13.3 RESISTANCE & OHM’S LAW; PP Objectives: 1) Explain the relationships between current, voltage, and resistance 2)Use Ohm’s Law to calculate current, resistance, or voltage. between conductors and insulators. 3) Distinguish between conductors and insulators.

Electrical Resistance Resistance is the measure of how strongly a wire or other object resists current flowing through it. Ex: copper wire has a low resistance thus easily carries a large current. Ex: a rubber band has a high resistance thus carries a tiny current. Ex. From lab 13A, the potentiometer acted as a resistor with varying degrees of resistance depending on the amount of resistance applied to the current.

Water Analogy (comparison) The relationship between electric current and resistance can be compared w/ water flowing from an open end of a bottle. *Large opening: resistance is low & water flows out easily. **Small opening: resistance is greater & water flow is slow.

Circuits The total amount of resistance in a circuit determines the amount of current for a given voltage. Every device that uses electrical energy adds resistance to a circuit. The more resistance the circuit has, the less the current. Ex: if we string several light bulbs together, the resistance in the circuit increases and the current decreases. Each bulb is dimmer than a single bulb would be.

Measuring Resistance: the ohm (Ω) Electrical resistance is measured in units called ohms. This unit is abbreviated w/ the Greek letter omega( Ω ). If a circuit has a resistance of one ohm, then a voltage of one volt causes a current of one ampere to flow.

Resistance of wires Wires used to connect circuits are made of metals such as Cu( copper) and Al (aluminum). The resistance of wires is usually so low compared to other devices in a circuit that you can ignore wire resistances when calculating the total resistance. Exception to above rule: when dealing with large currents.

Measuring resistance A Multimeter can be used to measure the resistance of wires, light bulbs, and other devices. Ex: Like the one used in Lab 13A. The meter will display resistance in ohms, Kilo-ohms (x1,000Ω), or mega-ohms ( x1,000,000 Ω).

Ohm’s Law The current in a circuit depends on the battery’s voltage and the circuit’s resistance. Voltage and current are directly related. Ex: double the voltage, doubles the current. 1V, 1amp; 2V, 2amps. Resistance and current are inversely related. Ex: double the resistance cuts the current in half. 4Ωohms; 2amps.

The formula for Ohm’s Law Current (amps, A); I = V/R,( V=volts,V), R=resistance (ohms, Ω) Ex: A toaster has a resistance of 12 ohms and is plugged into a 120-volt outlet. How much current does it draw? I= V/R Given: 120-volts=V and R=12 ohms, plug into formula I=120 Volts/12 ohms I= 10 volts/ohms or 10A Try a and b on page 308 for more practice

Ohm’s Law continued Equation…gives you……if you know… I=V÷RCurrent (I)Voltage and resistance V=I X R R= V÷I Voltage (V) Resistance (R) Current and resistance Voltage and Current

The Resistance of Common Objects The resistance of electrical objects ranges from small (0,001 Ω) to large (10 X 10^6 Ω). Every electrical device is designed w/ a resistance that causes the right amount of current to flow when the device is connected to the proper voltage. Resistances match operating voltage.

The resistance of skin The typical resistance of dry skin is 100,000 ohms or more. On average, nerves in skin can feel a current of around amps. You can get a dangerous shock from 120 volts from a wall socket because there is enough voltage to force amps( 120 V ÷100,000Ω) through your skin!

Water lowers skin resistance Wet skin has much lower resistance than dry skin! Due to the lower resistance, the same voltage will cause more current to pass through your body when wet! This occurrence can cause SERIOUS injury or may be Fatal!

Changing Resistance The resistance of many electrical devices varies with temperature and current. The resistance of many materials, including those in light bulbs, increases as temperature increases.

Conductors and Insulators A conductor can conduct, or carry, electric current. Most metals are good conductors, like Al & Cu. Insulators do not allow current to easily pass through them like glass, rubber, or wood. Semi-conductors are materials in between conductors & insulators. Their ability to carry current is higher than an insulator but lower than a conductor. Ex: computer chips, t.v.’s, & portable radios. Remember materials used in Lab 13A.

Resistors Are electrical components that are designed to have a specific resistance that remains the same over a wide range of currents. Used to control the current in circuits like our potentiometer in lab 13A. Two main types: Fixed and Variable. Fixed-have a resistance that cannot be changed. Variable-resistance can be changed. Was our potentiometer a fixed or variable type resistor? Please explain. Home Work: Text p. 311; #1-4