Electric Fields

Fields Matter contains atoms and molecules A VERY large number together make up large masses like planets which can produce gravitational fields. We do not ‘see’ what causes the force….but we see the apple accelerate towards the Earth.

Atoms have Protons and Electrons…carry electric charge Normally object is neutral…why? What happens here? Has a net charge. We do not ‘see’ the force picking up the paper, but we see the paper accelerate towards the comb. THE COMB PRODUCES WHAT IS CALLED AN ELECTRIC FIELD!

Both gravitational and electric force are mysterious because there is no visible link between the two bodies that accelerate towards each one another. Field in physics generally means a region in which invisible forces act.

MAGNETIC FIELDS Produced by magnets and electric currents. Magnetic fields create magnetic lines of force. In the same way as these lines represent the strength and direction, gravitational and electric fields can also be describes using lines of force.

Electric Forces

Unlike charges attract Like charges repel If an object carrying a small electric charge, Q, feels a force, F when placed close to the charged rod, we say the electric field strength produced there by the rod has a size F/Q

Definition Electric Field Strength E = electric force F on a small charge/small charge Q E=F/Q = force per unit charge The unit for electric field strength is the newton per coulomb, N/C

If a piece of paper was lifted by an electric force of 1.5x10 -5 N and carried a charge of 5.0x10 -8 C, then it must have been in an electric field of 300N/C. Electric field strength is a VECTOR so it has magnitude and direction. The direction of E is the same as the direction of the electric force, F, which is defined as the force on a positive charge.

Examples: A proton of charge +1.6x C is moving in an electric field of strength 500N/C. What is the electric force that acts on it? E=F/Q F=EQ 500x1.6x x N How does this force compare with the weight of a proton (mass=1.7x kg)? Weight=mg 1.7x x 9.8 =1.7x N So the force is 4.7x10 9 times bigger!

Examples: Calculate the magnitude and direction of the electric force acting on a charge of -1C placed in an electric field of 100N/C acting due north. E=F/Q F= -1x100 F= -100N north F= 100N south

A charge of 25μC experiences a force of 1x10 -4 N. What is the strength of the electric field producing this force? E=F/Q E= 1x10 -4 /25x10 -6 E= 4N/C What force will an electron feel when it is in the electric field of an X-ray machine which has a strength of 4.5x10 5 N/C? F=EQ 4.5x10 5 x -1.6x x N towards to positive end.

How fast will the electron be travelling if this field accelerates it from rest and it is within the field for a distance of 10cm? Using v 2 =u 2 +2as U=0m/s, v=?, s= 0.1m, a=EQ/m – HOW?? E=4.5x10 5 N/C, Q=-1.6x10 -19, m=9.11x So, v 2 = 2x(4.5x10 5 ) x (-1.6x ) x0.1/9.11x V = -1.26x10 8 m/s Direction towards the positive end of the field

An electron is traveling at 1.8x10 6 m/s when it enters an electric field that slows it. The field has a magnitude of 6200N/C. How far does the electron penetrate the field before it stops? V=0m/s, u=1.8x10 6 m/s, E=6200N/C, s=? m Mass electron = 9.11x kg Charge of an electron = 1.6 x C

Uniform Electric Fields Two oppositely charged plates placed in close proximity produce a uniform electric field between them.

When a small charge is placed in the field the force remains constant. In a uniform electric field the size of the electric field strength can be expressed as: E = V/d V= electric potential difference, V, scalar quantity only d = the separation of the two plates, m E is expressed in V/m which is equivalent to N/C These plates are commonly called capacitors!