A so-called lie detector works in part by measuring the resistance between two p

Question

A so-called lie detector works in part by measuring the resistance between two points on the subject's skin. Theoretically a person perspires more when he is lying and the resistance of his skin drops. What other reasons could you have for wanting to know the resistance of an object? Questions 1. Why did the current increase when you closed the knife-blade switch? 2. Why is it very important to keep the resistance per unit length as low as possible for cable used for house wiring and utility lines? 3. You have two light bulbs. Does the brighter one have higher or lower resistance if they both operate at the same voltage? (Hint: What are the equations for dissipated power?)

Explanation / Answer

Question (1) –

When you close the knife switch, you provide the path of the electric current. In this way the current flows through the closed loop. So, this is the reason why the electric current increases when we close the knife switch.

Question – (2) –

The expression for power loss through an electric circuit is –

P = I^ * R

where, P is the power loss, I is the current through the circuit and R is the resistance of the circuit.

In the above expression, we see that P is directly proportional to R.

Means, greater the value of R larger will be the power loss.

So, to minimize the power loss, per unit resistance is kept low for a circuit.

Question-(3) –

Power P = V^2 / R

so for the same value of V, the bulb which has lower value of R will have greater value of P.

So, the brighter bulb has lower value of resistance.   

Related Questions

Navigate

• Browse (All)
• Browse A
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.