Chapter 19- DC circuits

Question 1

What is emf of a source?

Answer 1

source of electromotive force

The potential difference between the terminals of such a source, when no current flows to an external circuit

Question 2

Why do the headlights of a car dim when you turn it on at first?

Answer 2

This happens because the starter draws a large current, and the battery voltage drops below its rated emf as a result. The voltage drop occurs because the chemical reactions in a battery cannot supply charge fast enough to maintain the full emf.

Question 3

When no current is drawn from the battery what does the terminal voltage equal to?

Answer 3

emf

Question 4

What is internal resistance?

Answer 4

charge must move (within the electrolyte) between the electrodes of the battery, and there is always some hindrance to completely free flow. Thus, a battery itself has some resistance

Question 5

What is a resistor in a series?

Answer 5

When two or more resistors are connected end to end along a single path as

Question 6

What is the total voltage equal to?

Answer 6

total voltage V is equal to the sum of the voltages across each resistor:

V = V1 +V2 +V3 = IR1 +IR2 +IR3.

Question 7

What is net resistance?

Answer 7

When we put several resistances in series, the total or equivalent resistance is the sum of the separate resistances.

Question 8

What happens when you add more resistance to a circuit?

Answer 8

hen you add more resistance to the circuit, the current through the circuit will decrease.

Question 9

What is a resistor in a parallel?

Answer 9

so that the current from the source splits into separate branches or paths

Question 10

What is the difference between a parallel and a series when you disconnect one device?

Answer 10

With parallel wiring, if you disconnect one device , the current to the other devices is not interrupted.

Compare to a series circuit, where if one device is disconnected, the current is stopped to all others.

Question 11

What occurs in a parallel circuit?

Answer 11

In a parallel circuit, the total current (I) that leaves the battery splits into three separate paths. We let I1 , I2 , and I3 be the currents through each of the resistors, R1 , R2 , and R3 , respectively. Because electric charge is conserved, the current I flowing into junction A (where the different wires or conductors meet) must equal the current flowing out of the junction

Question 12

What happens when you add more resistors to a parallel ?

Answer 12

The net (or equivalent) resistance is less than each single resistance. This may at first seem surprising. But remember that when you connect resistors in parallel, you are giving the current additional paths to follow. Hence the net resistance will be less.†

Question 13

How does adding more resistors to a seriesaffect the resistivity through the length?

Answer 13

placing resistors in series effectively increases the length and therefore the resistance;

Question 14

How does adding more resistors to a parallel affect the resistivity through the area?

Answer 14

putting resistors in parallel effectively increases the area through which current flows, thus reducing the overall resistance.

Question 15

Series or parallel? (a) The lightbulbs in Fig. 19–5 are identical. Which configuration produces more light

Answer 15

(a) The equivalent resistance of the parallel circuit is found, 1􏰕R = 1􏰕R + 1􏰕R = 2􏰕R. Thus R(eq) = R􏰕2. The parallel combination then has lower resistance (= R􏰕2) than the series combination AR = R + R = 2RB. There will be more total current in the parallel configuration , since I = V􏰕Req and V is the same for both circuits. The total power
+ 12 V – (1) Series
+ 12 V – (2) Parallel
transformed, which is related to the light produced, is P = IV, so the greater current in parallel means more light is produce

Question 16

Series or parallel? (b) Which way do you think the headlights of a car are wired? Ignore change of filament resistance R with current.

Answer 16

(b) Headlights are wired in parallel (2), because if one bulb goes out, the other bulb can stay lit. If they were in series (1), when one bulb burned out (the filament broke), the circuit would be open and no current would flow, so neither bulb would light.

Question 17

Bulb brightness in a circuit. The circuit in Fig. 19–8 has three identical lightbulbs, each of resistance R. (a) When switch S is closed, how will the brightness of bulbs A and B compare with that of bulb C?

Switch () is placed {parallel}
C -- {A,B}

Answer 17

(a)With switch S closed ,the current that passes through bulb C must split into two equal parts when it reaches the junction leading to bulbs A and B because the resistance of bulb A equals that of B. Thus, A and B each receive half of C’s current; A and B will be equally bright, but less bright than C (P = I2R).

Question 18

(b) What happens when switch S is opened? Use a minimum of mathematics.

Answer 18

b) When the switch S is open, no current can flow through bulb A, so it will be dark. Now, the same current passes through bulbs B and C, so B and C will be equally bright. The equivalent resistance of this circuit (= R + R) is greater than that of the circuit with the switch closed, so the current leaving the battery is reduced. Thus, bulb C will be dimmer when we open the switch, but bulb B will be brighter because it gets more current when the switch is open.

Question 19

What are two ways which a electric current can damage a human body?

Answer 19

(1) heating tissue and causing burns;

(2) stimulating nerves and muscles, and we feel a “shock.”