Chapter 18

Chapter 18

Direct Current Circuits

Sources of emf

• The source that maintains the current in a closed circuit is called a source of emf

• Any devices that increase the potential energy of charges circulating in circuits are sources of emf (e.g. batteries, generators, etc.)

• The emf is the work done per unit charge

• SI units: Volts

emf and Internal Resistance

• A real battery has some internal resistance r; therefore, the terminal voltage is not equal to the emf

• The terminal voltage: ΔV = Vb – Va

ΔV = ε – Ir• For the entire circuit (R – load resistance):

ε = ΔV + Ir

= IR + Ir

emf and Internal Resistance

ε = ΔV + Ir = IR + Ir• ε is equal to the terminal voltage when the current is

zero – open-circuit voltage

I = ε / (R + r)• The current depends on both the resistance external to

the battery and the internal resistance

• When R >> r, r can be ignored

• Power relationship: I ε = I2 R + I2 r

• When R >> r, most of the power delivered by the battery is transferred to the load resistor

Resistors in Series

• When two or more resistors are connected end-to-end, they are said to be in series

• The current is the same in all resistors because any charge that flows through one resistor flows through the other

• The sum of the potential differences across the resistors is equal to the total potential difference across the combination

III 21

21 IRIRV )( 21 RRI eqIR

Resistors in Series

• The equivalent resistance has the effect on the circuit as the original combination of resistors (consequence of conservation of energy)

• For more resistors in series:

• The equivalent resistance of a series combination of resistors is greater than any of the individual resistors

eqIRV 21 RRReq

...321 RRRReq

Resistors in Parallel

VRIRI 2211

21 III 21 R

V

R

V

• The potential difference across each resistor is the same because each is connected directly across the battery terminals

• The current, I, that enters a point must be equal to the total current leaving that point (conservation of charge)

• The currents are generally not the same

Resistors in Parallel

VRIRI 2211

21 III 21 R

V

R

V

21

11

RRV

eqR

V

21

111

RRReq

• For more resistors in parallel:

• The inverse of the equivalent resistance of two or more resistors connected in parallel is the algebraic sum of the inverses of the individual resistance

• The equivalent is always less than the smallest resistor in the group

Resistors in Parallel

321

1111

RRRReq

Problem-Solving Strategy

• Combine all resistors in series

• They carry the same current

• The potential differences across them are not necessarily the same

• The resistors add directly to give the equivalent resistance of the combination:

Req = R1 + R2 + …

Problem-Solving Strategy

• Combine all resistors in parallel

• The potential differences across them are the same

• The currents through them are not necessarily the same

• The equivalent resistance of a parallel combination is found through reciprocal addition:

...111

21

RRReq

Problem-Solving Strategy

• A complicated circuit consisting of several resistors and batteries can often be reduced to a simple circuit with only one resistor

• Replace resistors in series or in parallel with a single resistor

• Sketch the new circuit after these changes have been made

• Continue to replace any series or parallel combinations

• Continue until one equivalent resistance is found

Problem-Solving Strategy

• If the current in or the potential difference across a resistor in the complicated circuit is to be identified, start with the final circuit and gradually work back through the circuits (use formula ΔV = I R and the procedures describe above)

Chapter 18Problem 13

Find the current in the 12-Ω resistor in the Figure.

Kirchhoff’s Rules

• There are ways in which resistors can be connected so that the circuits formed cannot be reduced to a single equivalent resistor

• Two rules, called Kirchhoff’s Rules can be used instead:

• 1) Junction Rule

• 2) Loop Rule

Gustav Kirchhoff1824 – 1887

Kirchhoff’s Rules

• Junction Rule (A statement of Conservation of Charge): The sum of the currents entering any junction must equal the sum of the currents leaving that junction

• Loop Rule (A statement of Conservation of Energy): The sum of the potential differences across all the elements around any closed circuit loop must be zero

Junction Rule

I1 = I2 + I3

• Assign symbols and directions to the currents in all branches of the circuit

• If a direction is chosen incorrectly, the resulting answer will be negative, but the magnitude will be correct

