RESISTING THE MOVEMENT OF CHARGE Topic 3

Resisting Movement of a Charge Resistance is a property of a substance that hinders motion of electric charge and converts electric energy into other forms of energy. If current is described as the movement of electrons through conductors, and voltage is what makes the electrons move, then resistance is what opposes the motion of the electrons. A good conductor has low resistance. Poor conductors have high resistance.

Resistance Roadblock Nichrome wire is a resistor. The standard unit for resistance is the ohm ( Ω) Resistance can be measured with an ohmmeter Technicians are more likely to use a multi-purpose electronic measuring instrument (a “multimeter”) set to measure resistance Ohm’s law R=V/I ie. Resistance = Voltage (potential difference)/ Current Other forms of the equation are: V = IR and I = V/R

Example What is the resistance of an electric heater, if a current of 12.5 A runs through it when it is connected to a 120 V wall outlet?

Practice Please work on the next four practice problems in your notes independently.

Resistors In electronic circuits, resistance is useful for: Changing the energy of moving charges into heat. To control current or voltage to suit the specific needs of other electric devices in the circuit. Circuit resistors are usually made of a heat-conducting ceramic core coated with materials such as carbon, metal oxides, metal film, or resistance wire

Variable Resistors Variable resistors change in response to temperature, light, voltage, or some other variable. Varying the length of a Nichrome wire can act as a variable resistor. The longer the length, the more resistance there is. Variable resistors are used in light dimmers, motor speed controls, surge protectors, and refrigerator and stove temperature controls. Variable resistors are also called rheostats.

Types of Circuits Series circuits have only one current path. Parallel circuits have several current paths.

Series Circuit As more bulbs are added in a series circuit, the bulbs become dimmer because there are more resistors for the current to travel through-more bulbs mean less current. In a series circuit all loads must be functional & all switches closed for any of the loads to work

Series Circuit Advantages of series circuits: – Easy to construct – Less wire – Batteries last longer Disadvantages of series circuits: – Additional bulbs mean dimmer lights – If one bulb burns out, they are all out

Parallel Circuits Adding more bulbs to a parallel circuit does not decrease the brightness of the bulbs. More bulbs increases the current. In a parallel circuit not all loads need to be functional for the circuit to work because there are a number of paths the current can travel

Parallel Circuits Advantages of a parallel circuit – Additional bulbs does not reduce the brightness – Other loads continue to work even though one may not be working. Disadvantages of a parallel circuit – More wires to construct – Wires heat up with increased current flow – Batteries won’t last as long

House Wiring Practical electric circuits for the home are parallel circuits. The voltage across each load in a parallel circuit is the same. Turning on one appliance will not reduce the energy available to other loads.

House Wiring One potential problem does exist with parallel circuits. Current through wires connected to the source increases whenever another branch in the circuit is closed. When you turn on any appliance in your home, the current in the wires closest to the source increases. More current means the temperature of the conducting wire rises.

House Wiring The wires could become hot enough to start a fire. To guard against an electrical fire, household circuits always include fuses or circuit breakers.