Ohm’s Law and Electrical Safety

1. Ohm’s Law and Electrical Safety Lesson 11 November 23rd, 2010

2. A Fascination with Electricity • Georg Simon Ohm was born in German in the early 1800s. He taught math in a school in Switzerland. He found in an interest in electricity and his studies led to a deep understanding of how these different electrical concepts were related.

3. Ohms Law His discoveries are today known as Ohm’s law. • A law in science is a generalization based on collection of observable evidence. • Ohm’s law established the relationships between potential difference (V), current (I), and resistance (R). The symbol for resistance is called the ohm (Ω) in honour of Georg Ohm’s work in this field.

4. Quick Lab • Hook up the following in series: • Battery • Connectors • Switch • Lamp • Ammeter in parallel with the lamp • The lamps are 5 Ohms • Multiply the Ohms by the amps to find voltage.

5. Quick Lab • 5 Ohms x 0.3 A = 1.5 V

6. V I R Potential Difference, Current, and Resistance • He realized that the potential difference (V) in a circuit is equal to the current (I) multiplied by the resistance (R). • Ohm’s law states that, as long as temperature stays the same, V = IR • the resistance of a conductor remains constant • the current is directly proportional to the potential difference

8. Example 1 • A current of 4.0 A flows through a 40 Ω resistor in a circuit. What is the voltage?

9. Example 2 • A 30 V battery generates a current through a 15 Ω resistor. How much current does the battery generate?

10. Example 3 • An electric stove is connected to a 240-V outlet. If the current flowing through the stove is 20 A, what is the resistance of the heating element?

11. Ohm’s Law and Temperature • Ohm’s law works for most circuits. However, temperature affects resistance. Generally, resistance is lower when a conductor is cooler. As the temperature increases, resistance increases. • For example, a filament in an incandescent light bulb often has 10 times more resistance when the bulb is warm.

12. Short Circuits • A path that allows electrons to flow along a different path than the one intended. • A short circuit is an accidental low-resistance connection between two points in a circuit, often causing excess current flow

13. Short Circuit • This will cause your electrical device to stop working • Due to the low resistance, the current increases and the conducting wires can quickly become hot and can start a fire • Short circuits can also kill if a person becomes the ground

14. Electrical Safety and Safety Devices • All electrical appliances present a risk of electric shock. Some electronic devices, such as computers, retain electric charge even when they are unplugged. This is why many electrical devices have a “Do Not Open” warning printed on them.

15. Fuses and Circuit Breakers • A fuse is a safety device in an electric circuit that has a metallic conductor with a low melting point compared to the circuit’s wires. If the current gets too high, the metal in the fuse melts and the current flow stops. • This prevents further problems, such as damage to your electrical components or a possible fire. A blown fuse must be physically replaced as it can work only once. • The symbol represents a fuse in a circuit diagram.

16. Fuses and Circuit Breakers • A circuit breaker does the same job as a fuse except that the wire inside does not melt. Instead, the wire heats up and bends, which triggers a spring mechanism that turns off the flow of electricity. Once the breaker has cooled, it can be reset.

17. Three-Prong Plug • The third prong of a three-prong electrical plug connects the device to the ground wire of the building. The ground wire sends any unwanted current flow directly to the ground. Instead of electricity travelling to the metal body of the device and shocking a person using the device, the current is directed to the ground.

18. Ground Fault Circuit Interrupter • A ground fault circuit interrupter (GFCI) or residual current device is a device that detects a change in current and opens the circuit, stopping current flow. For example, if an appliance gets wet while you are handling it and some current starts to flow through the water, the GFCI opens the circuit so there is less chance of injury to you.

19. Questions – hand in • Practice Problems (finding voltage) • 1. A current of 1.5 A flows through a 30 Ω resistor that is connected across a battery. What is the battery’s voltage? • 2. If the resistance of a car headlight is 15 Ω and the current through it is 0.60 A, what is the voltage across the headlight? • 3. The current in a circuit is 0.50 A. The circuit has two resistors connected in series: one is 110 Ω and the other is 130 Ω. What is the voltage in the circuit? (Remember to add the resistances together)

20. Practice Problems (finding current) • 1. A fire truck has a searchlight with a resistance of 60 Ω that is placed across a 24-V battery. What is the current in this circuit? • 2. A bulb with 15 Ω of resistance is in a circuit powered by a 3-V battery. What is the current in this circuit? • 3. What would the current be in question 2 if you changed to a 45 Ω bulb?

21. Practice Problems (finding resistance) • 1. A current of 0.75 passes through a flashlight bulb that is connected to a 3.0-V battery. What is the bulb’s resistance? • 2. A current of 625 mA runs through a bulb that is connected to a 120-V power supply. What is the resistance of the bulb? • 3. A table lamp draws a current of 200 mA when it is connected to a 120-V source. What is the resistance for the table lamp?