5.2 Charge and Field

Question 1

Any object with charge has what around it?

Answer 1

An electric field.

Question 2

What is an electric field?

Answer 2

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

Question 3

What is electric charge measured in?

Answer 3

Coulombs, C.

Question 4

Oppositely charged particles…

Answer 4

Attract each other.

Question 5

Like charges…

Answer 5

Repel.

Question 6

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

Answer 6

It will experience a force.

Question 7

What is Coulomb’s law?

Answer 7

F = (k Q1 Q2) / r^2

Question 8

A negative F from Coulombs law means the force is…

Answer 8

Attractive.

Question 9

Compare the force on Q1 with the force on Q2.

Answer 9

Equal but opposite.

Question 10

What is electric field strength defined as?

Answer 10

Force per unit charge.

Question 11

What is electric field strength, E?

Answer 11

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.

Question 12

What are the units of field strength?

Answer 12

NC^-1

Question 13

What kind of field does a point charge have?

Answer 13

A radial field.

Question 14

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

Answer 14

E = F / q

Question 15

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

Answer 15

r^2

Question 16

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

Answer 16

E = kQ/r^2

Question 17

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

Answer 17

Towards Q.

Question 18

What is the definition of electric potential energy, EPE?

Answer 18

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.

Question 19

Give the equation for EPE in a radial field.

Answer 19

EPE = kQq/r

Question 20

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

Answer 20

Positive.

Question 21

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

Answer 21

The electric force at that point.

Question 22

What is electric potential defined as?

Answer 22

Potential energy per unit charge.

Question 23

What is the symbol for electric potential?

Answer 23

V

Question 24

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

Answer 24

V = EPE / q

Question 25

What is electric potential, V, measured in?

Answer 25

Volts

Question 26

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

Answer 26

V = kQ/r

Question 27

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

Answer 27

The field strength at that point.

Question 28

Describe the field strength in a uniform electric field.

Answer 28

The same everywhere in the field.

Question 29

How can you produce a uniform field?

Answer 29

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

Question 30

Give the equation for field strength in a uniform field.

Answer 30

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

Question 31

What are the units of E?

Answer 31

Vm^-1

Question 32

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

Answer 32

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

Question 33

Describe the equipotential surfaces in a uniform field.

Answer 33

Parallel to the plates and perpendicular to the field lines.

Question 34

What happens to an object moving in a fluid?

Answer 34

It experienced a viscous drag force.

Question 35

In which direction does viscous drag force act?

Answer 35

The opposite direction to velocity.

Question 36

Give the equation for Stoke's law.

Answer 36

F = 6πηrv

Question 37

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

Answer 37

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

Question 38

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

Answer 38

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

Question 39

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

Answer 39

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

Question 40

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

Answer 40

Weight of the drop and viscous force from the air.

Question 41

Give the equation for weight of the oil drop.

Answer 41

W = 4/3 π r^3 ρ g

Question 42

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

Answer 42

r^2 = 9ηv / 2ρg

Question 43

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

Answer 43

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

Question 44

Give two equations for electric field strength.

Answer 44

E = F/q = V/d

Question 45

Combine the two equations for electric field strength.

Answer 45

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

Question 46

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

Answer 46

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

Question 47

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

Answer 47

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

Question 48

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

Answer 48

F = qvB

Question 49

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

Answer 49

The opposite direction to the movement of the electron.

Question 50

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

Answer 50

Perpendicular to motion.

Question 51

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

Answer 51

Circular.

Question 52

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

Answer 52

r = mv/Bq