11.3 OHM’S LAW AND ELECTRICAL SAFETY pp

Ohms 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.

Potential Difference, Current, and Resistance  Ohm 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

Rearranging Equations  When you are missing one of the three variables, you need to rearrange the equation so that variable is by itself on one side of the equal sign  To do this, determine which operation is currently being done to the other variable and do the opposite operation to both sides:  e.g. V = I R : to solve for I, divide both sides by R  On the I R side, R will then divide by R to equal 1 and 1 x I = I so we say the R’s cancel.

GRASP Method  G – given  Write all the given information with its symbol and units (e.g. I = 5A)  R – required  Write the symbol for the required followed by = ? (e.g. V = ?)  A – analysis  Write the formula you are going to use and rearrange equation if necessary

GRASP Method  S – substitute / solve  Plug in numbers into your equation and solve  P – paraphrase  Write a sentence answer to the question

Example 1  A current of 4.0 A flows through a 40 Ω resistor in a circuit. What is the voltage? G I = 4.0 A R = 40 Ω S V = IR = (4.0 A)(40 Ω) = 160 V R Voltage V = ? P Therefore, the voltage in the circuit is 160 V. AV = IR

Example 2  A 30 V battery generates a current through a 15 Ω resistor. How much current does the battery generate? G V = 30 V R = 15 Ω S I = V/R = (30 V) / (15 Ω) = 2 A R Current I = ? P Therefore, a current of 2 A is generated. AV = IRI = V/R

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? G V = 240 V I = 20 A S R = V/I = (240 V) / (20 A) = 12 Ω R Resistance R = ? P Therefore, the resistance of the heating element is 12 Ω. AV = IRR = V/I

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.

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

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

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.

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.

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.

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.

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.

Homework  Read pp. 458 – 464  Answer the following questions:  The 9 practice problems on pp. 460 – 461  p. 467 # 1 – 3, 7 – 12