# Resistance: Electric Current and Potential Difference

**Topics:**Electric current, Electrical resistance, Ohm's law

**Pages:**5 (914 words)

**Published:**July 29, 2013

Resistance decreases

Question 2

The charge possessed by an electron is 1.6x10-19 coulombs. Find the number of electrons that will flow per second to constitute a current of 1 ampere. Ans.

Question 3

Name the instrument used to measure electric current in a circuit. Ans.

Ammeter

Question 4

In the following table the values of current I flowing in a given resistor for the corresponding values of potential difference V across the resistor are shown : I(amperes)| 0.5| 1.0| 2.0| 3.0| 4.0|

V(Volts)| 1.6| 3.4| 6.7| 10.2| 13.2|

Plot a graph between V and I and use this graph to find the value of current when potential difference across the resistor is 5V. Ans.

Finding the current corresponding to 5 V potential difference (from graph) =1.5 A (approx.)

Question 5

(a) What does the following circuit symbols represent?

(i) (ii)

(b) The potential difference between the terminals of an electric heater is 60 V when it draws a current of 4 A from the source. Find the resistance of heater when in use. Ans.

(a) (i) Wires not connected

(ii) Variable resistor/rheostat

Question 6

Mention any two factors on which the resistance of a cylindrical conductor depends. Ans.

Its length, its cross sectional area, nature of its material and temperature of the conductor. (Any two)

Question 7

An electric heater draws a current of 10A when connected to a 200 V supply. What current will the heater draw when it is connected to 220 V supply. Ans.

Question 8

How is an ammeter connected in a circuit to measure current flowing through it? Ans.

In series.

Question 9

What is meant by the statement that the potential difference between two points is 1 volt? Ans.

It means that one joule of work is done to move a charge of one coulomb from one point to another.

Question 10

Name the instrument used for measuring :

(i) Potential difference (ii) Current

Ans.

(i) Voltmeter (ii) Ammeter

How is a voltmeter connected in a circuit?

Ans.

In parallel

Question 12

A wire of length l and area of cross-section A was drawn into a wire of double its length by melting it. If its original resistivity and resistance were and R respectively, what will be its new resistivity and resistance ? Ans.

We know

When a wire is doubled by melting, its length would become double and area of cross- section would be halved. i.e. length of wire =2 l and area of cross section =A/2 Thus, we have

where R1 is the new resistance.

Therefore, The new resistance of the wire is 4 R.

As resistivity of a material is the characteristic of a given material, hence it remains unchanged.

Question 13

Calculate the resistance of a conductor when the current flowing through it is 0.2 A and the potential difference is 0.8 V? Ans.

V = I R or R = V / I = 0.8 /0.2 = 4

Question 14

Apply Ohm's law to define the unit of resistance.

Ans.

Ohm's law, = R.

If V = 1V and I = 1A then R = 1 ohm.

The resistance of a conductor is said to be one ohm if a current of one ampere flows through it when a potential difference of one volt is applied across its ends.

Question 15

How much current will an electric bulb draw from a 220V source, if the resistance of the bulb–filament is 1200 ohms? Ans.

Question 16

(a) The components of an electric circuit are 0.5 m long nichrome wire XY, an ammeter, a voltmeter, four cells of 1.5 V each, rhoestat and a plug key. Draw a diagram of the circuit to study the relation between potential difference across the terminals X and Y of the wire and current flowing through it. (b) State the law that relates potential difference across a conductor with the current flowing through it. Ans.

(a) Circuit Diagram

(b) Ohm's law - If physical conditions like temperature etc., of a conductor are kept unchanged, the strength of current flowing through it is...

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