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A2 Physics > Topic 8 > Flashcards

Topic 8 Flashcards

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1
Q

Equation: arc length

A

l = rθ

l = arc length

r = radius

θ = angle in radians

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2
Q

Equation: Time period of rotation *

A

T = 2π / ω

T = Time period of rotation (s)

ω = angular velocity (rad s-1)

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3
Q

Definition: Angular velocity

A

The angle swept out by an object with rotational motion in unit time.

(rad s⁻¹)

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4
Q

Definition: Time period of rotation

A

Time taken to complete one rotation.

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5
Q

Equation: Angular velocity

A

ω = Δθ / Δt

ω = angular velocity (rad s-1)

Δθ = angle turned through in radians

Δt = time taken (s)

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6
Q

What assumptions are made when using Δθ in rotational motion equations?

A

Δθ is in radians.

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7
Q

Equation: Tangential velocity *

A

v = ωr

v = tangential velocity (ms-1)

ω = angular velocity (rad s-1)

r = radius (m)

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8
Q

Definition: Centripetal acceleration

A

The acceleration produced by a centripetal force, causing an object to travel with circular motion.

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9
Q

Equation: Centripetal acceleration *

A

a = rω² = v²/ r

a = centripetal acceleration (ms-2)

r = radius (m)

ω = angular velocity (rad s-1)

v = tangential velocity (ms-1)

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10
Q

Equation: Centripetal force

A

F = mv² / r = mrω²

F = centripetal force (N)

r = radius (m)

ω = angular velocity (rad s-1)

v = tangential velocity (ms-1)

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11
Q

Definition: Centripetal force

A

The force providing an object with centripetal acceleration allowing it to have circular motion.

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12
Q

What is the feature of an object in circular motion?

A

It’s velocity is constantly changing meaning it accelerates centripetally due to a centripetal force.

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13
Q

What does rpm mean?

A

Rotations per minute.

Rpm / 60 = frequency

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14
Q

Why must there be a resultant force on an object which is changing direction?

A

It is changing direction so therefore velocity must also be changing. Therefore the object must be accelerating.

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15
Q

Definition: Electric field

A

A region in space where charged particles experience a force.

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16
Q

Definition: Electric field strength

A

The force per unit charge acting on a positive test charge at a point in space.

(NC⁻¹)

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17
Q

Equation: Electric field strength *

A

E = F / Q

E = electric field strength (NC-1)

F = force (N)

Q = charge (C)

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18
Q

What do the features of field lines represent?

A

Closeness = strength of field.

Direction = direction of force on positive charge.

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19
Q

What are the rules for drawing radial field lines?

A
  • Cannot cross
  • Start on positive
  • End on negative
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20
Q

What are uniform fields?

A

A region of space where a force acts on a charged particle, and the force is constant at all points.

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21
Q

What are the rules for drawing uniform field lines?

A
  • Parallel.
  • Equally spaced.
22
Q

Equation: Electric field strength between plates *

A

E = V / d

E = electric field strength (NC-1)

V = pd across plates (V)

d = distance between plates (m)

23
Q

Definition: Electric potential

A

The work done per unit charge to move a small positive test charge from one point to another.

(JC⁻¹ = V)

24
Q

Equation: Electric potential

A

V = EPE / Q

V = electric potential (JC -1 or V)

EPE = electric potential energy (J)

Q = charge (C)

25
Q

Definition: Equipotential

A

Lines perpendicular to field lines at which potential is constant, meaning no work needs to be done to move a charge along them.

26
Q

How do you increase electrical potential energy?

A

Apply a force to a charge, to do work on it and move it against the electrostatic force.

