topic 7

Question 1

what is an electric field

Answer 1

a region where a charged particle experiences a force

Question 2

electric field strength formula

Answer 2

E = F / Q

Question 3

define electric field strength

Answer 3

force per unit charge experienced by an object in an electric field

Question 4

electric field strength in uniform and radial field

Answer 4

• uniform - E is constant
• radial - E varies

Question 5

Coulomb’s law

Answer 5

• the magnitude of the force between two point charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them

Question 6

Coulomb’s law formula

Answer 6

F = Q1Q2 / 4πε0r^2

Question 7

what makes the force repulsive?

Answer 7

• if charges have the same sign

Question 8

what makes a force attractive?

Answer 8

• if charges are oppositely signed

Question 9

what do point charges form

Answer 9

a radial field

Question 10

electric field strength in a radial field formula

Answer 10

E = Q / 4πε0r^2

Question 11

absolute potential energy

Answer 11

• the potential energy per unit charge at a positive point charge at that point in the field

Question 12

at which point is the absolute magnitude of electric potential the highest?

Answer 12

• at the surface of the charge
• as distance from charge increases, potential decreases
• hence, electric potential at infinity is zero

Question 13

positive charge - what is potential?

Answer 13

• potential is positive
• charge is repulsive

Question 14

negative charge - what is potential?

Answer 14

• potential is negative
• charge is attractive

Question 15

how can you form an electric field

Answer 15

• two parallel plates
• potential difference across them

Question 16

how to calculate electric field strength formed between the plates

Answer 16

E = V / d
- V is the p.d. across the plates
- d is the distance between the two plates

Question 17

electric potential in a radial field

Answer 17

V = Q / 4πε0r^2

Question 18

what is electric potential difference

Answer 18

• the energy needed to move a unit charge between two points

Question 19

what do the field lines show

Answer 19

• the direction of force acting on a positive charge

Question 20

field lines - uniform vs radial

Answer 20

• uniform - same electric force is exerted everywhere (equally spaced parallel lines)
• radial - depends on the distance between the two charges

Question 21

what does the distance between the field lines represent

Answer 21

the magnitude of the force

Question 22

what surfaces to electric fields have

Answer 22

equipotential surfaces

Question 23

what happens when a charge moves along an equipotential surface

Answer 23

• no work is done
• this is because the potential on an equipotential surface is the same everywhere

Question 24

magnetic flux denisty

Answer 24

measure of the strength of the field

Question 25

magnetic flux linkage

Answer 25

magnetic flux multiplied by the number of turns in a coil

Question 26

Flehming's left hand rule - charged particles

Answer 26

- thumb - direction of motion/force - first finger - direction of field - second finger - direction of conventional current

Question 27

what direction is the force

Answer 27

- force is always perpendicular to the motion of travel - hence charged particles travel in a circular motion in magnetic fields

Question 28

calculating force on a charged particle

Answer 28

F = Bqv sinθ

Question 29

Flehming's left hand rule - current carrying conductors in a magnetic field

Answer 29

- motion - field - current

Question 30

what happens when a current passes through a wire

Answer 30

- a magnetic field is induced - field lines of induced magnetic field form concentric rings around the wire

Question 31

calculating magnitude of force on the wire

Answer 31

F = BIl sinθ

Question 32

how is an emf induced in a coil

Answer 32

- conducting rod moves relative to magnetic field - electrons in rod experience a force - build up of electrons on one side of the rod - e.m.f. is induced

Question 33

when is a current induced

Answer 33

- if the coil forms a complete circuit a current is induced

Question 34

Lenz's law

Answer 34

direction of induced current is such as to oppose the motion causing it

Question 35

induction of emf in a coil through the change in current of another coil

Answer 35

- magnetic field is induced around a coil current-carrying wire - if the current through the wire changes, magnetic field will also change - emf will be induced in a second coil - induced emf in the second coil is proportional to the change in current in the first coil - if second coil completes a full circuit, a current is also induced

Question 36

factors affecting the emf induced in the second

Answer 36

- magnetic flux density - distance between the two coils - number of turns in the second coil - cross-sectional area of second coil - time taken for change in current