Loop Rule• When applying the loop rule, choose

a direction for transversing the loop

• Record voltage drops and rises as they occur

• If a resistor is transversed in the direction of the current, the potential across the resistor is – IR

• If a resistor is transversed in the direction opposite of the current, the potential across the resistor is +IR

Loop Rule• If a source of emf is transversed in

the direction of the emf (from – to +), the change in the electric potential is +ε

• If a source of emf is transversed in the direction opposite of the emf (from + to -), the change in the electric potential is – ε

Equations from Kirchhoff’s Rules• Use the junction rule as often as needed, so long as,

each time you write an equation, you include in it a current that has not been used in a previous junction rule equation

• The number of times the junction rule can be used is one fewer than the number of junction points in the circuit

• The loop rule can be used as often as needed so long as a new circuit element (resistor or battery) or a new current appears in each new equation

• You need as many independent equations as you have unknowns

Equations from Kirchhoff’s Rules

Problem-Solving Strategy

• Draw the circuit diagram and assign labels and symbols to all known and unknown quantities

• Assign directions to the currents

• Apply the junction rule to any junction in the circuit

• Apply the loop rule to as many loops as are needed to solve for the unknowns

• Solve the equations simultaneously for the unknown quantities

• Check your answers

Chapter 18Problem 17

Determine the current in each branch of the circuit shown in the Figure.

RC Circuits• If a direct current circuit contains capacitors and

resistors, the current will vary with time

• When the circuit is completed, the capacitor starts to charge until it reaches its maximum charge (Q = Cε)

• Once the capacitor is fully charged, the current in the circuit is zero

Charging Capacitor in an RC Circuit

• The charge on the capacitor varies with time

q = Q (1 – e -t/RC )

• The time constant, = RC, represents the time required for the charge to increase from zero to 63.2% of its maximum

• In a circuit with a large (small) time constant, the capacitor charges very slowly (quickly)

• After t = 10 , the capacitor is over 99.99% charged

Discharging Capacitor in an RC Circuit

• When a charged capacitor is placed in the circuit, it can be discharged

q = Qe -t/RC

• The charge decreases exponentially

• At t = = RC, the charge decreases to 0.368 Qmax; i.e., in one time constant, the capacitor loses 63.2% of its initial charge

Chapter 18Problem 35

A capacitor in an RC circuit is charged to 60.0% of its maximum value in 0.900 s. What is the time constant of the circuit.

Chapter 18Problem 54

An emf of 10 V is connected to a series RC circuit consisting of a resistor of 2.0 × 106 Ω and a capacitor of 3.0 μF. Find the time required for the charge on the capacitor to reach 90% of its final value.

Electrical Safety

• Electric shock can result in fatal burns

• Electric shock can cause the muscles of vital organs (such as the heart) to malfunction

• The degree of damage depends on– the magnitude of the current– the length of time it acts– the part of the body through which it passes

Effects of Various Currents

• 5 mA or less– Can cause a sensation of shock– Generally little or no damage

• 10 mA– Hand muscles contract– May be unable to let go a of live wire

• 100 mA – If passes through the body for just a few seconds,

can be fatal

Answers to Even Numbered Problems

Chapter 18:

Problem 2

(a) 24 Ω(b) 1.0 A (c) 2.18 Ω, I4 = 6.0 A, I8 = 3.0 A, I12 = 2.0 A

Answers to Even Numbered Problems

Chapter 18:

Problem 6

15 Ω

Answers to Even Numbered Problems

Chapter 18:

Problem 18

5.4 V with a at a higher potential than b

Answers to Even Numbered Problems

Chapter 18:

Problem 22

0.50 W

Answers to Even Numbered Problems

Chapter 18:

Problem 36

(a) 10.0 μF(b) 415 μC

Answers to Even Numbered Problems

Chapter 18:

Problem 38

(a) 8.0 A (b) 120 V (c) 0.80 A (d) 5.8 × 102 W