Electric Fields and Circuits Ch’s 33,34,35 Created by: St. John

Single Positive Field Charge + This is a 2D picture of the field lines that surround a positive field charge that is either point-like or spherically symmetric. Not shown are field lines going out of and into the page. Keep in mind that the field lines radiate outwards because, by definition, an electric field vector points in the direction of the force on a positive test charge. The nearer you get to the charge, the more uniform and stronger the field. Farther away the field strength gets weaker, as indicated by the field lines becoming more spread out.

Single Negative Field Charge - The field surrounding an isolated, negative point (or spherically symmetric) charge looks just like that of an isolated positive charge except the field lines are directed toward the field charge. This is because, by definition, an electric field vector points in the direction of the force on a positive test charge, which, in this case is toward the field charge. As before, the field is stronger where the field lines are closer together, and the force vector on a test charge is parallel to the field.

Point Charges of Different Magnitudes Let’s compare the fields on two separate isolated point charges, one with a charge of +1 unit, the other with a charge of +2 units. It doesn’t matter how many field lines we draw emanating from the +1 charge so long as we draw twice as many line coming from the +2 charge. This means, at a given distance, the strength of the E field for the +2 charge is twice that for the +1 charge

- + Equal but Opposite Field Charges (cont.) Here is another view of the field. Since the net force on a charge can only be in one direction, field lines never intersect. Draw the electric force on a positive charge at A, the electric field vector and B, and the electric force on a negative charge at C. The net force on a + charge at D charge is directly to the left. Show why this is the case by drawing force vectors from each field charge and then summing these vectors. A B C D

Review of Induction Valence electrons of a conductor are mobile. Thus they can respond to an electric force from a charged object. This is called charging by induction. Note: not all of the valence electrons will move from the bottom to the top. The greater the positive charge brought near it, and the nearer it is brought, the more electrons that will migrate toward it. (See animation on next slide.) conductor

Review of Induction (cont.) Because of the displaced electrons, a charge separation is induced in the conductor.

Capacitors - Overview A capacitor is a device that stores electrical charge. A charged capacitor is actually neutral overall, but it maintains a charge separation. The charge storing capacity of a capacitor is called its capacitance. An electric field exists inside a charged capacitor, between the positive and negative charge separation. A charged capacitor store electrical potential energy. Capacitors are ubiquitous in electrical devices. They’re used in power transmission, computer memory, photoflash units in cameras, tuners for radios and TV’s, defibrillators, etc. q = vc

OHM’S Law Volts AmpsOhms esistance oltage Impedance Think of water here. Voltage is a dam holding back all the pressure of a Lake. Amps is the current of water that the dam is releasing and is flowing from one place to another. Resistance is all the trees and rocks slowing the current down. Current (flow)

Series Circuit In a Series circuit, the CURRENT (flow) is THE SAME at every part of the circuit… BECAUSE the RESISTANCE is the SUM of all the resistors and the electrons can only get through the circuit as fast as the TOTAL resistance allows. In a Series circuit, the CURRENT (flow) is THE SAME at every part of the circuit… BECAUSE the RESISTANCE is the SUM of all the resistors and the electrons can only get through the circuit as fast as the TOTAL resistance allows. AmpsOhms

Example Calculate the current of the circuit. Calculate the current of the circuit. 2.5 OHMS 3.6 OHMS 5 OHMS

Example Calculate the current of the circuit. Calculate the current of the circuit.

Generators convert mechanical energy to electrical energy. They produce the average potential difference of 120 volts in a wall outlet.

The two types of current are alternating current (AC) and direct current (DC).

In DC the current always flows the same direction. In AC the direction of current flows changes rapidly. If the changes were too slow, you would notice lights flickering, etc.

To prevent this, AC oscillates 60 times per second (60 Hz) in the U.S. Batteries produce DC; generators can produce AC or DC.

Current in a circuit is determined by the potential difference (volts). It is also determined by the resistance. Resistance is the opposition to the motion of charge.

The SI unit of resistance is the ohm (Ω). One ohm = 1 volt/1 ampere.

Ohm’s law is V = IR. It holds true for a wide range of materials and voltages, but is not true for all materials.

In a non-ohmic material, the slope of a graph of current vs. potential difference will not be a straight line.

A diode is a semiconducting device that is non-ohmic.

We will assume that all resistors follow Ohm’s law.

Some factors that affect the resistance of conductors are:

length - longer conductors have greater resistance, length - longer conductors have greater resistance, cross-sectional area - greater area produces less resistance, cross-sectional area - greater area produces less resistance, material - better conductors have less resistance, material - better conductors have less resistance, temperature - higher temperatures increase resistance. temperature - higher temperatures increase resistance.

By V = IR, changing the resistance of a circuit changes the current. Changing the voltage also changes the current, but this is not practical in household circuits.

The resistance of a steam iron is 19.0 Ω. What is the current in the iron when it is connected across a potential difference of 120 V?

Superconductors have no resistance below a critical temperature. By V = IR, if R is zero, a current can exist without a potential difference. These have been observed to exist for years!

Power is work/time or ΔPE/time. Electric power is calculated from this equation: P = I ΔV.

The SI unit of power is the watt, W. 1 W = 1 J/ 1 sec

P = IV and V = IR can be combined to form: P = I 2 R & P = V 2 /R.

An electric space heater is connected across a 120 V outlet. It dissipates 1320 W of power. Calculate the resistance of the heater.

Electric companies charge by units of energy, but they do not use joules. The unit used is kilowatthours.

A kilowatthour is the energy delivered in 1 hr at a constant flow of 1 kW.(kilo = 1000)

1 kWh = 3,600,000 J, or 3.6 X 10 6 J, or 3.6 MJ.

How much does it cost to operate a W light bulb for 24 h if electrical energy costs $0.080 per kWh?

Parallel Circuit The current has more than one path to take through the circuit The current has more than one path to take through the circuit Because of this, the RESISTANCE is reduced!!! Because of this, the RESISTANCE is reduced!!!

How to calculate Resistance in Series Circuits. R T = R 1 + R 2 + R 3 + … R T = R 1 + R 2 + R 3 + …

How to calculate Resistance in Parallel Circuits R t R 1 R 2 R 3 … R t R 1 R 2 R 3 … = ++