Electric Field

Question 1

Defn of electric field

Answer 1

Electric field is a region of space in which a charge placed in that region experiences an electric force

Question 2

Defn of electric field strength

Answer 2

Electric field strength at a pt is defined as the electric force exerted per unit positive charge placed at that pt

Question 3

Eqn for electric field strength, E

Unit, vector/scalar?

Answer 3

E = F/q

N C^-1 OR V m^-1, vector

Question 4

How to determine direction of electric force and direction of electric field strength?

Answer 4

Electric force: Depends on whether charge is +ve or -ve. If it’s -ve, then it is be opp from the direction of electric field strength - towards region of higher electric potential

Electric field strength: Higher potential to low potential = electric force acting on a +ve charge at that pt

Question 5

Defn of Coulomb’s Law (Hint: eqn of F)

Answer 5

Coulomb’s law states that the force b/w 2 pt charges is proportional to the product of the charges and inversely proportional to the sq of the distance b/w them.

Question 6

Eqn of Force

Answer 6

F = Q1Q2/4πε0r^2

Q1, Q2 - in Coulomb, C
ε0 = 8.85 X 10^-12 F m^-1

Question 7

Eqn for electric field strength due to a pt charge Q

Answer 7

E = Q / 4πε0r^2

Unit: N c^-1

Question 8

When considering the electric force acting on charge q, which field strength(s) to consider?

Answer 8

Only the field strength due to the other charge Q is considered

Question 9

The direction of field strength due to a charged particle depends on…

Answer 9

the sign of its charge:
it pts radially away from a +ve pt charge & radially towards a -ve pt charge

By defn, they are the directn of the electric forces acting on a +ve test charge placed at that pt

Question 10

How to find resultant field strength at a pt due to more than 1 charge

Answer 10

1st charge is just there u only start calc from 2nd one

found from vector sum of the indiv electric field strength due to each charge at that pt

Question 11

5 general guidelines for drawing field lines

+ for pt charge how many must u draw

Answer 11

1. Direction of the field line indicates the direction of the resultant force acting on a small +ve test charge
2. Pt from higher to lower potential
3. never cross one another
4. density of the field lines represents field strength
5. perpendicular to the surface of the source//

Draw 8 symmetrical lines - use ruler & compass

Question 12

How to draw field lines b/w 2 charges

Answer 12

outwards for +ve charge

inwards for -ve charge

Question 13

Defn of electric potential energy, U of a charge at a pt in an electric field
Unit
Scalar/vector?

Answer 13

wk done by an external force in in bringing the charge from infinity to that pt
Joule (J)
Scalar

Question 14

Defn of electric potential, V at a pt in an electric field
Unit
Scalar/vector

Answer 14

wk done per unit +ve charge by an external force in bringing a small test charge from infinity to that pt
Volt (V) or J C^-1

Question 15

Eqn for electric potential

Answer 15

V = U/q

Also, U = qV

Question 16

Eqn for potential diff n what is that (layman defn)

Eqn for potential energy

Answer 16

ΔV = Vfinal - Vinitial
Potential diff is the change in electric potential
ΔU = qΔV

Question 17

Defn of electron-volt

Answer 17

Energy gained by an e-, which carries a charge of 1.60X10^-19 C, when it is accelerated through a potential difference of 1V

Basically the charge of an e

Question 18

Eqn for potential energy of the system of 2 pt charges and how is the sign determined?

Answer 18

U = Q1Q2 / 4πε0r

Sign determined by the signs of Q1 n Q2

Question 19

Eqn for electric potential due to a pt charge Q (2)

Answer 19

V = U/q = Q / 4πε0r

Question 20

Graphical r/s of FE (eqn)

Answer 20

FE = - dU/dr

From a U-r graph, the -ve of the grad of the tangent at a pt on the graph gives the electric force at that pt

Question 21

What does the sign in the eqn for the r/s of FE… mean

Answer 21

electric force pts in the direction of decreasing electric potential energy

Question 22

Eqn for graphical r/s b/w E & _

Answer 22

E = - dV/dr

The -ve of the grad of the tangent at a pt on the graph gives the electric field strength at that pt

Question 23

Given that the electric potential at the 3rd corner X = -8.99kV, determine the change in electric potential energy of the system if a charge Q3 of +1.0µC is brought to corner X, determine the change in electric potential energy of the system.

Answer 23

1st method:
ΔU = qΔV = Q3 (Vf - Vi) = (+1.0 X 10^-6)(-8992-0) = -8.99 X 10^-3 J

2nd method:
Find initial potential energy of Q1 & Q2
Find final potential energy of Q, Q2 & Q3: Uf = U12 + U23 + U31 [using the Q1Q2 / 4πε0r]
ΔU = Uf - Ui

Question 24

V-r & E-r graphs for single pt charges: +ve & -ve charges

Answer 24

Check school notes pt 17

Start with scalar then move to vector

Scalar (V) follow charge sign
vector follow direction of field lines to the charge - +ve away from itself -ve towards

Question 25

Link wk done by Fe to EPE

when is wk done +ve/-ve?

Answer 25

When the charge is travelling in the direction of field lines, W by Fe = +ve, Wext force = -ve, EPE decreases

Vice versa

EPE depends on Wext force cuz that’s the defn