Electric Field Flashcards
Defn of electric field
Electric field is a region of space in which a charge placed in that region experiences an electric force
Defn of electric field strength
Electric field strength at a pt is defined as the electric force exerted per unit positive charge placed at that pt
Eqn for electric field strength, E
Unit, vector/scalar?
E = F/q
N C^-1 OR V m^-1, vector
How to determine direction of electric force and direction of electric field strength?
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
Defn of Coulomb’s Law (Hint: eqn of F)
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.
Eqn of Force
F = Q1Q2/4πε0r^2
Q1, Q2 - in Coulomb, C
ε0 = 8.85 X 10^-12 F m^-1
Eqn for electric field strength due to a pt charge Q
E = Q / 4πε0r^2
Unit: N c^-1
When considering the electric force acting on charge q, which field strength(s) to consider?
Only the field strength due to the other charge Q is considered
The direction of field strength due to a charged particle depends on…
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
How to find resultant field strength at a pt due to more than 1 charge
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
5 general guidelines for drawing field lines
+ for pt charge how many must u draw
- Direction of the field line indicates the direction of the resultant force acting on a small +ve test charge
- Pt from higher to lower potential
- never cross one another
- density of the field lines represents field strength
- perpendicular to the surface of the source//
Draw 8 symmetrical lines - use ruler & compass
How to draw field lines b/w 2 charges
outwards for +ve charge
inwards for -ve charge
Defn of electric potential energy, U of a charge at a pt in an electric field
Unit
Scalar/vector?
wk done by an external force in in bringing the charge from infinity to that pt
Joule (J)
Scalar
Defn of electric potential, V at a pt in an electric field
Unit
Scalar/vector
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
Eqn for electric potential
V = U/q
Also, U = qV
Eqn for potential diff n what is that (layman defn)
Eqn for potential energy
ΔV = Vfinal - Vinitial
Potential diff is the change in electric potential
ΔU = qΔV
Defn of electron-volt
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
Eqn for potential energy of the system of 2 pt charges and how is the sign determined?
U = Q1Q2 / 4πε0r
Sign determined by the signs of Q1 n Q2
Eqn for electric potential due to a pt charge Q (2)
V = U/q = Q / 4πε0r
Graphical r/s of FE (eqn)
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
What does the sign in the eqn for the r/s of FE… mean
electric force pts in the direction of decreasing electric potential energy
Eqn for graphical r/s b/w E & _
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
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.
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
V-r & E-r graphs for single pt charges: +ve & -ve charges
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
Link wk done by Fe to EPE
when is wk done +ve/-ve?
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
