chapter 17 electric field

Question 1

what is an electric field

Answer 1

a region in which a charged particle experiences a force

Question 2

in a uniform field what is the equation linking electric field strength, potential difference and separation of charges

Answer 2

E = V  /  D                                                                                  
E = electric field strength,   V = potential difference,   
D = separation of charges

Question 3

in a uniform field what is the equation linking electric field strength, electric force and charge

Answer 3

E = F  /  Q
E = electric field strength,    F = electric force,   
q = charge

Question 4

what is the angle between equipotentials and field lines

Answer 4

Equipotentials are always perpendicular to the field lines.

Question 5

what is the equation for electric field strength where the field lines curve, and what is the minus for

Answer 5

E = -dV / dr where r is the distance along the field line

the minus shows the direction of the electric field is in the opposite direction to the increase in V

Question 6

what is Coulomb’s law used for

Answer 6

to calculate the electric force on a charge, as well as to calculate the force of repulsion or attractiveness between 2 point charges.

Question 7

what are the requirements for modelling a charged object as a point charge

Answer 7

is the charged object is a sphere with evenly distributed charge, so it will act as if all of its charge is at its centre so that it can be modelled as a point charge

Question 8

what is the inverse square law between 2 point charges

Answer 8

1 / r^2, the further apart the charges, the weaker the forces between them

Question 9

what can be said about the forces on 2 interacting point charges

Answer 9

the force on one point charge is the equal and opposite of the force on the other one

Question 10

if F(electric) is positive in the coulomb’s law equation, what can be said about the 2 point charge’s interactions?

Answer 10

if the charges on the point charge’s are the same, there will be repulsive forces and so F will be positive

Question 11

if F(electric) is negative in the coulomb’s law equation, what can be said about the 2 point charge’s interactions?

Answer 11

if the charges on the point charge’s are opposing each other, there will be attractive forces between them and so F will be negative

Question 12

define electric field strength

Answer 12

force per unit charge

Question 13

what is coulomb’s law

Answer 13

the force between 2 charges q and Q is directly proportional to each of the charges, and inversely proportional to the square of the r, the distance between the centres of the 2 spheres

Question 14

when is the coulomb’s equation negative and when is it positive

Answer 14

negative for attractive forces, positive for repulsive forces

Question 15

what is the value of electric potential V for a charge sphere where there’s a radial distance of infinity

Answer 15

as r goes to infinity, electric potential goes to 0

Question 16

define electric potential energy

Answer 16

the work done to move a charge q from infinity to a distance r away from a point charge Q

Question 17

how does a point charge q gain potential energy in an attractive field

Answer 17

the charge q gains energy as r increases, because you need to do work overcome the attractive forces between the 2 attracted point charges so that you can move them further away

Question 18

how does a point charge q gain potential energy in an repulsive field

Answer 18

the charge q gains energy as r decreases because you have to do work against the repulsion to bring q closer to Q

Question 19

what is the equation to calculate the charge on an oil drop in Millikan’s experiment

Answer 19

q = mgd  /  v
q = charge
m = mass in kg
d is distance in metres
v = voltage

Question 20

what are the 2 equations that are equal when an oil drop is suspended between 2 charged plates

Answer 20

F = qE,  w = mg,
qE = mg

Question 21

define equipotential surface

Answer 21

a surface where the electric potential is constant

Question 22

what was the milikan’s oil experiment used to determine

Answer 22

the charge on an electron e

Question 23

what is the process in the milikan’s oil drop experiment, and what were the results

Answer 23

oil drops fall through a hole in the top plate, the positively charged plate, where they were ionised between 2 oppositely charged plates
the pd was then adjusted until a drop was suspended so that the uniform gravitational and electrical fields were equal, mg = qe
from this equation we can work out the charge on an electron with the different pd results
All of the values for q were multiples of 1.6 * 10^-19C.
this gave us the value of charge for electron as -1.6*10^-19C

