Electricity Fields

electric charges can exert forces on each other A force is that it is a push or pull. What exactly is causing the push or pull between electric charges ? Remember there are two types of forces: contact forces and non-contact forces

Micheal Faraday Michael Faraday developed a concept called the field which we can use to explain action at a distance forces Michael Faraday According to Faraday each charge creates an electric field which extends out and through all space. Other charges feel forces because they interact directly with the field. Fields are a concept which was invented to help describe action at a distance forces.

Mathematical Description of Fields place a small positive test charge in a field and measuring the force it experiences due to that field. use a small test charge because we don't want it to exert enough force to move the charges which have created the field. The test charge is positive simply for historical reasons.

Testing a Electrical Field strength and direction of the electric field at the various points is indicated by drawing an arrow. When the test charge is close to the positive charge the electric field is stronger so we draw a longer arrow. The field would also repel the test charge so we draw the arrow accordingly.

Drawing Fields To visualize fields around electric charges, we draw a series of lines to indicate the direction of the field at various points. These lines are called electric field lines and we draw them so that they indicate the direction of the force that the field would apply to a positive test charge.

Electric field lines for a single positive charge and a single negative charge Notice that where the field is stronger (near the charge) the lines are closer together

A little more complicated when more that one charge is involved Unlike Charges Imagine placing a positive test charge in the field This helps with direction

Like Charges

Common Characteristics of Every Electric Field 1. Electric field lines start on positive charges and end on negative charges 2. Electric Field lines do not cross 3. The density of the electric field lines is proportional to electric field strength

Field Strength There is one problem with simply measuring the force on test charge and using that as a measure of the electric field strength. The problem is that there is no standard for exactly how large the test charge should be. For example I might use a 1 µC test charge but you may use a 2 µC test charge. Your test charge would experience twice the force as mine. However there is one quantity that will be the same for each of our charges, that is the ratio of the force to the actual charge on the test charge.

This leads us to the following formula for electric field strength. E = F/q – E = electric field strength (N/C) – F = force experienced by test charge (n) – q = charge of the test charge ( C) Comparison – electric field near a charged rubber rod is approx 1000 N/C

Examples 1. A positive test charge of 2.0x10-5 C is placed in an electric field where it experiences a net force of N. What is the electric field strength at that point? 2. A charge of -200 µC is placed at a point in an electric field where the strength is 5000 N/C. What force will it experience?

Electrical Field Strength and Coulombs Law By using (1) E = F/q and (2) F = kq 1 q 2 / d 2 You can create a formula that calculates the electric field strength at any distance from a point charge Rearange (1) F = Eq and sub. Into (2)

Field Strength E = kQ d 2 Q – charge that creates the field d – distance between field charge and test charge

Here you go 1. What is the electric field strength at a point 30 cm from a charge of 5.0 µC 2. The electric field strength at a particular point near a 50 µC charge is found to be 4.5x10 4 N/C. What is the distance from this charge to the point in question? 3. A positive charge of 2.0 x C experiences a force of N when placed in an electric field. What is the magnitude of the electric field? 4. What charge exists on a test charge that experiences a force of 1.4 x N at a point where the electric field intensity is 2.0 x N/C? 5. What is the electric field strength 24 cm from a point charge of 6.0 mC? 6. What is the charge on a point charge if the electric field strength is 3.8 x 10 5 N/C at a point of 59 cm from the point charge? 7. How far from a 4.2 mC charge is the electric field intensity 2.6 x 10 5 N/C?