Electricity: Electric Current and New Resistance

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COMMON MISTAKES AND HOW TO AVOID THEM Phy X: ELECTRICITY

1.

Topic Direction of flow of current.

Common slips made Confusion between direction of flow of current and direction of movement of electrons Confusion in use of symbols- Is ampere written as A or amp. V=W/Q is often remembered wrongly. In questions where you have to check the resistance from the graph see what is plotted along the x and y axis. Reason why alloys are used to make elements of electric irons and heaters is not because they are insulators.

2.

Units

3. 4.

Formula Graphs

5.

Resistivity

6.

Change in resistance

If a wire is folded(doubled on it) and made half its length, then the resistance does not become half.

7.

Resistors in series

Is the current or the voltage same in resistors in series.

How to avoid The direction of current is opposite to the direction of movement of electrons. In a circuit current always flows from positive to negative terminal and you need to put arrows in this direction. The symbol for ampere is A and not amp. If you wish you can use “ampere” instead of “A”. Make sure you memorize this formula correctly. If V is plotted along y axis and I is plotted along x axis then the slope of the graph gives R. If V is plotted along x axis with I along the y axis then the slope of the graph gives 1/R. Had the alloys been insulators then current would not flow at all. We want current to flow in these devices as well as heat up the wire, so we cannot use a very good conductor like copper, aluminium etc. and the best choice is to use an alloy that has a high resistivity. Remember that the area of cross-section also becomes double, so the new resistance is ¼ the old resistance. If the length of the wire was made 1/3rd by folding it then the new resistance will be 1/9th the original. If the length of the wire is made 1/4th then the new resistance is 1/16th the original. Can you see the pattern? The current is the same. Current that starts from

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