27
Q

Equation: Coulomb’s law *

A

F = kQ₁Q₂ / r²

k = 1 / 4πε₀

F = Q₁Q₂ / 4πε₀r²

F = electrostatic force between two charged particles (N)

Q₁ = charge of first particle (C)

Q₂ = charge of second particle (C)

ε₀ = permittivity of free space (8.85x10⁻¹² Fm⁻¹)

r = distance between two charges (m)

28
Q

Equation: Electric field strength around a point charge

A

E = Q / 4πε₀r²

E = electric field strength (NC-1)

Q = charge of particle (C)

ε₀ = permittivity of free space (8.85x10⁻¹² Fm⁻¹)

r = distance between two charges (m)

29
Q

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

A

Similarities:

• They both obey the inverse square law.

Differences:

• Gravitational fields only have forces of attraction, electrical fields have forces of repulsion too.

30
Q

How can a uniform electric field be demonstrated in a laboratory?

A

Apply a potential difference across two parallel plates, and fire a beam of electrons between them.

31
Q

What is a test charge?

A

A small, positively charged particle.

32
Q

Why is energy needed to push two positively charged particles together?

A

Because work must be done against the repulsion force produced between the charges in order to move the particle.

33
Q

What is a capacitor?

A

A component which stores charge, consisting of two conductors seperated by a dielectric.

34
Q

What is a dielectric?

A

An insulator.

35
Q

What is a feature of a capacitor?

A

The charge stored is directly proportional to the pd across it.

36
Q

Definition: Capacitance

A

The charge stored per unit pd.

(F)

37
Q

Equation: Capacitance *

A

C = Q / V

C = capacitance (F)

Q = charge (C)

V = potential difference (V)

38
Q

How do you work out the energy stored on a capacitor from a Q-V graph?

A

Energy stored is equal to the area below a Q-V graph.

39
Q

Equation: Energy stored in a capacitor *

A

W = 1 /2QV

W = energy stored on capacitor (J)

Q = charge stored (C)

V = potential difference (V)

40
Q

How is energy stored on a capacitor?

A

It is stored as electric potential energy on the charged capacitor.

41
Q

Howis energy released from a capacitor?

A

All of the energy stored on the capacitor is dissipated as heat as the discharge current passes through the resistor.

42
Q

What does the current-time graph look like when charging or discharging a capacitor?

A
43
Q

What do the graphs for charge and pd against time look like when discharging a capacitor?

A

Exponential decay

44
Q

What do the graphs for charge and pd against time look like when discharging a capacitor?

A

Exponential decay

45
Q

How do you make charge from a capacitor flow for longer?

A
  • Store more charge on the capacitor.
  • Decrease the rate at which the capacitor discharges.
46
Q

Definition: Time constant

A

During discharge, the charge on a capacitor falls to 63% of its original value in a time equal to one time constant.

47
Q

Equation: Time constant

A

τ = RC

τ = time constant (s)

R = resistance (Ω)

C = capacitance (F)

48
Q

Equations: Decay equations *

A

Q = Qo e-t/RC

V = Vo e-t/RC

I = Io e-t/RC

Q = charge remaining on capacitor (C)

Qo = initial charge on capacior after 0 seconds (C)

t = time (s)

R = resistance (Ω)

C = capacitance (F)

49
Q

How do you identify the initial charge Qo from a charge-time graph?

A

Find the charge at t=0 (where the line intercepts the y-axis)

50
Q

What happens when you close the switch in a capacitor circuit?

A

Current flows through the circuit and the capacitor charges up causing one plate to be positively charged, and the other to be negatively charged of the same magnitude.

51
Q

What happens to the flow of current as a capacitor charges?

A

1) Initially, the pd across the capacitor is zero meaning there is maximum current.
2) However, as charge builds up on the capacitor, its pd increases, and therefore the pd of the resistor decreases, reducing the current.
3) Eventually, the pd of the capacitor reaches a maximum and the pd of the resistor becomes zero. Therefore, no current flows through the circuit.

52
Q

What happens f you move the plates of a capcitor closer together?

A

Capacitance increases meaning the charge on thecapacitor increases as Q = CV

A2 Physics (14 decks)

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