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OCR Physics B Unit 5 > 5.2 Charge and Field > Flashcards

5.2 Charge and Field Flashcards

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1
Q

Any object with charge has what around it?

A

An electric field.

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2
Q

What is an electric field?

A

A region around a charged object that can attract or repel other charges.

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3
Q

What is electric charge measured in?

A

Coulombs, C.

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4
Q

Oppositely charged particles…

A

Attract each other.

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5
Q

Like charges…

A

Repel.

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6
Q

What happens to a changed object if it is placed in an electric field?

A

It will experience a force.

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7
Q

What is Coulomb’s law?

A

F = (k Q1 Q2) / r^2

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8
Q

A negative F from Coulombs law means the force is…

A

Attractive.

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9
Q

Compare the force on Q1 with the force on Q2.

A

Equal but opposite.

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10
Q

What is electric field strength defined as?

A

Force per unit charge.

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11
Q

What is electric field strength, E?

A

The force that a charge of +1C would experience if placed in the electric field. E is a vector pointing in the direction that the charge would move.

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12
Q

What are the units of field strength?

A

NC^-1

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13
Q

What kind of field does a point charge have?

A

A radial field.

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14
Q

Give the equation for field strength in terms of force, F on a test charge, q.

A

E = F / q

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15
Q

In a radial field, what is E inversely proportional to?

A

r^2

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16
Q

Give the equation for E in a radial field, distance r away from the point charge Q.

A

E = kQ/r^2

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17
Q

Where would the field lines point in a field caused by a negative Q?

A

Towards Q.

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18
Q

What is the definition of electric potential energy, EPE?

A

The worst hat would need to be done to move a small charge, q, from infinity to a distance, r, away from a point charge.

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19
Q

Give the equation for EPE in a radial field.

A

EPE = kQq/r

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20
Q

What would the sign of the EPE be for a test charge in a repulsive field be?

A

Positive.

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21
Q

What does the gradient of an EPE vs r graph tell you?

A

The electric force at that point.

22
Q

What is electric potential defined as?

A

Potential energy per unit charge.

23
Q

What is the symbol for electric potential?

A

V

24
Q

Give the equation for V in terms of EPE and q.

A

V = EPE / q

25
Q

What is electric potential, V, measured in?

A

Volts

26
Q

What is the equation for electric potential in a radial field?

A

V = kQ/r

27
Q

What does the gradient of tangent to a V against r graph tell you?

A

The field strength at that point.

28
Q

Describe the field strength in a uniform electric field.

A

The same everywhere in the field.

29
Q

How can you produce a uniform field?

A

Connect two parallel plates to the opposite poles of a battery.

30
Q

Give the equation for field strength in a uniform field.

A

E = V/d where V is the p.d. between the plates and d is the distance between the plates.

31
Q

What are the units of E?

A

Vm^-1

32
Q

Describe the field lines in a uniform field between two plates.

A

Parallel to each other, going from the more positive plate to the other plate.

33
Q

Describe the equipotential surfaces in a uniform field.

A

Parallel to the plates and perpendicular to the field lines.

34
Q

What happens to an object moving in a fluid?

A

It experienced a viscous drag force.

35
Q

In which direction does viscous drag force act?

A

The opposite direction to velocity.

36
Q

Give the equation for Stoke’s law.

A

F = 6πηrv

37
Q

What do η, r and v represent in Stoke’s law?

A

η is the viscosity of the fluid. r and v are the radius and velocity of the object respectively.

38
Q

What did the atomiser do in Millikan’s Oil-Drop experiment?

A

Created a fine mist of oil drops, charged by friction.

39
Q

What was the purpose of the charged plates in Millikan’s Oil-Drop experiment?

A

They produced a field that exerted a force on the charged particles.

40
Q

Give two forces acting on an oil drop before the electric field is switched on.

A

Weight of the drop and viscous force from the air.

41
Q

Give the equation for weight of the oil drop.

A

W = 4/3 π r^3 ρ g

42
Q

Rearrange Stoke’s law and the weight of the oil drop to make r^2 the subject of the equation.

A

r^2 = 9ηv / 2ρg

43
Q

Give two forces acting on a stationary oil drop after the electric field had been turned on.

A

Weight of the drop and force due to the uniform electric field.

44
Q

Give two equations for electric field strength.

A

E = F/q = V/d

45
Q

Combine the two equations for electric field strength.

A

F = QV/d where Q is the charge on the oil drop.

46
Q

What was Millikan’s Oil-Drop experiment evidence for?

A

Charge is quantised since the charge on any oil drop was a multiple of -1.6 x 10^-19

47
Q

Give three equations used to derive the equation for force on a singly charge particle moving through a magnetic field.

A

F = BIl, I = q/t, t = l/v

48
Q

Give the equation for force on a singly charged particle moving in a magnetic field.

A

F = qvB

49
Q

In which direction is current when an electron travels in a magnetic field?

A

The opposite direction to the movement of the electron.

50
Q

Which direction does the force act on a charged particle moving through a magnetic field?

A

Perpendicular to motion.

51
Q

Describe the motion of a charged particle moving through a perpendicular magnetic field.

A

Circular.

52
Q

Rearrange two equations and make r the subject to find the radius of the circular motion of a particle travelling in a magnetic field.

A

r = mv/Bq

OCR Physics B Unit 5 (4 decks)

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