October 2, 2017 Ohm’s Law

Drill A series circuit has 165- of total resistance with 14 V as the power supply. What is the total current of this circuit? A copper wire has a length of 133 m and a cross-sectional area of 1.32 x 10-6 m2. If the wire is connected to a 9-volt battery, how much current flows through the wire? The resistivity of copper is 1.68 x 10-8 m.

A student connects a single resistor to a variable voltage source, then uses an ammeter to measure the current through the resistor at increasing voltages. The student’s data is shown in the graph below. Calculate the value of resistance of the resistor.

The graph below shows the graph of V vs I for four resistors A, B, C and D. Which resistor has the highest resistance?

Partner Check Homework!

Electric Current Current =  rate at which charge flows past a point in a circuit I = Q/t 1 ampere = 6.24 x 1018 electrons flowing in 1 second Ben Franklin created the inaccurate convention that we still use today: Current flows from the positive terminal  negative terminal. Electrons actually move from the negative terminal  positive terminal.

Electric Potential Difference Potential Difference = Voltage https://www.youtube.com/watch?v=TBt-kxYfync (2:30)

Ohm’s Law Ohm’s Law can be manipulated into many different equations: Resistance = voltage/current (R = V/I) Voltage = current x resistance (V = IR) Current = voltage/resistance (I = V/R) Remember resistance in the circuit is constant Resistance is the opposition to the flow of current Determined by the resistor + components in circuit (wires, loads)

Ohm’s Law Current = voltage/resistance (I = V/R) Current =  rate at which charge flows past a point on a circuit The rate of flow (current) depends upon the amount of energy supplied to the circuit (voltage) and the amount of opposition to its flow (resistance).

Ohm’s Law If the voltage in a circuit doubles, causing the current to double, what happened to the resistance value of the resistor in the circuit? If the voltage across a fixed resistance is increased to a value five times larger than the original, what will happen to the current? If the resistance in a circuit with constant voltage increases, the current will ________. The total amount of charge that passes through a cross section of wire per unit of time is referred to as _________. If a resistor suddenly decreases in value (resistance decreases), what will happen to the current through the resistor?

Power Power = rate at which electrical energy is transferred by an electrical circuit P = VI Unit = watt (W) 1 watt = 1 joule/second What are examples of common wattages?

Power A potential drop of 50.0 volts is measured across a 250- ohm resistor. What is the power developed in the resistor? An electrical appliance draws 9.0 amperes of current when connected to a 120-volt source of potential difference. What is the total amount of power dissipated by this appliance?

Combining Equations – Example 1 The tungsten heating element in a 1500-W heater is 3.00 m long, and the resistor is to be connected to a 120-V source. What is the cross-sectional area of the wire? The resistivity of tungsten is 5.6 x 10-8 m. Power equation – find current Ohm’s Law – find resistance Resistance equation #1 – find cross-sectional area

Combining Equations – Example 2 A copper wire within a 60-W lightbulb has a cross-sectional area of 3.14 x 10-3 m2. The resistivity of copper is 1.68 x 10-8 m. Assuming the lightbulb is connected to a 125-V power source, what is the length of the wire? Power equation – find current Ohm’s Law – find resistance Resistance equation #1 – find length

Create a Vocabulary Chart Term Symbol Unit Equation Definition Resistance Voltage Current Charge N/A Power

Lab Report Include all required sections Graphs Axis labels Units Equation Slope calculation – what does it represent? Analysis Explain relationship between voltage, current, resistance How does data prove that V/I is equal to resistance? Conclusion Was your hypothesis correct? Sources of error Future experiments