Physics is Life1 Chapter 23 Electric Circuits Regents Physics Mr. Rodriguez

Physics is Life2 There are 3 basic things that you note when thinking about electric circuits: Voltage Current Resistance

Physics is Life3 Voltage: a force that pushes and pulls the current through the circuit (in this picture it would be equivalent to gravity) Current: the actual “substance” that is flowing through the wires of the circuit (electrons!) Resistance: friction that impedes flow of current through the circuit (rocks in the river)

Physics is Life4 Electrical current flows in a loop, or a circuit. The image shows a simple electrical circuit. The circuit has four important parts: a power source (a battery, in this case), a conductor (the wire), the load (the light bulbs), and a switch. Current flows from the positive side of the battery, through the bulbs, and back to the negative side of the battery. As the electric current makes a complete loop, both bulbs will light. The complete loop condition is known as a closed circuit. A current that flows continuously in one direction is called a steady state current and is abbreviated DC. An open circuit condition occurs when the path of current flow is interrupted. This can happen if the wire is severed at any point on the circuit, or if the wire is disconnected from either side of the battery. With an open circuit, current will not flow, and the bulb will not light A switch is a device that enables a person to open or close the circuit whenever they choose. Circuit Characteristics

Physics is Life5 In the last unit, you learned that when two conducting spheres touch charges flow from the sphere at a high potential to the one at a lower potential. The flow continues until there is no potential difference between the two spheres. A flow of charged particles is an electric current. I = q/t The SI unit of current is the Ampere. Since one ampere is the flow of one coulomb of charge per second, it is a current in which 6.25 x electrons pass through the circuit per second. Producing Electric Current

Physics is Life6 Direction of Current Conventional Current: Flow of current from positive terminal to the negative terminal. Electron Current: Flow of current from the negative terminal to the positive terminal.

Physics is Life7 Current and Voltage Defined Think of voltage as what pushes the electrons along in the circuit, and current as a group of electrons that are constantly trying to reach a state of equilibrium. If there is no voltage, electrons don’t move, therefore there is no current. Difference in electrical charge between two points creates difference in potential energy, which causes electrons to flow from an area with lots of electrons (negative terminal) to an area with few electrons (positive terminal), producing an electric current.

Physics is Life8 Sample Problem A current of 5.00 A flowed in a copper wire for 20.0s. (a) How many coulombs of charge passed through the wire at this time? (b) how many electrons flowed through this wire at this time? Solution (a) I = Q/t 5.00 = Q / 20.0 s Q = 100 C (b)Number of electrons per sec is (5)(6.25 x electrons/s )= 3.12 x 1019 electrons per sec. Multiply 3.12 x electrons by 20 sec = 6.25 x electrons/s OR use N e = Q/e 100 C / 1.6 x = 6.25 x electrons Electric Circuits

Physics is Life9 Relation between Current and Resistance Suppose two conductors have a potential difference between them. If they are connected with a copper rod, a large current is created. On the other hand, putting a glass rod between them creates no current. The property of determining how much current will flow is called resistance. In this diagram, Let the amount of liquid passing through the neck of the container be the current, and let the pressure of the water above the neck be the voltage and let the resistance depend upon the charateristics of the container (width of the neck of the container).

Physics is Life10 Resistance The resistance depends on material and geometry (shape). For a wire, we have: R =  L / A where  is called the resistivity. The units of resistance is Ohms (  ) and measures how hard it is for current to flow through the material, L is the length of the wire, and A is the cross-sectional area of the wire.

Physics is Life11 Definition of Resistance By experiment we find that if we increase the voltage, we increase the current: V is proportional to I. When a metal wire is connected to a battery supplying a constant difference of potential, the current flowing through it depends on the degree of opposition which the electrons encounter in the wire conductor as they collide with the atoms in their paths. This opposition is called the resistance of the wire. The total resistance of a circuit can be related to the ratio of the potential difference applied to the ends of the wire to the current flowing through it, or R =  L / A = V/I By experiment we find that if we increase the voltage, we increase the current: V is proportional to I. When a metal wire is connected to a battery supplying a constant difference of potential, the current flowing through it depends on the degree of opposition which the electrons encounter in the wire conductor as they collide with the atoms in their paths. This opposition is called the resistance of the wire. The total resistance of a circuit can be related to the ratio of the potential difference applied to the ends of the wire to the current flowing through it, or R =  L / A = V/I

Physics is Life12Resistance Just as with fluid flow, the amount of resistance does not depend on the voltage (pressure) or the current (volume flow). The formula V=IR relates voltage to current. If you double the voltage, you will double the current, not change the resistance. As was the case in fluid flow, the amount of resistance depends on the materials and shapes of the wires.

