Electrical Resistance

Gr 9 Science

• Electrical resistance = the property of a substance that hinders electric current and converts electrical energy to other forms of energy.

• Resistor = a device used in an electric circuit to decrease the current through a component by a specific amount.

• Load = a resistor or any other device that transforms electrical energy into heat, motion, sound or light.

Ohm`s Law

• Georg Ohm was curious about the relationship between current and force. He noticed in experiments that when the force was cut in half, the current was also cut in half.

• The resistance remained the same. • So Ohm`s Law states that:

force (V) = current (A) x resistance (Ω)or V = I x R

• Therefore,

current (A) = force (V) ÷ resistance (Ω)or I = V ÷ R

resistance (Ω) = force (V) ÷ current (A)or R = V ÷ I

• The power of an electrical machine is measured in watts.

watts = voltage (volts) x current (amps)

So,

• Ohm`s Law also states that the ratio of potential difference to current is a constant called resistance.

• The unit of resistance is the ohm (Ω), which is equivalent to one volt per ampere (V∕A).

Factors that affect the resistance of wires:

• 1. The type of material affects the resistance. Ex., a copper wire has less resistance than an iron wire of the same length and diameter.

• 2. A shorter wire has less resistance than a longer wire of the same diameter and made of the same material.

• 3. A thicker wire has less resistance than a wire of the same length and material.

• 4. Resistance increases with temperature. A cold filament has less resistance than a hot filament.

• A superconductor is a material through which electric charge can flow with no resistance.

• A superconducting wire does not transform electrical energy into heat. Therefore, using a superconducting wire would vastly increase the efficiency of supplying electrical energy.

• A resistor that obeys Ohm s Law has constant ′resistance.

• A resistor that does not obey Ohm s Law is ′called a non-ohmic conductor.

Ex., in the filament of an incandescent bulb resistance increases as the filament s ′

temperature increases