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Ohm’s Law Much like how gravitational potential energy depends on a masses location with in the gravitational force field of another mass, electric potential energy depends on a charges location with in the electric force field of another charged object. Potential Just like if you increase the mass of one of the objects the PE increases, if you increase the charge of one of the objects the PE also increases. The ratio of electric PE to amount of charge is known as electric potential, or voltage (V). V = PE/q Potential So, if the location of a charge(q) doesn’t change, but the actual charge changes, Voltage remains the same. Voltage is independent of charge in the field. Electrons move from a point of high electric potential to a point of low electric potential. Comparing potential difference in a battery to gravitational potential energy: 12 volts q 0 volts Note: this actually has nothing to do with Gravitational PE Potential Difference When there is a difference in potential between two points in a circuit, electrons will flow until there is no longer a difference. Voltage, or potential difference is the driving force for electrons in a circuit. Unit of potential difference or “Electromotive Force” is the Volt (V) Electromotive The Force (EMF) maximum difference of potential between the terminals of a battery or generator Current ( I ) What do we know about electrons in a conductor? – Conduction “free” electrons are free to move from atom to atom. – Current is simply the flow of charges or the net movement of these electrons Voltage does not go anywhere, the charges move. Current When there is a Potential Difference between two ends of a circuit, charges will move until both ends reach a common potential. This is similar to how water flows from high pressure to low pressure areas until there is no longer a difference. (figure 34.1 in text) Current Current is measured in amperes (A) (amps) I = q/t… so the units are 1A = 1C/s The way we say current flows through a circuit is actually the opposite direction that electrons move… I know… its weird Resistance Water Pipe Re-visit What do you notice about the flow of water through different pipes? Resistance () Resistance in a wire depends on four different properties: Thickness, length, and conductivity(how well electrons flow in a material), and temperature. It is measured in Ohms () Resistance Thick wire <resistance Thin wire Resistance Short wire <resistance Long wire Resistance Cool wire <resistance Hot most materials) wire (for Ohm’s Law V = IR Revisit the Do Now What hurts you when you get shocked? The faster the current passes through you, the more damage it does (pg 537 in text) Ohm’s Law and Electric Shock Damaging effects of electric shock are from current traveling through the body. The initial cause of electric shock is the voltage, however the current does the damage. Your body typically offers 100,000 Ohms of resistance. A current of 0.01 Amperes could cause muscle spasms. Power What is a short circuit? Battery example (note: don’t try this at home!) Power Electrical Power (P) = current x voltage (P = I x V) 1Watt = 1amp x 1volt Power Relate to our old concept of power: 1watt = 1amp x 1volt = (q/t) x (PE/q) = PE/t = power