ELECTRICAL RESISTANCE

Electrical Resistance: is a property of a substance that hinders motion and converts electrical energy into other forms of energy.

For example, the resistance of the tungsten wire filament in a light bulb is over 400 times greater than the copper connecting wires.

When a current flows through the high resistance filament of the light bulb, the filament converts much of the energy carried by the current into light and heat.

• Wires with high resistance are used in appliances to convert electrical energy into heat energy

1. Outline of Glass bulb2. Low pressure inert gas (argon,

nitrogen, krypton, xenon)3. Tungsten filament4. Contact wire (goes out of stem)5. Contact wire (goes into stem)6. Support wires (one end embedded

in stem; conduct no current)7. Stem (glass mount)8. Contact wire (goes out of stem)9. Cap (sleeve)10. Insulation (vitrite)11. Electrical contact

TV O Resistance video (9 min.)

• https://www.youtube.com/watch?v=YGvu9iqjJq4

Ohm’s Law• For most wires, the ratio of potential

difference (V) to current (A) is constant

• The constant is called resistance (R)

• Ohm: one ohm is the resistance of a conductor through which a current of one ampere flows when a potential difference of one volt is applied.

Electric Resistance Math Formula

•

R

V

A

• The larger the resistance in a circuit, the smaller the current.

Symbol for Ohm

• A resistor is designed to resist the flow of electrons.

• The coloured stripes on resistors indicate their resistance

Ohmic Conductors• Any resistor that has constant resistance

regardless of potential difference is called Ohmic because it follows Ohm’s Law

• Examples: are copper, iron and silver wires at normal temperatures

Graphing Resistance• If you plot the potential difference on the

vertical axis and the current on the horizontal axis, and connect the points, there is a straight line.

• The slope of the line is the resistance.

• The steeper the slope, the greater the resistance.

Ohm’s Law: The slope is the Resistance

Non-Ohmic Conductors• An incandescent light bulb is non-ohmic

because its resistance increases with temperature

Factors that Affect the Resistance of a Wire

1. Type of material: For example, copper is less resistant than silicon

• Length of the wire: the longer the wire, the greater the resistance

• The Diameter of the Wire: the greater the diameter, the less the resistance

• The temperature of the wire: The higher the temperature, the greater the resistance

• Can you explain this using Particle Theory?

• The molecules of good conductors have less resistance to flow of electrons

• The molecules of poor conductors have greater resistance to flow of electrons.

Superconductors

• When electric charge can flow through a material with no resistance, the material is a superconductor.

• If wires that were superconducting at room temperature could be made, they would vastly increase the efficiency of supplying electrical energy.

Large Hadron Collider for Nuclear Research (CERN)

• Is the largest superconducting magnet ever built. It uses a current of 21 000A but has a potential difference of only 9 volts.

• Calculate its resistance…..

CERN 4 minutes

• http://ishare.rediff.com/video/entertainment/large-hadron-collider/419443