electrostatics and electric field

Electrostatics Part I

Objectives: 1.1-1.2/ Objectives 3.1-3.11

ObjectivesDefine static electricityDescribe how static electricity developsState the law of conservation of electric chargeDefine the coloumbRelate charge to electric current Q= ItApply Coulomb’s law and vector approach to find the

resultant Electric ForceDescribe properties of an electric fieldDetermine the electric field strength of a point ChargeApply equations of motion to describe linear and

projectile motion in an uniform electric field

Static Electricity

• Static electricity is an excess of electric charge trapped on the surface of an object

Static Electricity Recall that matter consists of Atoms and atoms in turn

consist of electrons, protons and neutrons. Electrons = negative charge Protons= positive charge

1. Neutral charge has protons = electrons2. Negative charge: electrons > protons3. Positive charge: protons > electrons

How is Static Electricity Produced? When two non-conductors are rub together electrons

(negative charges) of one dry material are transferred to and build-up on another

Conservation of Charge (Q)• Law of Conservation of Electric Charge: “ The net amount of any charge produced in a system is zero. “• The amount of electric charge remains constant

in a closed system. • Electrons are simply transferred from one

material to another.

Coulomb (Q) : SI Unit of Charge

• Coulomb: the amount of charge transferred in one ampere in one second.

Q = ItCoulomb = Amp-sec

• The electric charge on an electron is very small and it is NEGATIVE:

e = -1.6x 10-19 Coulombs• Protons have equal and opposite charge of

electronse = + 1.6 x 10-19C

Activity

• A steady current of 2.5 A exists in a wire for 4.0 min

(a) How much total charge passed by a given point in the circuit during those 4.0 min? (b) How many electrons would this be

Coulomb’s (Inverse Square) Law• The law looks at the forces created between

two charged objects. • This force can either be attractive or repulsive.• Magnitude of Force is :

– Directly proportional to the product of the charges– Inverse proportional to the square of the distance

between the chargesK = 9.0 x 109 Nm2/C2

K = 1/4πεo

Coulomb’s Law

Like charges:Force is directed away from each other

Applying Coulomb’s Law To Two Fixed Charges

1. Find the electric force a proton exerts on an electron in a hydrogen atom, given that the average distance between them is 0.53 x 10-10 m.

o Tips: Ignore the signs when applying Coulomb’s law. o Determine the direction of force based on attraction

or repulsion.2. Which charge exerts a greater force on the other: Q1

= 1 uC and Q2 = 50uC, separated by a distance l?

Using Vector Addition to Find Resultant Electric Force

• Recall that force is a vector quantity : both magnitude and direction

• The net force is vector sum of all forces acting on the object.

•

Using Vector Addition to Find Resultant Electric Force

ELECTRIC FIELD

Electric Field Strength (Intensity)

• Electric field is any region where a charged object experiences an electric force.

• Electric field: Force per unit charge

•

• Direction of field lines depends on the direction of acceleration of a small positive test charge in that field

Electric Field due to a One-Point Charge

• E depends only the charges creating the field, not the test charge.

• Electric field is best expressed in terms of the permittivity space constant

USE THIS EQ.

Doing Work in Electric Field

Doing Work in an Electric Field

Force Acting on A Charged Particle in an Electric Field

• E = F/q

• F = qE

Motion of Charged Particles in an Electric field

• When a particle of charge q and mass m is placed in an electric field E, the electric force exerted on the charge is qE.

• If this is the only force exerted on the particle:

• The acceleration of the particle is therefore:

• E is uniform (that is, constant in magnitude and direction), then the acceleration is constant.

Motion/Electric Field

Projectile Motion in Electric Field

Equations of Motion

Keep in Mind

No acceleration in x direction: therefore time taken to go through the field =

sx = ux t + 1/2axt2 [no ax] Initial velocity in y direction = 0 Voy or Uy

= 0Vertical displacement or deflection:

Sy = uyt + ½ ayt2 , where ay = qE/m

Question

• An electron enters an uniform electric field as shown in fig 23.25 with vi = 3.00x106 m/s and E= 200 N/C. the horinzontal length of the plate is l = 0.100m.

a) Find the acceleration of the electron.b) Find the time it takes for the electron to travel

through the fieldc) What is the vertical displacement of the electron in

the fieldd) Find the speed of the electron as it emerges from the

field

Projected at an Angle• An electron and a proton are fired at a velocity of an

angle of above the horizontal into a uniform electric field created by two charged plates with a separation of 2cm and a length of 10cm and a potential difference of 2kV. The charges enter the field 0.5cm above the bottom plate, as shown in the figure below. Locate the positions at which the proton and electron collide with the plates.

• The mass of an electron is 9.11x 10-31 kg , the mass of a proton is 1.67x10-27 kg. The charge on an electron is 1.67x 10-19 C, and the charge on a proton is 1.67x 10-

19 C .

Solution

Applications of static Energy in everyday life

Photocopier machine uses static electricity to copy print to a page. The ink is

charged so that it will stick to the paper in the designated areas.

Spray painting cars: The paint is charged. Because the paint particles have the

same charge, they repel each other so they spread out evenly. The charged paint particles are attracted to the car and stick to the body

Reduce pollution in furnaces: They give the smoke an electric charge; when it passes by

walls of the opposite charge, most of the smoke particles cling to the wall. This reduces the pollution entering into the atmosphere.

METHODS OF REDUCING STATIC ELECTRICITY

Static electricity can be useful but also annoying or even damaging ( think of lightning)

Below are two primary ways to reduce static electricity.

1. Increasing Moisture/Reduce dryness:

Recall that static electricity favours dryness. Adding moisiture (water or oil) prevents build up of charges.

e.g. moisturising hair will prevent sparks while combing.

2. Grounding: the process of making an object neutral or uncharged by connecting to a very large object – most times the ground.

Example: the use of lightning rods channel the electricity of the lightning to the ground.