**Answer: Option C**

# Category Archives: 17 Current Electricity

## Common Electrical Symbols

These are some of the necessary electrical symbols that you must know. Those in grey background are for Pure Physics.

## Finding total effective resistance

## Finding unknown resistor R and setting up the electrical circuit

To find the unknown resistor R, the following apparatus are setup.

Refer to the video below for the setting up of the apparatus.

**Why do you need a variable resistor (rheostat)?**

- Without the variable resistor, you will have only one set of current I and potential difference V readings. Using the formula R = V/I, you are able to find the unknown resistor. But this method is not so accurate.

- Hence, to make it more accurate, we include a variable resistor to control the size of the current through the circuit. Thus having different readings of the potential difference V across the unknown resistor.
- Instead of just one set of readings of I and V, we now have about 5 sets.
- This allows us to plot a graph of V against I.
- By finding the gradient of the best fit line, we are able to find the resistance more accurately.
*[gradient = V / I = R, hence the gradient of V-I graph represents resistance R]*

For** pure metallic conduc****tor**, like the fixed resistor R, it obeys the **Ohm’s Law**, hence it is an ohmic conductor.

From the graph,** the current I flowing the conductor is directly proportional to potential difference V** across the conductor, provided physical conditions like temperature remains constant. *[the graph is a straight line with constant gradient, and passes through the origin]*

## 2013SPp1q17 What is the new P when V across resistor is doubled?

**Solutions: C**

Alternatively, you may put values to work out using the 2 basic formulae of P = IV and V = IR. Refer the video below.

## Rules of Series and Parallel Circuits

Rules of Series and Parallel Circuits

To understand direct current (DC) circuits, the best way is to think in terms of river system.

Series Circuit

Parallel Circuit

Series and Parallel Circuit

Examples: