Technology 3 course

Calculate the equivalent resistor and the current in the battery.

Remember: In a series circuit, the total resistance of the circuit (also called effective resistance) is equal to the sum of the individual resistances, so Re = R1 + R2 + R3 …

In a parallel circuit, a quicker method of finding  the equivalent resistance is to use the general formula:

1 / Re = 1/R1 + 1/R2  + 1/R3 …..
Solution: We have three resistances in serial (10, 10 and 20 ) in the first branch.

After that, we have two branches with a resistor of 40 each one (in parallel). Then

Re = 20 Ω ,  Now using Ohm`s Law I = V/R, I =5V/20Ohm = 250 mA

Re =   64,18 Ω    I = 77,9 mA

Re = 58,41 Ω     I = 85,6 mA

1º In this circuit, the value of the battery is 10 Volts and the current measured in the ammeter is 2 mA.

a)  Calculate the value of Resistor 1 in this  circuit.
b)  How much current would flow if the value of  R was doubled?

Solutions: R  = 5 KΩ   and b) I = 1 mA

2º Calculate the current flowing in this circuit.

b) What would be the ammeter reading if the resistor’s value was halved?

Solutions: a) 200 mA  b) 400 mA

3º  a) Calculate the current flowing through  the 40 Ohms  resistor.

Total resistor is 100, then the current in the circuit  is 0.15A

Using Ohm`s Law V=IR = 0.15A40Ohm = 6V 

 b) Calculate the current flowing through  the 60 Ohms  resistor.

Total Voltage is 15 V and I use 6V in the first resistor. So, I use 9V in the second resistor

c) What will be the reading on the  current in the battery?
d ) What is the total resistance in this circuit?
Solutions: V1 = 6 volts b) V2 = 9 volts c) 150 mA d) Rt = 100 Ω

4º What will be the value of the current flowing through the 100 Ohm resistor?

And what about the current in the other resistor?

Solutions: 50 mA  and 100 mA

6º We have a two parallel resistors. Calculate the total resistance of the circuit and the current flowing through each resister.

Solutions: Rt = 33,33 Ω, I1 = 0,05 A   and I2 = 0,1 A

7º A series circuit has 4 resistances of 20, 40, 10 and 5 Ohms. Calculate the Total resistance and the current flowing through each one if the battery has a value of 10 Volts

Solutions: Total resistance = 75 Ω.  I = 0,13 Amps

8º In this circuit, calculate:

a )  The total resistance in the circuit

b) The  current flowing in the circuit.

c) The voltage across  every resistor

Solutions:

a) = 40 Ω  b) = 0,225 A   c ) V1 = 2,25  V2 =1,125  V3 = 5,625

Note: If you ad V1 plus V2 plus V3, you get the Battery voltage

9º  5 ,10 and 25 Ohms resistors are connected in parallel. Calculate the  total resistance and the current flowing through each one

Solutions: Rt = 2,94 Ω  I1 = 1,8 A  I2 = 0,9 A  and I3 = 0,36 A

10º In the circuit on the left, we have 3 series resistors. We measure 8 volts in the voltmeter ( represented by V ).

Calculate the voltage across the 20 Ω  resistance

V = 32 V

11º  In the next circuit , calculate the voltage across the 20 Ohm resistor.

Solution: 1,64 volts

12º What is The electric power?. How it is measured?

13º A hair dryer has a resistance of 100 Ω and it is plugged to a 220 mains supply. If it is  operating for 40 minutes, calculate how many kilowatts per hour of energy does it use and how much do you pay if 1kwh = 0,20€.

14º  What resistor values are indicated by the following colour bands? (A) Blue, black, yellow