Equivalent Resistance, Volts, Amps

Volts and Amps in a Series Circuit

In a series circuit, the Amps remain constant throughout the whole circuit

Amps = 2A

Amps = 2A

Amps = 2A

Volts and Amps in a Series Circuit

However, in a series circuit at each resistor the volts change. When you add up all the voltage at each resistor, you end up getting the total volts going through the circuit.

Amps = 2A

Amps = 2A

Amps = 2A

Volts2 = 3V

Volts1 = 2V

Volts3 = 2VVolts

total = 7V

Volts and Amps in a Parallel Circuit

In a parallel circuit, each line of wire has a different amount of amps. When added the amps together, you get the total amount of amps in the entire circuit. Amps3

= 1A

Amps2 = .5A Amps3

= .5A

Amps tot = 2A

Volts and Amps in a Parallel Circuit

However, in a parallel circuit if you have a total of 3 volts in the circuit, each resistor will only have 3 volts

Amps3 = 1A

Amps2 = .5A Amps1

= .5A

Volts Total = 3V

Volts1 = 3V

Volts2 = 3V

Volts3 = 3V

Amps tot = 2A

Equivalent or Total Resistance - Req

Equivalent Resistance is the TOTAL resistance of a circuit Symbol: Req or Rt

Units: Ω Ohms Formula:

Series:

Req = R1 + R2 + R3 + …

Parallel:

...3

1

2

1

1

1

Re

1

RRRq

5

3 10

Type of circuit: Series

Formula: Req = R1 + R2 + R3

Req = 10 + 5 + 3

Req = 18

What is the equivalent resistance (Req) of the circuit below?

30

75

Type of circuit: Series

Formula: Req = R1 + R2

Req = 75 +30

Req = 105

What is the equivalent resistance (Req) of the circuit below?

Type of circuit: Parallel

Formula:

What is the equivalent resistance (Req) of the circuit below?

10 Ω

15 Ω

21

11

Re

1

RRq

1 ÷ 0.1666 = 6Ω

15

1

10

1

Re

1

q

0666.01.0Re

1

q

1666.0Re

1

q

6Req

Type of circuit: Parallel

Formula:

What is the equivalent resistance (Req) of the circuit below?

321

111

Re

1

RRRq

1 ÷ 0.3333 = 3Ω

15

1

5

1

15

1

Re

1

q

0666.02.00666.0Re

1

q

3333.0Re

1

q

3Req

15

5

15

KEY POINTS

321 R1

R1

R1

Req1

Symbols of circuit parts: Voltmeter, Ammeter, Power Supply, Resistor

Two types of circuits (series and parallel) Formula for Req (equivalent resistance)

Series – Req = R1 + R2 + R3

Parallel -

All the Information you need

Simulations

Open up applications for simulation Or go to

http://phet.colorado.edu/en/simulations/category/physics

Colour coding on resistors

Resistors are coated with ceramic.

They have color coded bands to indicate resistance

1st & 2nd band first two digits of resistance

3rd band Number of zeros to add after the first two numbers. I.E. 101, or 2, or 3….

Resistor   

Example: If you have a resister that is red, orange and yellow

Black 0

Brown 1

Red 2

Orange 3

Yellow 4

Green 5

Blue 6

Violet 7

Gray 8

White 9

First colour is red, thus first digit is 2 and the

second colour is orange, thus the second digit is 3

= 23

The third colour is yellow, and this digit is 4 we therefore find 104 =

10000

This resistance would be 10000 X 23 = 230 000