Electric fields

Question 1

What is a free electron

Answer 1

An electron that can move about and isn’t attracted to a singular atom

Question 2

First step to charge an object

Answer 2

• Must first be isolated from the earth - otherwise any charge gained is neutralised by electrons transferring between the object and the earth

Question 3

Why are certain insulators easy to charge

Answer 3

Their surface atoms can easily gain/lose electrons e.g. perspex and polythene

Question 4

Electric field line

Answer 4

Path a positive test charge follows

Question 5

Field lines for oppositely charged points

Answer 5

Concentrated at the points - positive test charge flows along curved path to negative

Question 5

Field lines for point to oppositely charged plate

Answer 5

Concentrated at the point, but at right angles where they meet the plate. Field is stronger where lines are concentrated

Question 5

Field lines for oppositely charged plates

Answer 5

Run parallel (slightly eccentric)
meet plates at right angles (apart from edges)
Field is uniform between plates as field lines are parallel

Question 6

Electric field strength

Answer 6

Force per unit charge on a positive test charge at that point

(Newtons per coulomb)

Question 7

Properties of electric field strength

Answer 7

In same direction as electric field if object is positive (viceversa)

A vector

+Test charge must be negligible compared to 1 coulomb as it would affect the charges that cause the field - affecting electric field strength

Question 8

Proportionalities of E between parallel plates

Answer 8

depends on concentration of charge on surface of plates, proportional to charge per unit area

Question 9

Does work done in moving a particle in a field depend on the path it takes?

Answer 9

No

Question 10

Using eq to see if force is attractive or repulsive

Answer 10

Multiplying 2 positive charges gives a positive force - repulsive as it acts in the same direction as increasing separation

Question 11

Adding field strength vs potentials

Answer 11

Field - vector sum as field strength is a vector, potential is normal sum as is a scalar

Question 12

Why insulators can’t conduct

Answer 12

Electrons are attached to individual atoms (not free).

Question 13

What should thread be made out of in shuttling ball exp

Answer 13

Insulator, otherwise wld divert charge

Question 14

Calculating current in shuttle ball exp

Answer 14

I=Qf where f is frequency ball shuttles back and forth at (1/time taken to go there AND back)

Question 15

Electric force on a neutron

Answer 15

Zero - chargeless

Question 16

Air as an insulator

Answer 16

Can be treated as an insulator (gases are poor conductors of heat+lack of free e). However in strong electric fields, molecules can be ionised by having es removed.
Lighting where a cloud becomes more and more charged and E in air becomes stronger - charge discharges through air to Earth.

Question 17

How a lightning rod works

Answer 17

Provides path of least resistance for charged cloud - tallest and most conductive thing, as opposed to discharging through air to the ground - less conductive + needs to travel greater length.

When a charged cloud is overhead, strong E at the tip of rod - ionises air molecules around it, discharge charge from cloud to rod then through earth.

Question 18

Uniform field

Answer 18

Same magnitude + direction everywhere in field

Question 19

Field factors

Answer 19

E field exists near any charged body, greater the charge, stronger the field. More concentrated charge is on surface, stronger E at surface.

Field between 2 parallel plates depends on concentration on charge at the surface, spread evenly across. Can be shown that E is proportional to Q/A where A is the area.

Q/A = Eε0 - Don’t need to kno eq

Question 20

How a Van de Graff generator works

Answer 20

Charge created when rubber belt rubs against a pad - is carried up to metal dome. Potential difference between dome and earth builds up as charge accumulates until sparking occurs.

Spark transfers energy from dome, work must be done to charge the dome since a force is required to move charge up the belt to the dome (has same charge).

In general, work must be done to move a charged object closer to a like charge.

Question 21

Electric potential

Answer 21

For 2 like charges X and Y

Electric potential increases from 0 as X is moved from infinity towards Y. Work done to overcome repulsion.

At a point in field, defined as work done per unit positive charge on another positive test charge in moving charge from infinity to that point.

By definition, position of 0 potential is infinity

W=Q(V2-V1)

Question 22

Equipotentials

Answer 22

Surfaces of constant potential, no work done moving along them for a test charge, at right angles to lines of force of field

Question 23

Potential gradient

Answer 23

Change in potential per unit distance.
In non uniform field, gradient varies with position - closer together equipotentials are, greater potential gradient (at right angles to equipotentials)

If field is uniform, equipotentials are equally spaced

E=-V/d

Question 24

Why point charges are convenient

Answer 24

Where distances under consideration are much larger than diameter of object

same idea as a distant star - diameter negligible compared to distance

Question 25

Summing electric field strengths

Answer 25

On a line
Et = E1+E2 (prove)

perpendicular
Et^2=E1^2+E2^2

Question 26

Concentration of charge on objects

Answer 26

For a curved object/irregular, charge concentrated at sharpest/most curved points.

For a plate, concentrated at surface

Why charge gathers at the tip of a lightning rod when a charged cloud is overhead. Also why fatal accidents can occur if conducting rod is held near a high voltage cable

Question 27

What makes an object a perfect conductor

Answer 27

If the electric field strength inside is 0

Question 28

F and E def

Answer 28

F arrow = qE arrow
Force and E in same direction

Question 29

Why equipotentials are negative for a negative point charge

Answer 29

Electric potential is taken to be zero at infinity.
A positive test charge would gain potential
energy as it was moved away from the negative charge and towards infinity.

Question 30

Why an electron getting repelled from a negative changes its Ek, as well as de broccoli wavelength

Answer 30

• The electron loses potential energy as it
  moves away from Q.
• As it moves away its speed will increase.
• Its de Broglie wavelength will decrease…
• because its momentum will increase.

repulsive force
accelerates the electron, causing its
kinetic energy to increase and its
potential energy to decrease.