charge accepter gains it. It is initially easy to move theses electrons off the capacitor but more work is needed over time.

I-t: Exponential decay.
- Initially there is a large current due to the large pd across the plate. Current drops as pd drops.

25
Q

What is the time constant?

A

The time taken for the charge/voltage/current to fall to 37% of the original.

26
Q

Why 37% when considering the time constant?

A

because it is when tRC = 1.

27
Q

Derive I = I₀e⁻ᵗ/ᴿᶜ from Q = Q₀e⁻ᵗ/ᴿᶜ.

A 
Q = Q₀e⁻ᵗ/ᴿᶜ
Q = It
      substitute 
It = I₀t₀e⁻ᵗ/ᴿᶜ
      final time, t₀ = t
     ∴ ÷t
I = I₀e⁻ᵗ/ᴿᶜ
28
Q

Derive V = V₀e⁻ᵗ/ᴿᶜ from Q = Q₀e⁻ᵗ/ᴿᶜ.

A 
Derive I = I₀e⁻ᵗ/ᴿᶜ
P = VI ∴ I = VP
      substitute
VP = V₀P₀e⁻ᵗ/ᴿᶜ
      power is constant.
      ∴ ÷ P
V = V₀e⁻ᵗ/ᴿ
29
Q

What is the equation for time constant?

A

τ = RC

τ: time constant.
R: resistance.
C: capacitance.

30
Q

When might you see an isolated magnetic pole?

A

NEVER!!!

31
Q

What is the standard notation when drawing field arrows within and out of a page?

A

⊙ Circle with dot:
- Out of page.
⨂ Circle with cross:
- In to page.

32
Q

What are two rules to keep in mind about magnetic fields?

A
  • Magnetic fields cause moving charges to feel a force.

- Moving charges produce a magnetic field.

33
Q

What are the parts of the right hand rule?

A

Thumb: Current

Fingers: Magnetic field.

34
Q

What is magnetic flux density?

A

Magnetic flux density is the density of magnetic field line in a given area — is the strength of the magnit.

UNIT: Tesla, T

SYMBOL: B

35
Q

What is flux?

A

A scalar measure of the magnetic field (B) interacting with a surface-area (A).

UNIT: Weber, Wb

SYMBOL: ϕ

36
Q

What is flux linkage?

A

Flux interacting with multiple loops in a coil, rather than just one wire/surface.

UNIT: Weber-turns

SYMBOL: Nϕ
(N = number of loops).

37
Q

What equation links flux to magnetic flux density?

A

ϕ = BAsinθ

ϕ: flux.
B: mag flux density.
A: area.
sinθ: to find component perpendicular to current.

38
Q

When do you use each of these equations:

F = Bqvsinθ

F = BIlsinθ

A
  • Individual charged particles:
    F = Bqvsinθ
  • Charges in a wire:
    F = BIlsinθ
39
Q

What are the parts of Fleming’s left hand rule?

A

Thumb = Force.

First finger = Magnetic field.

Second finger = Current.

( conventional charges flow in the opposite direction to the electrons)

40
Q

What is the case for moving chargd particles in a magnetic field?

A
  • Moving charged particles will feel a force.
  • The force will be expressed by F= Bqvsinθ
  • The force will act perpendicular to both the velocity of the particle and magnetic feild.
41
Q

Why does a particle spin in a magnetic field the is coming out of the page?

A
  • The moving feild will feel a force, F=Bqvsinθ.
  • This must act perpendicular to the velocity of the particle and the magnetic field.
  • As a result the force acts centripetally.
  • And thus the particles spins.
42
Q

How can you work out the radius of centrapetal motion from the equation of force on a moving charged particle?

A

F=Bqvsinθ
F= mv² / r

θ = 90 so sinθ = 1

mv² / r = Bqv
mv/r = Bq
r = mv/Bq

p = mv

r = p/Bq

43
Q

What happends to wires in a magnetic field?

A
  • Wired carrying a current will feel a force in a magnetic field.

= The force will be expressed by F = BIlsinθ.

  • The force will act perpendicular to both the direction of current and the magnetic field.
44
Q

Why does the pottentail difference across a wire (not along) cause the wire to move?

B = in page.
I = Right.
e = Left.
A
  • The moving charge particles (electrons) in the wire feel the fore F= Bqvsinθ acting up (due to Flemmings left-hand rule).
  • As a result electrons move towards the top of the wire.
  • This creates a potential difference across the wire.
  • The metal ions of the wire are attracted to the net negativity at the top.
  • At the same time the electrons are both attracted to the metal ions and feel its upwards force due to Bqvsinθ. This means the electrons remain the same distance from the metal ions.
  • So when the metal ions move up so do the electrons.
  • And so the wire moves.
45
Q

What is Faraday’s Law?

A

The emf induced in a wire is proportional to the rate of change of flux linkage through the wire.

46
Q

What is Lenz’s Law?

A

The polarity of an induced emf is such that it produces a current whose magnetic field opposes the change which produces it.

(current is only induced by and emf when in a closed system)

47
Q

How does a temporary magnetic feild change?

A

If the magnitude or direction of charge flow (current) changes.

48
Q

Using Faraday’s law, explain how a transformer works.

A
  • The primary coil carries an a.c current, thus inducing a magnetic field.
  • This magnetic field acts on the secondary coil.
  • This field varies continuously, proportional to the frequency of a.c current.
  • This results in a rate of change of flux linkage over the secondary coil, inducing an emf.
49
Q

Why is Vᵣₘₛ used?

A

Used instead of the average.

  • Because the power supply in alternating, V varies sinusoidally between V₀ and -V₀.
  • This means its average would be 0 - not helpful.
  • Therefore the root-mean-squared provides a more true ‘average’ value.

A-Level Physics [Edexcel] (9 decks)

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