1. Aim: How do we explain Ohm’s Law?

2. Ohm’s Law
∆V = IR The potential difference across a conductor is equal to the current in a conductor multiplied by the resistance of a conductor. Ohm’s Law is not an empirical law of nature meaning it is not true for all materials.

3. Ohmic vs Non-ohmic
What makes a material non-ohmic? The material is Ohmic if its resistance doesn’t change (Voltage and Current are linearly related)

4. Ohm’s Law Questions
1. Which of the following will cause the current through an electrical circuit to decrease? Choose all that apply. a. decrease the voltage b. decrease the resistance c. increase the voltage d. increase the resistance

5. Questions (cont)
2. 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

6. 3. You have likely been warned to avoid contact with electrical appliances or even electrical outlets when your hands are wet. Such contact is more dangerous when your hands are wet (vs. dry) because wet hands cause ____. a. the voltage of the circuit to be higher b. the voltage of the circuit to be lower c. your resistance to be higher d. your resistance to be lower e. the current through you to be lower

7. 4. 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.

8. 5. If the voltage across a circuit is quadrupled, then the current through the circuit would be ____. a. one-fourth as much b. four times as much c. unchanged d. ... nonsense! There would be no way to make such a prediction.

9. Circuit Symbols

10. Ammeters
Ammeters measure current. They are connected in series with the circuit.

11. Voltmeters
Voltmeters measure the potential difference between two points in a circuit. They are connected in parallel.

12. Calculations Draw sketches of each circuit and solve for the unknown.
An electrical device with a resistance of 3.0 ohms will allow a current of 4.0 amps to flow through it if a voltage drop of ________ Volts is impressed across the device.
R= V/I
3Ω=V/4A
V=12 V
2. When a voltage of 120 V is impressed across an electric heater, a current of 10.0 amps will flow through the heater if the resistance is ________ .
R=120 V/10 A
R= 12 Ω
3. A flashlight that is powered by 3 Volts and uses a bulb with a resistance of 60 ohms will have a current of ________ Amps.
R=V/I
R=3V/I
60Ω=3V/I
I=0.05 A

13. Calculations (cont)
2. Sketch the circuits using the appropriate circuit symbols and solve for the missing values
Diagram A
R=3Ω
I=2 A
Diagram B
∆V=6V
R=6Ω
Diagram C
V=2V
I= 2 A
Diagram D 2V
∆V=2V

14. Electrical Power
What are the units of power? Check that Equations for Power: P = VI P = I2R P = V2/R

15. A few notes on Power and Energy
The electrical energy and/or power supplied by the battery is equal to the energy and/or power used by the resistors in the circuit. Also if the circuit is turned on for a circuit amount of time, the energy generated by the battery or used by the resistor E = Pt = VIt = I2Rt=(V2/R)t

16. Electrical Power
Calculate the resistance and the current of a 7.5-Watt night light bulb plugged into a US household outlet (120 V).
P=VI 7.5 W =120 V I I= A
R=V/I R=120V/0.0625A R=1920Ω
2.Calculate the resistance and the current of a 1500-Watt electric hair dryer plugged into a US household outlet (120 V).
P=VI 1500 W=120 V I I=12.5 A
R=V/I R=120V/12.5 A R=9.6Ω
3.The sticker on a compact disc player says that it draws 288 mA of current when powered by a 9 Volt battery. What is the power (in Watts) of the CD player?
P=VI=9V(.288 A)=2.6 W

17. Electrical Power
4. A color TV has a current of 1.99 Amps when connected to a 120-Volt household circuit. How much electrical energy is consumed by the Tv in 60 s? W=VIt=120V(1.99 A)(60s)=14,328 J 5. A 27 Ωtoaster is connected to a 120-V household outlet. How much energy does the toaster consume in 10 s? W=(V2/R)t=[(120V)2/27Ω]10s= J