Electric Fields Flashcards
Describe Coulomb’s law.
The force between two point charges:
Can be attractive or repulsive.
Is directly proportional to the product of the charges.
Is inversely proportional to the square of the distance between them
Define electric field strength in words.
Force per unit charge.
On a positive test charge due to an electric field.
Give the equation that defines electric field strength.
F = EQ
Give two units for electric field strength.
N C-1
V m-1
Is electric field strength scalar or vector? What significance does this have when combining field strengths?
Vector quantity.
Direction must be considered when combining.
What do electric field lines show?
The direction of the force,
that a positive test charge would feel in an electric field.
Sketch a radial electric field around a positive and negative point charge and describe the key features.
Field strength decreases with distance.
Shown by field lines getting further apart.
Sketch a uniform electric field between two parallel charged plates and describe the key features.
Field lines are:
Parallel to each other.
Perpendicular to plates.
From more positive to more negative plates.
Equally spaced as electric field strength is equal at all points.
Give the equation for electric field strength in a uniform field.
E = V / d
An electron enters a uniform field at a constant horizontal velocity perpendicular to the field lines. Explain the path of the electron.
Electrostatic force acts vertically upwards at all points.
Causes vertical velocity to accelerate upwards.
No horizontal force so horizontal velocity is constant.
Moves in a parabolic path towards positive plate.
How can the acceleration of the vertical component be obtained?
F = EQ = V / d
a = F / m = VQ / md
Define electric potential at a point in an electric field.
Work done per unit charge
On a positive test charge,
To move it from infinity to that point.
Why is electric potential positive for a positive charge?
At infinity, electric potential is defined as zero.
And work needs to be done to move a positive test charge away from infinity (i.e. V increases away from infinity).
Why is electric potential negative for a negative charge?
At infinity, electric potential is defined as zero.
And work needs to be done to move a positive test charge towards infinity (i.e. V increases towards infinity).
Give the equation for calculating work done moving an object in a electric field.
ΔW = QΔV
What are electric equipotentials?
Lines or surfaces that join together all of the points with the same electric potential.
How much work is done moving an object along an equipotential?
Zero as there is no change in electric potential.
What do you know about the direction of equipotentials relative to field lines?
Equipotentials and field lines are always perpendicular.
Explain the spacing of the equipotentials in a radial field.
Get further apart as distance from charge increases.
For a given energy per unit charge, a charge can be moved further, as field strength decreases.
Explain the spacing of the equipotentials in a uniform field.
Equally spaced between the plates.
For a given energy per unit mass, an object can be moved an equal distance, as field strength is constant.
Give the equation that relates electric potential V and electric field strength E.
E = ΔV / Δr
Complete: electric field strength can also be referred to as…
Electric potential gradient.
What does the magnitude of the gradient of a graph of electric potential V against distance r represent?
Electric field strength (E).
If equipotentials are closer together, what does this tell you?
Greater rate of change of electric potential V with distance r (i.e. greater ΔV/ Δr).
Which means a greater electric field strength E.
What does the area under a graph of electric field strength E against distance r represent?
Electric potential difference ΔV.
Is electric potential scalar or vector? What significance does this have when combining potential?
It is a scalar quantity.
So direction does not need to be considered (but sign of charge still does!).
Complete: between two like charges there is a point where ________________ equals zero.
Electric field strength (field strengths in opposite directions cancel out).
Complete: between two opposite charges there is a point where ________________ equals zero.
Electric potential (one potential is positive, one is negative so they add to zero).
