Electrical Resistance - Examples

Electrical Resistance

































Find the electrical resistance of a light bulb which passes 2A of current when a potential difference of 120V is placed across it.

R = V/I = (120V)/(2A) = 60



Suppose a bulb passes 3A of current when 120V is put across it, what then is its resistance?

R = V/I = (120V)/(3A) = 40


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Metal wires usually obey Ohm's Law. If a potential difference of 1.5V causes 3A of current to flow in a wire, how much current will flow when 6V is applied?

Voltage and current are proportional --- the voltage is multiplied by a factor of 4 so the current is also multiplied by 4: 12A of current will flow.

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A diode shows very low resistance to current flowing in one direction and very high resistance to current flowing in the opposite direction.

Transistors are useful because their resistance to current flowing between two of their terminals depends on the current which flows in a third, control terminal.

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An electrician holds a (grounded) aluminum ladder with one hand and a live electric light terminal (at 120V) with the other. What will happen to her?

Case 1: Dirty hands. Use V = IR to find
I = V/R = (120V)/(1,000,000) = 0.00012A
--- She does not even notice.

Cases 2: Clean hands.
Find I = V/R = (120V)/(500) = 0.24A
--- She is killed instantly.

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