Chapter 21 Electric Fields Rionna Greene, Jasmine Thomas, Roderick McCullough, and Maureshia Knowlin

Creating and Measuring Electric Fields Electric force, like gravitational force varies inversely as the square of the distance between two point objects. BOTH forces can act from great distances. A, creates a force on another charged object, B, anywhere in space, object A must somehow change the properties of space. Object B somehow senses the change in space and experiences a force due to the properties of the space at its location. We call the changed property of space an electric field. An electric field means that the interaction is not between two distant objects, but between an object and the field at its location.

Creating and Measuring Electric Fields(continued)) The forces exerted by electric fields can do work, transferring energy from the field to another charged object. How can you measure an electric field? Place a small charged object at some location. If there is an electric force on it, then there is an electric field at that point. The charge on the object that is used to test the field, called the test charge, must be small enough that it doesn’t affect other charges. The strength of an electric field is equal to the force on a positive test charge divided by the strength of the test charge.

Creating and Measuring Electric Fields(continued) The direction of an electric field is the direction of the force on a positive test charge. The magnitude of the electric field field strength is measured in newtons per coulomb, N/C An electric field should be measured only by a very small test charge. This is because the test charge also exerts a force on q. Thus, F=Eq. The direction of the force depends on the direction of the field and the sign of the charge. Each of the lines used to represent the actual field in the space around a charge is called an electric field line. Another method of visualizing field lines is to use grass seed in an insulating liquid, such as mineral oil. The electric forces cause a separation of charge in each long, thin grass seed.

Chapter 21 Section 2

Chapter 21 Section 2(continued) Whenever the electrical potential difference between two or more positions is zero, those positions are said to be at equipotential. Only differences in potential energy can be measured. The electric potential difference point A to point B is defined as ∆V=V subscript B-V subscript A. There is a repulsive force between negative and positive charges. A uniform electric force and field can be made by placing two large, flat, conducting plates parallel to each other.

Chapter 22 Section 1 Electric energy provides the means to transfer large quantities of energy great distances with little loss. Because electric energy can so easily be changed into other forms, it has become indispensable in our daily lives. Even quick glances around you will likely generate ample examples of the conservation of electric energy. Example: Inside, lights to help you read at night, microwaves, and electric ranges to cook food, computers, and stereos all rely on electricity for power. Outside, street lights, store signs, advertisements, and the starters in cars all use flowing electric charges. A flow of charged particles is an electric current. A flow of positive charges that move from higher potential to lower potential is a conventional current.

Chapter 22 Section 1(continued) Battery- a device made up of several galvanic cells connected together that converts chemical energy to electric energy. Electric Circuit- any closed loop or conducting path allowing electric charges to flow. A charge pump creates the flow of charged particles that make up a current. Example: A generator driven by a waterwheel. -The water falls and rotates the waterwheel and generator. -The kinetic energy of the water is converted to electric energy by the generator. Charges cannot be created or destroyed, but they can be separated. Power is defined in Watts: (W). Power; P=IV - Equal to the current times the potential difference

Chapter 22 Section 1(continued) The property determining how much current will flow is called resistance. R=V divided by I. -Resistance is equal to potential voltage divided by current. -The resistance of the conductor is measured in ohms. One ohm is the resistance permitting an electric charge of 1 amp to flow when a potential difference of 1 potential voltage is applied across the resistance.

Chapter 22 Section 2 Energy that is supplied to a circuit can be used in many different ways. A motor converts electric energy to mechanical energy, and a lamp changes electric energy into light. Power: P=I^2R -Power is equal to current squared times resistance. Power: P=V2/R -Power is equal to the potential difference squared divided by the resistance.

Chapter 22 Section 2(continued) Thermal Energy 1.E=Pt - Thermal energy is equal to the power dissipated multiplied by the time. 2. E=I^2Rt - It is also equal to the current squared multiplied by resistance and time. 3. E=(V2/R)t - Voltage squared divided by resistance multiplied by time.