Overview
The objectives of today's lecture is to give you a quick introduction and/or review of:

• Electric force (Coulomb's Law)
• Electric field
• Electric Potential
• Ohm's Law
  

Notes on Chapter 2
pp 12 -17 of your lab book

Recall the following three equations from mechanics:

1. Newton's Law of Gravity: this describes how matter is attracted to other matter.

2. The definition of the gravitational field strength: this defines a physical object called a field. In this case, it is the field of gravity:

3. The definition of work: this equation helps you relate an applied force to the amount of energy that is done on or by the system due to this force. (272 students. for you this is an integral of F ds)

Electric Force:
The first equation describes how matter is attracted to other matter; in electricity and magnetism, we want to know how charges are attracted or repelled from one another The equation which tells you how charges are attracted or repelled from one another is Coulomb's Law:

You will notice that this law is very similar to the law you learned for the force of gravity. One of the differences between the equations is the fact that there is no minus sign. The minus sign in the gravitational force indicated that the gravitational force was always attractive. The electric force on the other hand, can be attractive or repulsive, depending on the charges involved. Objects can be positively charged, negatively charged, or neutral.  Charged objects exert forces on one another.

• Like charges repel each other
• Opposite charges attract each other

Electric Field:
Like gravity, we also want to define a field, called the electric field . The equation for the electric field is also very similar to the gravitational version.

Electric Potential:

We would also like to calculate the amount of work done on a charge by the electric field (or the work that must be done on a charge to move it in an electric field). Given that the electric force is F = Eq from the equation above, the equation for work is:

W = -(Eq) s cos 

(the minus sign indicates whether or not energy is gained or lost).  Recall that s is the displacement vector and theta is the angle between the displacement vector and the force. Rearranging this yields:

W = -q (Es cos )

The value in the parenthesis is a new physical quantity we will call the potential difference (or by its common name, a voltage). Voltage turns out to be an easy quantity to measure. We have many devices in the lab which measure voltage.  In our lab we will use two main voltage-measuring devices: a voltmeter (or multimeter) and an oscilloscope. The idea is that we can easily relate a measured voltage to the amount of energy that a charge has gained or lost.

Ohm's Law:
Some components in a circuit take away energy and some components give energy.  

• A resistor takes away potential energy from the electron.
• A battery gives energy to the electron.

An example: an electron gains e*5V worth of energy from one terminal of a 5V battery to the other.
 
How much energy is lost in a resistor? We still can measure the voltage across a resistor, but it turns out that the voltage changes as we change our circuit. The energy lost depends on the current and resistance of the resistor.  

So we should define the word current (I):
Normally we talk about many electrons at once travelling through a component, or the current of electrons through a component. 

• Current is the amount of charge passing a single point per unit time.

The resistance (R) of a substance is based upon the material and geometry of a substance. For some substances, the resistance is a constant for a given geometry. Such substances are called Ohmic substances. The resistors we use in a circuit are such substances. If we know the current going through the resistor and the resistance of a resistor, then we can calculate the amount of voltage lost across the resistor by using Ohm's Law:

V = IR

Now that you know these four things, you are almost ready to learn about DC Circuits (our next lab). Next time we will learn Kirchhoff's Rules (which actually are restatements of the conservation of energy and the conservation of charge).

We will be doing the DC Circuit lab next week. The DC Circuit lab is very long (3 hours). Unless you want to stay 3 hours or later, I suggest you come prepared to class.  Your first pre-lab assignment is given on the DC Circuits notes found under the schedule. Everyone should do and turn in the DC Circuits pre-lab, since we don't have our groups yet (and eveyone has to know how to use the equipment anyway).