CBSE Class 10 Physics Electricity Notes Set 02

Ohm's Law

1. Connect a resistance coil, an ammeter (in series), a voltmeter (in parallel with the coil), a rheostat and a key to a battery. Move the rheostat to get different currents and note the current I and potential difference V. Keep the key open when no reading is being taken so that the wire does not heat.
2. V/I is constant for the coil, and the resistance R = V/I. A graph of V against I is a straight line through the origin whose slope is R.
3. Ohm's law holds only if the temperature is constant. A filament bulb does not obey it, because its resistance rises as it heats.

Resistors in Series and in Parallel

In the experiment, the resistances r\( _1 \) and r\( _2 \) are measured separately by V/I and then together; the measured series value is r\( _1 \) + r\( _2 \), and the measured parallel value agrees with the formula within experimental error.

Joule's Law and the Mechanical Equivalent of Heat

1. A current I at potential difference V for time t does work W = VIt joules. In a calorimeter with a heating coil this appears as heat H = (mass of water + water equivalent of calorimeter) \( \times \) rise of temperature, in calories.
2. J = W/H, which comes to about 4.2 J per calorie. Heat produced in a resistor is I\( ^2 \)Rt, so doubling the current gives four times the heat.
3. Heat loss is reduced by insulating the calorimeter and by stirring gently.

Electrochemical Equivalent of Copper

1. In a copper voltameter, a copper cathode is cleaned, dried and weighed and then placed with a copper anode in copper sulphate solution containing a little sulphuric acid. A steady current I is passed for a time t.
2. The cathode is dried and weighed again. The gain in mass m divided by the charge It gives the electrochemical equivalent Z = m/(It). For copper Z is about 0.000329 g per coulomb.
3. The current must not change direction, or copper would be taken from the cathode.