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SERIES CIRCUITS

Unit 8: Electricity and Magnetism

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SERIES CIRCUITS

  • In a series circuit the current only takes one path
  • All the current must flow through every part of the circuit
  • A single opening in a series circuit will stop the flow of electricity in the circuit

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SERIES CIRCUIT DIAGRAM

closed circuit

open circuit

+

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-

+

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-

gap = no current

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CURRENT IN �SERIES CIRCUITS

  • In a series circuit, current is the same at all points
  • All devices in the circuit receive the same amount of current regardless of the resistance

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RESISTORS IN �SERIES CIRCUITS

  • Multiple resistors connected in series may be treated as single resistor
  • Rtotal = R1 + R2 + …

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VOLTAGE CHANGES IN SERIES CIRCUITS

  • As current flows along a series circuit, each resistor uses up some of the energy
  • As a result, the voltage gets lower after each resistor
  • Each separate resistor creates a “voltage drop”
  • The voltage drops add up to the total voltage coming from the battery
  • Vtotal = V1 + V2 + …

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VT= 12V

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IF YOU ADD RESISTORS TO A SERIES CIRCUIT

Voltage

decreases across each resistor (total voltage is the same)

Current

remains the same

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SAMPLE PROBLEMS

for each problem, draw a circuit diagram

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1. Three 20 Ω resistors are connected in series across a 120-V generator. �a. What is the total resistance of the circuit? �b. What is the current in the circuit?

Givens

R1 = R2 = R3 = 20 Ω

Rtotal = ?

V = 120 V

I = ?

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a. Rtotal = R1 + R2 + R3

Rtotal = 20 + 20 + 20

Rtotal = 60 Ω

b. I = V/R

I = 120/60

I = 2 A

R1 = 20Ω

R2 = 20Ω

R3 = 20Ω

120 V

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2. A 10 Ω, 15 Ω, and 20 Ω resistor are connected in a series across a 90-V battery. �a. What is the total resistance of the circuit? �b. What is the current in the circuit

Givens

R1 = 10 Ω

R2 = 15 Ω

R3 = 20 Ω

V = 90 V

I = ?

a. Rtotal = R1 + R2 + R3

Rtotal = 10 + 15 + 20

Rtotal = 45 Ω

b. I = V/R

I = 90/45

I = 2 A

90 V

10 Ω

15 Ω

20 Ω

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2. A 10 Ω, 15 Ω, and 20 Ω resistor are connected in a series across a 90-V battery. �c. What is the voltage drop across each resistor?

Givens

R1 = 10 Ω

R2 = 15 Ω

R3 = 20 Ω

V = 90 V

I = ?

**note: when you calculate voltage drop, you use the value of resistance at each resistor, BUT current is the same throughout

R1 = 10 Ω

I = 2 A

V = ?

I = V/R

2 = V/10

V = 20 V

90 V

10 Ω

15 Ω

20 Ω

R2 = 15 Ω

I = 2 A

V = ?

I = V/R

2 = V/15

V = 30 V

R3 = 20 Ω

I = 2 A

V = ?

I = V/R

2 = V/20

V = 40 V

20 V + 30 V + 40 V = 90 V

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8.2 LESSON CHECK