Electric Fields

Forces on charges

 * Electric charge Q  is measured in coulombs (C) and can be either positive (carried by particles like protons) or negative (carried by particles like electrons)
 * The smallest unit of charge is 1.6 × 10^-19C (the size of the charge on an electron or proton)
 * Oppositely charged particles attract each other. Like charges repel.(The force experienced is given by Coulomb's Law)

Calculate forces using Coulomb's Law

 * F=kQ1Q2/r^ 2
 * F is the force on either charge (N)
 * Q1 and Q2 are the two charges (C)
 * r is the separation of the centres of the two charges (m)
 * k is a constant; its value is 9 × 10^9 Nm^2 C^-2
 * k = 1/(4πε0 ), where ε0 is the permittivity of free space.

Electric Field Strength
Where E is electric field strength(NC^-1)， F is the force(N) and Q is the charge (C)
 * Electric field strength ,E, is defined as the force per unit positive charge.(It's the force that a charge +1 C would experience if it was placed in the electric field.)
 * E=F/Q 
 * Note that since force is a vector and charge is a scalar, electric field strength must be a vector.(pointing in the direction that a positive charge would move)

Representing electric fields
Fields are represented by drawing field lines (lines with arrows)
 * The closer together the lines are the stronger the field.
 * Field lines point in the direction a positive test charge would move.Positive and negative.jpg

Types of electric field
Point charges The electric field for such a charge can be found from Coulomb's law F=kQq/r^2    F = qE                       This gives E=kQ/r^2
 * A point charge, or any body that behaves as if all its charge is concentrated at the centre, has a radial field.

Parallel plate capacitor
 * A uniform field can be produced by connecting two parallel plates to the opposite poles of a battery
 * In a uniform field, the field lines are parallel so they're always the same distance apart. This means that the field strength is the same at all points within the field ( a test charge would experience the same force wherever it was.)


 * The field strength between two parallel plates depends on the potential difference, d, between them, according to the equation E=V/d (V is the potential difference between the two plates. d is the distance between plates. This shows that electric field strength can also have units of V   m^-1)

Variation of field with distance
Point charge
 * The decline of E    with distance follows an inverse square law (E ∝ 1/r^ 2)

Parallel plate capacitor
 * E is constant whatever the position between the plates

Electric potential

 * potential difference - a potential difference between two points means there is a voltage drop between them. Accordingly, electrical potential is measured in volts.
 * The potential at a point in an electric field is defined as the work done in bringing a test unit positive charge from infinity to that point.(The potential of a charge at infinity is always defined to be zero.)
 * The definition of potential in terms of work tells us that electric potential is a measure of the potential energy per unit charge( The potential energy on a charge due to a potential V is: P.E. = qV  P.E. = potential energy (J) q = charge (C) V = electric potential (V)
 * Since energy is a scalar and charge is a scalar, potential must also be a scalar quantity. The work done when a charge moves through a potential difference is given by W = Q∆V   W = work done (J) Q = charge (C) ∆ V = potential difference (V)
 * The electrical potential energy increases if a positive charge moves to a point of higher potential or a negative charge moves to a point of lower potential

Potential for point charge, parallel plate capacitor

 * For a point charge Q V =kQ/r
 * For a parallel plate capacitor with spacing d and potential difference V between the plates, the potential decreases linearly between the positive and negative plates