Flow of Charge Electric Current Voltage Sources Resistance – Ohm’s Law – Electric Shock Direct Current and Alternating Current – AC-DC Conversion Introductory Physics1 Electric Current: Ch-23

Agenda Speed and Source of Electric Current – Drift Velocity Power Circuits – Complete Circuit – Series Circuit – Parallel Circuit Parallel Circuits and Overloading Introductory Physics2

Flow of Charge Introductory Physics3 Water flows from the high side to the low side Water will stop when the sides are the same height Putting a pump in to move the water back to the high side completes the circuit.

Flow of Charge In order to have continuous “flow” you need three things – A complete circuit Wire, pipe, other means to transfer something – A source of power All circuits have friction of some sorts and a power source is needed to keep the flow going Pump, battery, generator, solar cell, fuel cell – Something to flow Water, ions, electricity, etc Introductory Physics4

Electric Current Electric current is the flow of electrons, the closed circuit is made up of wires and other circuit elements such as resistors, capacitors, transistors, diodes Moving protons are not considered electric current, however the same rules apply Current is measures in Amperes which are defined as the number of coulombs per second Introductory Physics5

Electric Current Introductory Physics6

Voltage Sources Introductory Physics7 Voltage or potential sources are generally found by converting other types of energy to a potential In this instance chemical potential is being converted to electrical potential

Voltage Sources Introductory Physics8 The pump provides potential for the flow of water and the battery provides potential for the electric circuit

Electrical Resistance Introductory Physics9 A resistor is a circuit element made up of a material which “resists” the flow of current. It is similar to the friction that is found in pipes. It converts flow into heat.

Ohm’s Law Introductory Physics10 The flow of current through a resistor causes a potential drop across the resistor The majority or resistors have straight (linear) curves of the Current versus Voltage curve. The resistance is the inverse of the slope. The unit of resistance is the ohm ( Ω)

Ohm’s Law Introductory Physics11 Ohm’s Law states that the ratio of the voltage to current across a resistor is a constant. Some materials such as semiconductors constant slopes in one direction and infinite resistance in the other. These devices are called diodes.

Ohm’s Law: Example Introductory Physics12 A 12 volt battery is connected to a 2ohm resistor, how much current goes through the resistor.

Ohm’s Law and Electric Shock Introductory Physics13 Current (A)Effects 0.001Can be felt 0.005Painful 0.010Involuntary muscle contraction 0.015Loss of muscle control 0.070Possible Fatal

Shock Introductory Physics14 If your bodies resistance is 100,000 Ω how much current flows through your body if you touch a 12 V battery 12V/100,000 Ω = A If your skin is wet and your resistance drops to 1000 Ω how much current flows 12V/1000 Ω = A

Shock Introductory Physics15 In order to get a shock the bird need to complete the circuit. Since she perches on only one of the wires,, she will receive no shock

Shock Introductory Physics16 In order to not accidentally complete a circuit, many appliances have a ground wire The ground wire is connected to ground and any accidental circuit will take the path of least resistance. Since you have a high resistance little current will flow through you

Direct and Alternating Current Introductory Physics17 Voltage and current are constant Direct CurrentAlternating Current Current forms a sine wave

Converting AC to DC Introductory Physics18 Diodes are used to pass only current in one direction

Converting AC to DC Introductory Physics19 A device called a capacitor “smoothes” out the current similar to a tank can smooth out a pulsating flow of water

Speed and Source of Electrons in a Circuit Introductory Physics20 Normally electrons move randomly in a conductor using thermal energy (solid lines) if there is a potential across the wire then there is an electric field parallel to the wire that “pushes” the electrons to the right

Speed and Source of Electrons in a Circuit Introductory Physics21 This is similar to a ball rolling down an incline (gravitational potential instead of electric potential) bouncing between poles

Electrical Power Introductory Physics22

Electrical Power-Example Introductory Physics23 This 100 watt light bulb is part of a 120 Volt circuit. What current flows through it and what is its resistance P=VI=100 Watts = (120 V)I I=100 watts/120 V=.833 Amps P=I 2 R R=P/I 2 =(100 Watt)/(.833 A) 2 =145.2 Ω

Electric Circuits-Series Introductory Physics24 Current flows through each element one at a time. If they are resistors the equivalent resistance is the sum of the individual resistances

Electric Circuits-Series Introductory Physics25 If one light bulb is removed then the circuit is not complete and current cannot flow

Electric Circuits-Parallel Circuits Introductory Physics26 The voltage across each element are the same Total current is divided between each element The inverse of the equivalent resistance is the sum of the inverse of the individual resistances

Electrical Circuits-Parallel Introductory Physics27

Electric Circuits-Parallel Introductory Physics28 The advantage is that if one circuit is broken then the rest are still lit, however the current increases in the rest.

Parallel Circuits and Overloading Introductory Physics29 Parallel circuits like those found in the house are prone to overloading

Parallel Circuits and Overloading Introductory Physics30