### Parallel and Series Circuits

Electrical devices can be connected in two different ways in a circuit.

1) In series
2) In parallel

Series Circuits

Rtotal = R1 + R2

• The current is the same through each resistor.
• The voltage splits up.
• Total resistance exceeds the highest resistance of the resistors.

Parallel Circuits

1/Rtotal = 1/R1 + 1/R2

• The voltage is the same through each resistor.
• The current splits up.
• Total resistance is less than the lowest resistance of the resistors

In the following circuit, the resistors are in red. Put in the values that you deal with in Red boxes. Treat the boxes as if they were real resistors, in doing so, and then calculate the total resistance.

Resistance Calculator

R  =  Ω

1. The dimensions of a solid iron cuboid are 6cm, 4cm and 2cm respectively. If the resistivity of the substance is 0.002 units, find the resistance in the following cases:
• If the current enters through the smallest surface
• If the current enters through the largest surface
2. Explain the following:
• The copper cable of a lightning conductor is very thick
• The wire inside a hair dryer is thin and long
3. The resistance of a piece of wire is 9 Ohms. Find the new resistance, if it is folded in two and then twisted.
4. Can Ohmâ€™s law be applied to a light bulb which is hotter when in use, as the physical conditions do not seem to be constant?
5. You are provided with three resistors of 12 Ohms each. How many different combinations can be obtained from them? Calculate the total resistance in each case.
6. If two resistors are connected in series the total resistance is 25 Ohms and if they are connected in parallel the resistance is 6 Ohms. Find the resistance of each.
7. The power of a light bulb is 60W at 240V. What is its new power if the voltage is dropped down to 200V?
8. Explain as to why the electricity is transmitted at a very high voltage through thick copper cables through large distances.
9. If a 2 kW kettle is used 2 hours a day for a period of one month, how many kWh are used up?
10. Explain the following:
• A switch is always connected across the live wire
• TV does not need an earth wire
• Ammeters are connected in series whereas voltmeters are connected in parallel