Question 24

evidence for discreteness of charge on

electron

Answer 24

milikan’s results showed that the charge on an electron was 1.6 * 10^-19C, and as all his results were whole number multiples of this charge exists in packets of 1.6 * 10^-19C

Question 25

what does the area under an electric force, F-electric graph between 2 values on the x-axis give

Answer 25

change in electrical potential energy, E

Question 26

how do you work out change in electric potential energy, E from an electric force, F-electric, graph

Answer 26

find the area under the electric force graph between 2 values on the x-axis to find the change in potential energy

Question 27

what does the gradient of an electric potential, V, give

Answer 27

the electric field strength, E-electric

Question 28

how do you work out electric field strength, E-electric, from a graph of electric potential, V-electric

Answer 28

the gradient of the electric potential graph gives us the electric field strength

Question 29

what does the area under an electric field strength, E-electric, graph between 2 points give

Answer 29

The area under an electric field strength graph gives the electric potential, V-electric

Question 30

how do you work out electric potential, v-electric from a graph of electric field strength, E-electric

Answer 30

find the area under the graph of electric field strength between 2 values on the x-axis to give electric potential

Question 31

what does the gradient of an electrical potential energy, E, graph give

Answer 31

the electric force, F-electric

Question 32

how do you work out electric force from a graph of electrical potential energy

Answer 32

the gradient of an electric potential energy gives the electric force

Question 33

how do you work out electric force for the oil drop in Millikan’s experiment

Answer 33

F-electric = q V / d

Question 34

what path do charged particles in a magnetic field take

Answer 34

deflected in a circular path

Question 35

An infinite distance from Q, what potential energy does a charged particle q have?

Answer 35

Zero potential energy

Question 36

Describe the relationship between potential energy and distance in a repulsive field? What sign are the charges of q and Q?

Answer 36

Q and q are both positive (or both -ve). You have to do work against the repulsion to bring q closer to Q. The charge q gains potential energy as r decreases

Question 37

Describe the relationship between potential energy and distance in a attractive field? What sign are the charges of q and Q?

Answer 37

Q is negative and q is positive. The charge q gains potential energy as r increases (epe becomes less negative)

Question 38

Charged particles in a magnetic field are deflected in a [ ] path

Answer 38

circular

Question 39

Describe the motion of charged particles in a magnetic field

Answer 39

By Fleming’s left hand rule the force on a moving charge in a magnetic field is always perpendicular to its direction of travel

Question 40

Different charged particles will have paths with different [ ] - the higher the ratio of m go q, the [ ] the radius of the path

Answer 40

Radii

Larger

Question 41

Why does a current carrying wire experience a force in a magnetic field?

Answer 41

The negatively charge electrons are affected by magnetic fields

Question 42

What sign is electric potential for repulsive and attractive forces?

Answer 42

Repulsive force = positive potential

Attractive force = negative potential

Question 43

Why is there a negative sign in the relationship between electric field strength and the rate of change of electrical potential with distance?

Answer 43

To increase the charge’s potential, and potential energy, you have to do work against the force - they ‘act’ in the opposite direction

Question 44

Explain the effect of moving a unit charge between two distances R (bigger) and r. Use force, graphs and energy in your answer

Answer 44

You change the charge’s electric potential energy when you move it between two distances. In order to do this, you have to apply a force and do work.

The force applied is equal to the electric force. For a point charge q, you can plot the electric force against distance from the charge Q which is producing the electric field.

This is an inverse square law and the area under the curve between R and r gives the change in electric potential energy

Question 45

similarities between electric and magnetic

Answer 45

have a force per unit:
force per unit charge for electic
force per unit mass for grav

follow the inverse square law

Question 46

differences between electric and magnetic

Answer 46

electric can be attractive or repulsive, grav can only be attractive

objects can be shielde3d from electric fields, can’t be shielded from grav

electric field strength depends on medium, weak over air etc
grav field strength independent of medium