HEAT will flow if there is a difference in temperature
WATER will flow if there is a difference in pressure. Is the pressure inside the jug (due to the weight of the water) the same as the pressure outside the jug (due to the weight of the atmosphere)? If the pressure outside was the same as the pressure inside, would the water flow out? At which location is there a larger difference between the pressure inside and outside, near the top of the water level or near the bottom of the water level? What evidence can you see that demonstrates a larger difference in pressure at the two locations?
Electrons will flow if there is a difference in electric pressure. Electric pressure is called “Potential”, and is measured in Volts. If there is no difference in pressure from one location to another, the electrons will not flow. In other words, if there is no “Potential Difference, V” from one location to another, there will be no electric “current”.
Current CURRENT: a flow of charged particles (electrons) through a conductor Current, I, is measured in amperes, A, or “amps”. Andre Ampere e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e-
Example: What is the current, I, if 8 C of charge passes through a wire in 2 seconds? I = q / t I = 4 amps How long would it take for 3.2 x 10 18 electrons to pass through the wire? t = q / I But… what is the charge, q? q = (3.2 x 10 18 )(1.6 x 10 -19 C) = 0.512 C t = 0.512 C / 4 A = 0.128 seconds
The Damage Caused by Electric Shock 1 mA = 0.001 A Mild shock can be felt 5 mA = 0.005 A Shock is painful 15 mA = 0.015 A Muscle control is lost 100 mA = 0.1 A Death can occur 60W light bulb - 0.5 A Starter motor – 210 A Clothes dryer – 18 A Iron- 3 A
Circuits ELECTRIC CIRCUIT: Charges moving in a closed loop A circuit requires a both a conductor, usually metal wires, and a “charge pump”. CHARGE PUMP: a device that provides a potential difference so that charges keep moving. Alessandro Volta The Potential Difference, V, provided by the charge pump is called its VOLTAGE.
If the voltage of a battery is 9 V, this means there is a difference of 9 V of potential (pressure) between the positive terminal and the negative terminal. If the voltage of a battery is 1.5 V, this means there is a difference of 1.5 V of potential (pressure) between the positive terminal and the negative terminal. The pressure DIFFERENCE, the VOLTAGE, is required for charges to flow! So, charge pumps such as batteries, are called voltage sources.
Circuits This potential difference is sometimes called the emf, (electromotive force) Examples of charge pumps: batteries, solar cells, generators, power supplies
The source of the electrons moving in the circuit is NOT the battery or the wall outlet! The free electrons are contained within the wire itself. An individual electron does not actually travel all the way around a circuit. One electron bumps into the next that bumps into the next that bumps into the next ….. It is the ENERGY that gets transferred all the way around the circuit. You are not buying electrons from your electric company- you already have them! You are buying energy!
Resistance all conductors offer some resistance to the flow of charges, even metal wires. RESISTANCE = The unit for resistance is the OHM, . This equation is often called OHM’S LAW
George Ohm- first determined the math relationship now called Ohm’s Law
Example What is the resistance of an appliance if 2 amps of current run through it when supplied with 120 V? R = V / I R = 120 V / 2 A R = 60
WATER ANALOGY WaterElectricity Flow of water current flow of charge Water pump keeps flow going charge pump psi.pressure voltage Pipes of differentresistancewires of diameterdifferent diameter
Small electrical components called “resistors” are inserted into circuits to control the amount of current flowing through that part of the circuit.
Certain metals offer less resistance to the flow of charges than others. Example: Copper is a better conductor than iron The resistance of a wire of length L and cross sectional area A is given by RESISTANCE, R = where is the resistivity of that particular metal.
The resistivity increases as temperature increases. So, a hot wire resists the flow of charges more than a cooler wire. RESISTANCE, R =