Chapter 5.3 Notes Electrical Energy

A Capacitor is an electrical device that stores energy in an electric field. Leyden Jar

Capacitor Demo

A power supply removes electrons from one plate in a capacitor and places them on a second plate.

This causes the plate that loses electrons to become positively charged and the plate that gains electrons to become negatively charged. The work done to create this electric field is equal to the potential energy stored in the field.

Equation for Work in an electric field = ½ x charge x Voltage W = ½ q V

If the charge is 10 and the change in Voltage is 6, what is the work done? W = ½ q x V W = .5 x 10 x 6 W = 30 J

The charge on either plate of a capacitor is equal to the Capacitance x voltage Capacitors have fixed values of capacitance.

The constant for a capacitor is called capacitance and is represented by the letter C. Charge of a plate in a capacitor = Capacitance x voltage q = C x V

If the Capacitance is .0005 and the Voltage is 200 V, what is the charge? q = C x V q = .0005 C/V x 200 V q = .1 C

Potential energy in a capacitor = ½ (Capacitance) (Voltage)2 PE = ½CV2

If the Capacitance is .0005 and the Voltage is 200 V, what is the Potential Energy stored? PE = ½CV2 PE = ½ (.0005)(200)2 PE = ½ (.0005)(40,000) PE = 10 J

If an electric charge is moving, it will create a magnetic field.

A magnetic field is a circle with the current in the center:

Ferro Fluid Demo

If two currents are in the same direction, the wires are attracted.

If two currents are in opposite directions, the wires are Repulsed.

The Earth’s magnetic field is caused by currents in the molten iron core found at the center of the earth.

The Earth’s Magnetic Field causes the Northern Lights by attracting electrically charged particles given off by the sun.

A magnetic field can also create an electric field A magnetic field can also create an electric field. This is called electromagnetic induction (EMF).

If a loop of wires move through a magnetic field, a current is produced in the wire.

Demonstration

To increase the electricity created by a magnetic field: Use a larger magnet Use a coil (wire) with a larger diameter Use more turns of the wire Move the magnet faster

A capacitor stores energy in an electric field and an inductor is an electrical device that stores energy in a magnetic field.

The amount of energy that an inductor can hold is called inductance. Use the symbol L to represent Inductance. Equation for EMF = -inductance (current/time) EMF = -L (I/t)

A coil with an inductance of A coil with an inductance of .0045 Hertz a current of 1000 amps in 2 seconds. What is the EMF? EMF = -L (I/t) EMF = -.0045 (1000/2) EMF = -.0045 (500) EMF = -2.25 V

Potential energy in inductors (Equation) = ½ inductance (current)2 PE = ½ LI2

A coil with an inductance of 8 Hertz and a current of 15 A has how much PE stored? PE = ½ LI2 PE = ½ (8) 152 PE = ½ (8)(225) PE = 900 J