Evans SPH 4U1

Unit 6: Electricity

Note 4: Electric Fields

Reference: Chapter 7.3

Electric Fields

Field is a sphere or area or region of influence.

An electric field may be defined as the region around a charged body (a body carrying static charge) or charged bodies in which electric forces may be detected.

These electric forces can be detected by neutral or charged bodies.  By convention, scientists have agreed to using a small positive charge as a test charge.

The electric field around a single positive charged body:  (Picture will be shown on blackboard-  radial field)

Electric fields are shown by field lines or lines of force. The direction of the field line points to the direction of the electric force on a test charge at a specific location.  The spacing between the field lines is indicative of the strength of the field.

The electric field around a single negatively charged object  (Picture will be shown on blackboard-  radial field)

The electric field between & around 2 equally but oppositely charged bodies placed a short distance from each other.  (Picture will be shown on blackboard. Note individual test point charges add up vectorially to create electric field lines).

The electric field between & around 2 equally charged bodies (same sign) placed a short distance from each other. (Picture will be shown on blackboard)

A Very Special Field

·         For an electric field between 2 oppositely charged parallel metal plates and separated by a short distance:

It is a uniform field.  The strength is unchanged and the direction is unchanged between the plates.  Let’s show why…

The plates are too large to be considered point charges.  The net electric force on a small, positive test charge at any point would be the vector sum of the electric forces acting on it from each of the 2n point charges.

The direction of each force is along the straight line joining the point charge to the positive test charge (diagram on blackboard will show this by adding force vectors from a positive test charge).

§          outside the parallel plates (except for bulging near the edges)

§         Electric field intensity is constant everywhere between the plates.  The electric field lines are straight, equally spaced & perpendicular to the plates.

§         Magnitude of the  intensity at any point between the plates depends only on the magnitude of the charge on each plate, nothing else.

where q is the charge per unit area on each plate.

Electric Field Intensity is defined as the force experienced per unit of positive charge at a specific location in an electric field.  It is a vector quantity in which the direction is the same as the direction of the electric force for a positive charge.

Defining Equation:

SI Units: NC-1

Comparison of Force and Field intensity of Gravitational forces vs Electric forces.

 Graviational Electric Force Field Intensity

Sample Problems:

Text Page page 343 #'s 1-7

Be careful to watch if questions ask for electric field intensity or electric force.

1.      An object with a net electric positive charge of 4.0 X10 –9 C is fixed on a horizontal surface.

a)    Find the field intensity at a point 10cm to the east.  (3.6 x 103 N/C)

A second body carrying 2.0 X 10-9 C of positive charge is now placed at a distance of 10 cm to the east of the 4.0 X10 –9 C fixed body. Find:

b)      The electric force exerted on the smaller charge (7.2 x 10-6 N[E]).  Repeat if the second body has a 2.0 X 10-9 C of negative charge

(7.2 x 10-6 N[W])

2.      The electric field intensity between the two parallel metal plates is 2.0 x 10 2 N/C. What is the effect on the electric field intensity of each of the following changes considered separately?

a.       The charge on each plate is doubled. (Answer: 4.0 x 10 2 N/C).

b.      The plates are moved farther apart by 4 times. (no effect)

3.      What would the electric field intensity midway between two positively charged particles of 2.0 x 10 –9 C and 4.0 x 10 –9 C that are 20 cm apart?

(Answer: 1.8 x 103 N/C away from 2)

A Couple of Applications of Electric Fields:

Electrostatic Precipitators (pg 345)- use electric fields to remove ash, soot and particle emissions from fossil fuel burning. Negative wires cause air to become ionized and electrons move away from the wire toward the positive plate.  On their way the electrons encounter the soot/ash particles making them negative and the soot/ash particles now become attracted to the positive plate. The particles now collect on the plates and can be shaken or cleaned off.

Shielding - For a conductor, the electric field is zero inside the conductor and the charge is on the surface.  The electric field is perpendicular to the surface of the conductor.