Understanding Units of Electricity Measurement

EQ: How is electricity measured? LO: We will understand what the different units of measurement for electricity mean CT: I will make an analogy of the meaning of the different measurements of electricity to the flow of water

KEY POINTS • Electric Potential Difference (Voltage) • Current • Power • Ohm’s Law • Resistance • Types of circuits https://www.youtube.com/watch?v=q58P_39UFHw

Static electricity vs Electric current • Static Electricity: build up of charges that stay still • Electric Current: continuous flow of charges through a conductor

Electric Current and Circuits Requirements: A closed conducting loop terminal. An energy supply Task: Brainstorm and sketch an unsuccessful circuit to light the bulb

Electricity and Voltage Electricity- is the flow of electrons (-) • Electric Potential Difference = Voltage • The measure of the strength of current based on the strength of the push/pull that causes charges to move and is measured in volts (V) • Electricity-Water Analogy: Charges (-) flow from HIGH voltage areas to LOW voltage areas analogous to water flowing UPHILL to DOWNHILL, the greater the difference from high to low determines the strength of flow

Voltage like Gravity • Work has to be done on rock to move it upward, against the force of gravity • Work has to be done on a + test charge to move it against the force of the electric field = gain in potential energy • Since energy is imparted to the test charge/rock in the form of work, the positive + charge/rock would be gaining potential energy as the result of the motion

Voltage Equation Change in Potential Energy (Joules-J) Change in potential difference (voltage-V) Work (Joules – J) Charge (Coulombs – C)

Voltage Example • Calculate the amount of work it takes to move 1.0 C of charge from the - to the + terminal of a 1.5-volt battery.

Voltage Check Point To move a (+) test charge from D to A in the internal circuit = work done or work not done When work is done on the charge, this changes its potential energy to a: higher value or lower value

Voltage Check Point 1) Moving an electron within an electric field would change the ____ the electron. a. mass of b. amount of charge on c. potential energy of 2) If an electrical circuit were analogous to a water circuit at a water park, then the battery voltage would be comparable to _____. a. the rate at which water flows through the circuit b. the speed at which water flows through the circuit c. the distance that water flows through the circuit d. the water pressure between the top and bottom of the circuit e. the hindrance caused by obstacles in the path of the moving water 3) If the electrical circuit in your Walkman were analogous to a water circuit at a water park, then the battery would be comparable to _____. a. the people that slide from the elevated positions to the ground b. the obstacles that stand in the path of the moving water c. the pump that moves water from the ground to the elevated positions d. the pipes through which water flows e. the distance that water flows through the circuit 4) Which of the following is true about the electrical circuit in your flashlight? a. Charge moves around the circuit very fast - nearly as fast as the speed of light. b. The battery supplies the charge (electrons) that moves through the wires. c. The battery supplies the charge (protons) that moves through the wires. d. The charge becomes used up as it passes through the light bulb. e. The battery supplies energy that raises charge from low to high voltage.

Electricity and Current • The rate at which charge flows past a point in a circuit, depends on diameter of wire and is measured in Amperes (Amps-A) • Electricity-Water analogy: flow of charge analogous to water pipes. By increasing diameter of pipe this increases flow of water

Current and speed of charge Direction and velocity of currents Velocity (drift speed) The path of a typical electron through a wire is rather a chaotic zigzag path characterized by collisions with fixed atoms. A typical drift speed might be 1 meter per hour. That is slow!

Current Equation Current (Ampere – Amps – A) Charge (Coulombs – C) Time (Seconds – s)

Current Example Amtrak introduced an electric train in 2000 that runs between New York and Boston. With a travel time of 3.00 h, the train is not superfast, but it is comfortable, very safe, and environmentally friendly. Find the charge passing through the engine during the trip if the feeding current is 1.80 ×102 A.

Current Check Point 1) A current is said to exist whenever _____. a. a wire is charged b. a battery is present c. electric charges are unbalanced d. electric charges move in a loop 2) Current has a direction. By convention, current is in the direction that ___. a. + charges move b. - electrons move c. + electrons move 3) The drift velocity of mobile charge carriers in electric circuits is ____. a. very fast; less than but very close to the speed of light b. fast; faster than the fastest car but nowhere near the speed of light c. very slow; slower than a snail 4) If an electric circuit could be compared to a water circuit at a water park, then the current would be analogous to the ____. A. water pressure B. gallons of water flowing down slide per minute C. water D. bottom of the slide E. water pump F. top of the slide

Electricity and Power Poweris the rate at which electrical energy is supplied to a circuit or consumed by a load. - Short circuit high rate of charge flow and no load to apply it to Change in voltage (Volts – V) Current (Amps – A) Power (Watts – W)

Power Equations

Power Examples In 1994, a group of students at Lawrence Technological University, in Southfield, Michigan, built a car that combines a conventional diesel engine and an electric direct-current motor. The power delivered by the motor is 32 kW. If the resistance of the car’s circuitry is 8.0 Ω, find the current drawn by the motor.

Power Examples Fuel cells are chemical cells that combine hydrogen and oxygen gas to produce electrical energy. In recent years, a fuel cell has been developed that can generate 1.06 × 104 W of power. If this fuel cell has a current of 16.3 A, what is the potential difference across the fuel cell?

Power Examples If an alarm clock is plugged into a 120 V outlet, the electric current in the clock’s circuit is 4.2 x 102 A. How much power does the alarm clock use?

Ohm’s Law This equation demonstrates the relationship of how current is affected by voltage and resistance. -The current (I) is directly proportional to the voltage. -The current (I) is inversely proportional to the resistance of the circuit. Voltage (Volts - V) Resistance (Ohms -Ω) Current (Amperes or Amps – A)

Resistance *friction that impedes flow of electrons through the circuit (rocks in the river) *The electric potential difference encourages charge movement while resistance discourages it *Factors that affect resistance: -Wire length -wire width -material

Ohm’s Law Review A certain electrical circuit contains a battery with three cells, wires and a light bulb. Which of the following would cause the bulb to shine less brightly? Choose all that apply. a. increase the voltage of the battery (add another cell) b. decrease the voltage of the battery (remove a cell) c. decrease the resistance of the circuit d. increase the resistance of the circuit

Ohm’s Law Review If the resistance of a circuit were tripled, then the current through the circuit would be ____. a. one-third as much b. three times as much c. unchanged d. ... nonsense! There would be no way to make such a prediction.

Ohm’s Law Example A medical belt pack with a portable laser for in-the-field medical purposes has been constructed. The laser draws a current of 2.5 A and the circuitry resistance is 0.6 Ω. What is the potential difference across the laser?

Understanding Units of Electricity Measurement