Physics is Life13 If a series of increasing potential differences is applied to a given wire whose temperature is kept constant, the current in the wire increases in direct proportion to the difference in potential. Plotting the currents I against the corresponding differences in potential V gives a straight line. This means that the ratio V/I is constant, and therefore that the resistance R of the metal wire also remains constant in spite of the change in V and I. George Simon Ohm discovered that this constancy of resistance is true for all metal conductors provided their temperatures are also kept constant. His discovery, known as Ohm’s Law, is written: V/I = R = constant Ohm’s Law

Physics is Life14 Ohm’s Law Current through an ideal conductor is proportional to the applied voltage –Conductor is also known as a resistor –An ideal conductor is a material whose resistance does not change with temperature For an ohmic device, V = Voltage (Volts = V) I = Current (Amperes = A) R = Resistance (Ohms = Ω)

Physics is Life15 Ohm’s Law Sample Problem (a) What current flows through a 80  coil of wire when it is connected to the terminal of a generator supplying a potential difference of 120 V? (b) How many electrons are passing through the coil per second? Solution (a) I = V/R = 120V/ 80  = 1.5 A (b) 1 A = 1 C/s = 6.25 x electrons/s I = 1.5 A = (1.5) (6.25 x electrons/s) = 9.4 x electrons

Physics is Life16 Electrical Power The change of electrical potential energy (W) of a charge is:  PE = W = qV. Power is the change in energy with respect to time:Power =  PE /  t. Putting these two concepts together we have: Power =  (qV) / t = V q / t = VI.

Physics is Life17 Electrical Power Besides this basic equation for power: P = I*V remember we also have Ohm’s Law: V = I*R. Thus we can write the following equations for power:P = I 2 *R = V 2 /R = I*V. The units of power are watts. From our previous knowledge, Power is also defined as Work(Energy)/Time. Thus: P = W/t = I 2 *R = V 2 /R = I*V.

Physics is Life18 Physics is Life18 Sample Problem Calculate the resistance of a 40W automobile headlight designed for 12V. Solution Since we are given P=40W and V=12V, we can use the equation P = I 2 R and solve for R: R= V 2 /P = (12V) 2 /(40W) = 3.6  Electrical Power

Physics is Life19 Ohm’s Law Formula Chart

Physics is Life20 Heat Generated in Resistors The heat, Q, (electrical energy) generated in a resistor in t seconds is equal to the power funished by the resistor during that time: Q Energy = Pt = IVt =I 2 Rt = V 2 /Rt The units of Q are Joules or Watt- seconds

Physics is Life21 The Light Bulb and its Components Has two metal contacts at the base which connect to the ends of an electrical circuit The metal contacts are attached to two stiff wires, which are attached to a thin metal filament. The filament is in the middle of the bulb, held up by a glass mount. The wires and the filament are housed in a glass bulb, which is filled with an inert gas, such as argon.

Physics is Life22 Light bulbs do not obey Ohm’s Law Bulbs are non-linear conductorsBulbs are non-linear conductors (R increases with temperature)

Physics is Life23 As light bulbs warm up, their resistance increases. If the current through them remains constant: They glow slightly dimmer when first plugged in. Why? R increases but I remains constant  P increases Most ohmic resistors will behave non-linearly outside of a given range of temperature, pressure, etc. The filaments of light bulbs are made of Tungsten, which is a very good conductor. It heats up easily. Light bulbs do not obey Ohm’s Law

Physics is Life24 Voltage versus Current for Constant Resistance The light bulb does not have a linear relationship. The resistance of the bulb increases as the temperature of the bulb increases.

Physics is Life25 Superconductivity As the temperature of certain materials approaches absolute zero, their resistance drops to zero. The property whereby a metal loses its electrical resistance when cooled to a sufficiently low temperature is called superconductivity. The metals known to be superconductive include aluminum, lead, titanium, niobium, vanadium, and technecium.

Physics is Life26 What the heck is going on?

Physics is Life27 Conclusion The flow of charged particles is called the an electric current. It is defined as I = Q/t Ohmic resistors obey Ohm’s Law: V =IR The resistance of a metal varies directly with its length and inversely with its cross section: R =  L/A where  is the resistivity of the wire. Resistance is affected by temperature. The resistance of a conductor increases as its temperature increases Light bulbs do not obey Ohm’s Law –Tungsten is such a good conductor that their resistance depends on their temperature –As their temperature increases, the power dissipated by the bulb increases i.e. They are brighter when they are hotter

Physics is Life28 Conclusion The power of a resistor in a circuit is given as P = IV = V 2 /R = I 2 R A superconductor is a perfect conductor of electricity; it carries direct current with 100% efficiency because no energy is dissipated by resistive heating. Once induced in a superconducting loop, direct current can flow undiminished forever. The Energy or Heat generated by a resistor in a gvien time t is Q = Pt. Conventional current is defined as the direction opposite the flow of electrons in a closed circuit