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

2. 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)

3. 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!

4. 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

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

6. Voltmeters are ALWAYS wired in parallel!

7. 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.

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

9. 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.

10. 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

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

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

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

14. 50 Ω
120 Ω
150 Ω R1 R2 R3
1.5A
? V

15. 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

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