Current Electricity Parallel Circuit Series Circuit What You

Current Electricity Parallel Circuit Series Circuit

What You Will Learn n n Transfer of energy in circuits. Conversion of energy. Electric Current – Conventional vs. Flow of Electrons Resistance and Ohm’s Law Basic Circuits

What You Already Know n n n You flip a switch to turn on a light, TV or computer. To turn on the car, you turn the ignition switch. MP 3 players, cell phones and flashlights have on/off switches and use batteries. n In each of these cases, you have a closed circuit in which electricity flows.

What You Already Know n Charge by Conduction – The process by which electrons are transferred from one object to another because of differences in excess number of electrons on one surface compared to the other.

What You Already Know Electric Potential n the Electric Potential Difference is equal to the Work required to move a test charge in an electric field divided by the magnitude of the test charge. F is constant since the electric field is constant from one plate to the other. + + + + A B qo F = qo. E Uniform Electric Field Vtotal = W/qo = Fd/qo = Ed -

Creating a Circuit Plate with number of Two equal and oppositely charged plates + + + + Plate with What would happen if a connected to both plates? of - was

Creating a Circuit The would flow from the charged plate to the until the amount of charge was the plates and the wire. + -+ + -+ + - - - + - + -+ - + - + - How do we maintain the flow of charged plate for both -+ + + _+ + + ?

Creating a Circuit + + + + - • • Circuit: • A in which electric can flow. • It generally includes a device such as a that reduces the. • It also includes a device to increase potential energy ( ).

What is Current? n Current is the rate of flow of charge. I= n n / =1 Conventional Current = flow of. (Note that charges do flow in metallic conductors. ) is simply the flow of. ( )

Ohm’s Law n German Georg Simon Ohm discovered that the ratio of the potential difference to current is a constant for a given conductor. = Where: = = = n n /___ in in ( in ) ( ( ) ) is the hindrance to the flow of. Most metallic conductors obey.

Ohm’s Law n n n The ( ) represents the ( ) of a curve where is plotted against. What is R? For materials, the curve is a straight line. m= None. g. = . .

Examples: Ohm’s Law n How much current flows through a 12 flashlight bulb operating at 3. 0 volts? n What is the voltage drop in a 5 resistor that has 2 amperes of current running through it? n What is the resistance of a heating element in a toaster operating at 120 volts with a current flow of 2 amperes?

What causes resistance? n E-field in conductor (resistor) is provided by a battery or voltage source. n n Charges (electrons) are put in motion due to influences of the , but in a very short time from things that get in the way , ( ), etc The more , the greater the and the fewer the , the less the. Imagine the following two scenarios. 1. 2. 3. Running down the hallway in between periods Running down the hallway after the late bell when there is nobody in them. Under which scenario would you experience less resistance?

Resistivity & Resistance n n Resistivity is a measure of the material. Resistivity is an intrinsic ( ) property of a material. The higher the , the higher the and vice versa. For a conductor of length L (m) and cross-sectional area A (m 2), the resistance can be determined by: R= Where = L= A= ( )

Ex. : Resistance & Resistivity 1. What would happen to the resistance in a wire if the length were increased? A. B. C. 2. What would happen to the resistance in a wire if the crosssectional area were increased? A. B. C. 3. It would decrease. It would increase. It would remain the same. What would happen to the resistivity the length were increased? A. B. C. It would decrease. It would increase. It would remain the same.

Low Resistance vs. High Resistance n To Summarize: wires make good conductors. n & I n While conductors. = Low Resistance wires make poor I & = High Resistance

Resistance vs. Length and Resistance vs. X-Sectional Area n Resistance and Length? Resistance and XSectional Area? Length X-Sectional Area What is the relationship between:

Resistivity vs. Temperature Note: The Resistivity is at therefore, the resistance is also , .

How fast do the electrons travel? n n A simple observation would tell an observer that the flow of electricity appears to be instantaneous when flipping on a light switch. Does that mean the electrons travel at the speed of light?

Drift Velocity n + + + + + When an electric field is applied to a conductor, it will set the electrons in motion in an overall direction the applied field. e n - While the electric field travels at nearly the of , the overall speed of the electron from one end of the conductor to the other is quite slow and random in direction due to.

Determining the drift velocity in a wire. n The total charge in a section of wire can be determined as follows: n Where: • • n= A= L= e=

Determining the drift velocity in a wire. n Since all the electrons move along the conductor at the same average drift speed, the total amount of charge that moves through a cross section of wire is: n Where the time it takes for the total charge to move through any cross section can be found by: n Where vd = .

Determining the drift velocity in a wire. n Substituting (1) into (2) for q, and (3) into (2) for q, and then solving for vd gives us: n The number of charge carriers per unit volume (n) can be found as follows: n Where: NA = M= =

Example – Drift Velocity n What is the drift velocity in the copper wires leading to a kitchen appliance that operates at 1 Ampere? n n n Note: wire in your kitchen has to be capable of carrying 20 amps of current, therefore, it is specified to be 12 gauge. The cross-sectional area of 12 gauge wiring is 3. 31 x 10 -6 m 2 Assume that 1 electron is donated by each atom. The density is 8. 96 x 103 kg/m 3. The atomic mass is 63. 546 g/mole.

Finding the Drift Velocity in a Copper Wire n First determine the number of charges per unit of volume (n) n Now determine the drift velocity (vd) n That’s only m/hr!

Power n Power = Rate at which work is done where: P=1 P= V= / =1 / = (1 / =1 ) • (1 I= )=1 / Since V = and I = P= = =1 : /

Example (Power) What is the power rating of a lightbulb in circuit where the current is 0. 50 A and the voltage is 120 V?

Power vs. Current and Power vs. Voltage (Ohmic Materials) n What is the relationship between: n power and current? Current P= Power and voltage? Voltage P=

Energy n Since power is the rate at which work is done the amount of energy required to complete a task is as follows: Total Energy = x W=

Example (Energy) How much energy is consumed by a lightbulb operating in circuit where the current is 0. 50 A and the voltage is 120 V for 1 hour?

Key Ideas n n n n A circuit is a closed path where current can flow. Current is the flow of charge. Resistance is the hindrance to the flow of charge. Ohm’s Law = voltage to current ratio (V/I) = Resistance. Resistivity is an intrinsic property of a material that is proportional the resistance. An increase in length of a conductor will increase resistance. An increase in cross-sectional area of a conductor will decrease resistance. Power equals the rate at work is done and is represented electrically by P = IV.