1. Conceptual Physics: Electricity: What is an electric field? - The space surrounding a charged object is affected by the presence of the charge; an electric field is established in that space. A charged object creates an electric field How do different electric fields interact? These two examples are when a test charge is the same sign as that of the electric field. These two examples are when a test charge is the opposite sign as that of the electric field. How do charged objects create electric potential? Gravitational PotentialV.s Electric Potential 1) Mass - a property of the object experiencing the gravitational field. 2) Height - the location within the gravitational field. 1) Electric charge - a property of the object experiencing the electrical field. 2) Distance from source - the location within the electric field. While electric potential energy has a dependency upon the charge of the object experiencing the electric field, electric potential is purely location dependent. Electric potential is the potential energy per charge. Electric Potential PE = Potential Energy Q = Charge What is the electric potential difference? Consider the task of moving a positive test charge within a uniform electric field from location A to location B as shown in the diagram below. In moving the charge against the electric field from location A to location B, work will have to be done on the charge by an external force. The work done on the charge changes its potential energy to a higher value; and the amount of work that is done is equal to the change in the potential energy. As a result of this change in potential energy, there is also a difference in electric potential between locations A and B. This difference in electric potential is represented by the symbol V and is formally referred to as the electric potential difference. Relation to circuits Relationship to batteries What is a circuit ? Consider the following situation. There are two metal plates parallel to each other and each being charged with an opposite type of charge - one being positive and the other being negative. This arrangement of charged plates would create an electric field in the region between the plates that is directed away from the positive plate and towards the negative plate. A positive test charge placed between the plates would move away from the positive plate and towards the negative plate. Now suppose that the two oppositely charged plates are connected by a metal wire. What would happen? The wire serves as a sort of charge pipe through which charge can flow. The fact that the light bulb lights and remains lit is evidence that charge is flowing through the light bulb filament and that an electric circuit has been established. An example of a simple circuit How can we make a circuit? What's required? What is a light bulb? The difference between a successful and not successful circuit.. closing the loop! Electrons doing Work! An electrical circuit is simply an energy transformation tool. Energy is provided to the circuit by an electrochemical cell, battery, generator or other electrical energy source. And energy is delivered by the circuit to the load at the location of the load. The rate at which this energy transformation occurs is of great importance to those who design electrical circuits for useful functions Power - the rate at which mechanical work is done Power is the rate at which electrical energy is supplied to a circuit or consumed by a load. UNIT OF POWER! WATTS!!! 1 watt = 1 joule / second EX: When it is observed that a light bulb is rated at 60 watts, then there are 60 joules of energy delivered to the light bulb every second. A 120-watt light bulbs draws 120 joules of energy every second. The ratio of the energy delivered or expended by the device to time is equal to the wattage of the device. How do you calculate power? EX: Determine the... a.... current in a 60-watt bulb plugged into a 120-volt outlet. b.... current in a 120-watt bulb plugged into a 120-volt outlet. c.... power of a saw that draws 12 amps of current when plugged into a 120-volt outlet. d.... power of a toaster that draws 6 amps of current when plugged into a 120-volt outlet. e.... current in a 1000-watt microwave when plugged into a 120-volt outlet. a. The current in a 60-Watt bulb plugged into a 120-Volt outlet is 0.5 A. b. The current in a 120-Watt bulb plugged into a 120-Volt outlet is? c. The power of a saw that draws 12 amps of current when plugged into a 120-Volt outlet is? d. The power of a toaster that draws 6 amps of current when plugged into a 120-Volt outlet is? e. The current in a 1000-Watt microwave when plugged into a 120-Volt outlet is? What is a Kilowatt-hour? Electrical utility companies who provide energy for homes provide a monthly bill charging those homes for the electrical energy that they used. A typical bill can be very complicated with a number of line items indicating charges for various aspects of the utility service. But somewhere on the bill will be a charge for the number of kilowatt-hours of electricity that were consumed. Exactly what is a kilowatt-hour? Is it a unit of power? time? energy? or some other quantity? And when we pay for the electricity that we use, what exactly is it that we are paying for? A Kilowatt hour is a unit of Power time. If Power = Energy / time, then Power time = Energy. So a unit of power time is a unit of energy. The kilowatt hour is a unit of energy. Example: Remember 1000 watts in a Kilowatt Power Rating (Watt) Time (hrs)Energy Used (kilowatt- hour) Costs (cents) Costs ($) 60 Watt Bulb 10.060 kW*hr 0.6 ¢$0.006 60 Watt Bulb 40.24 kW*hr 2.4 cents$0.024 120 Watt Bulb 20.24 kW*hr 2.4 cents$0.024 100 Watt Bulb 10010 kW-hr100 cents$1.00 60 Watt Bulb 1000 ¢$10 10060 kW-hr P = Watts V = Volts I = Amps 10¢/kilowatt-hour (i.e., $.10/kWhr) for their electrical energy.