How do we measure current and potential difference (voltage) in a circuit? HW:

What are the three variables in Ohm’s Law? Current (I) in Amps Voltage (V) in Volts Resistance (R) in Ohm’s How does a simple circuit look in order to function? Closed Loop Battery Resistor (sometimes light bulb)

Circuit Elements Other Elements Resistances Measurement Devices Voltage sources Fixed resistor – resistance does NOT change once installed Opens and closes the circuit turning current flow on or off Resistance CAN be changed after installation Voltmeter is a device for measuring VOLTAGE How hard are electrons being pushed? Ammeter is a device for measuring CURRENT How many electrons flow per second? Provide electrical potential for electrons – “PUSHING” or “PUMPING” them through the circuit Special resistor – used to produce light!

Measurements V V V A A V SERIES PARALLEL Voltmeter measures RELATIVE differences from OUTSIDE the circuit PARALLEL V Ammeter measures flow INSIDE the circuit SERIES 100V R = 10Ω V V 0V A 0V 10A A 10A V = 100V V 100V

Ammeters are ALWAYS wired in series! Light Bulbs A Voltage Source A

Voltmeters are ALWAYS wired in parallel!

SUMMARY How is the voltmeter connected to a circuit? Parallel with high resistance, so no current goes through. How is an ammeter connected to a circuit? Series with little resistance, so all the current passes through.

Ohm’s Lab Goal: Investigate the relationship between Voltage and Current.

Two types of Circuits Series Circuit: When two or more resistors connect end-to-end in series with each other forming a loop. Parallel Circuit: Current from the power source splits into separate branches such that the resistors are not in series, but parallel.

Series Rules ITotal = I1 = I2 = I3 = …= IN The equivalent resistance for a series circuit is the SUM of all RESISTANCES in the circuit RTotal = R1 + R2 + R3 + …+ RN The total voltage in the circuit is equal to the SUM of the VOLTAGE DROPS across the resistors. VTotal = V1 +V2 + V3 + …+ VN The current in the circuit is constant at ALL points in the circuit. ITotal = I1 = I2 = I3 = …= IN All COMPONENTS and the WHOLE CIRCUIT obey OHM’S LAW

Req = R1 + R2 + R3 = 400Ω 100 Ω 75 Ω 225 Ω R1 R2 R3 10 V I = V / R I = 0.025 A 10 V

VIR Chart V I R R1 R2 R3 Req 100Ω 75Ω 225Ω 10V 0.025A 400Ω 0.025A 2.5V

POWER Similar to Ohm’s Law, power applies to each resistor in a circuit. P = IV P-Power Unit: Watts (W)

50 Ω 120 Ω 150 Ω R1 R2 R3 1.5A ? V

VIRP Chart V I R P R1 R2 R3 Req 50Ω 120Ω 150Ω 1.5A 75 V 1.5A 180 V 112.5W 180 V 1.5A 270.0W 225 V 1.5A 337.5W 480 V 320Ω 720W

SUMMARY What does a series circuit look like? In a series circuit, what are the rules for… Current? Voltage? Resistance?