Electrical resistance

• Resistance measures how difficult it is for current to flow.

Electrical Resistance

• The total amount of electrical resistance in a circuit determines the amount of current that in the circuit for a given voltage.

• The more resistance the circuit has, the less current that flows.

Measuring resistance

• Set the meter to measure resistance ().

• Set the black and red leads on opposite ends of the objects.

The ohm

• Resistance is measured in ohms ().

• One ohm is the resistance when a voltage of 1 volt is applied with a current of 1 amp.

Ohm's law

• German physicist Georg Ohm experimented with circuits to find an exact mathematical relationship between voltage, current and resistance.

• Ohm's law can be used to predict any one of the three variable if given the other two.

Calculate current

• A light bulb with a resistance of 2 ohms is connected in a circuit that has a single 1.5-volt battery.

• Calculate the current that flows in the circuit. • Assume the wires have zero resistance.

The resistance of electrical devices

• The resistance of electrical devices ranges from very small (0.001 Ω) to very large (10×106 Ω).

• Each device is designed with a resistance that allows the right amount of current to flow when connected to the voltage the device was designed for.

Changing resistance

• The resistance of many materials, including those in light bulbs, increases as temperature increases.

• A graph of current versus voltage for a light bulb shows a curve.

• A device with constant resistance would show a straight line on this graph.

Electrical Conductivity

• The electrical conductivity describes a material’s ability to pass electric current.

19.3 Conductors and insulators

• A material such as copper is called a conductor because it can conduct, or carry, electric current.

• Materials that insulate against (or block) the flow of current are classified as electrical insulators.

• Some materials are neither conductors nor insulators.

• These materials are named semiconductors.

19.3 Resistors• Electrical components called

resistors can be used to control current.

• Resistors have striped color codes to record their "values" (writing on them is difficult).