Chapter 18

Question 1

What is the source of emf?

What is the unit?

Answer 1

The source that maintains the current in a closed circuit
Any devices that increase the potential energy of charges circulating in circuits are sources of emf

SI unit: volt
Emf is work done per unit charge

Question 2

What is kirchhoffs rules?

Answer 2

Junction rule:
Consequence of conservation of conservation of charge
Sum of currents entering any junction must equal sum of currents leaving that junction
Total current in=total current out
Loop rule:
Sum of potential differences across all elements around any closed circuit loop must be zero
In a closed loop, net change in potential is 0

Slides 20 and 21 nov 26

Question 3

For a real battery, is terminal voltage equal to emf?

Answer 3

No because real batteries have internal resistance

Terminal voltage = ΔV=Vb-Va
ΔV=ε-Ir

For the entire circuit:
ε=IR + Ir 
Slide 23 nov26!!
R= load resistance (external resistance)
r=internal resistance

Question 4

What is the power relationship?

Answer 4

P=Iε=I^2 R + I^2

When R»r, r can be ignored since most of the power delivered by the battery is transferred to the load resistor

Question 5

What is the battery a source of?

Answer 5

Source of constant emf not source of constant current

Question 6

What are resistors in series?

Answer 6

When two or more resistors are connected end to end
Current is same in all resistors (any charge that flows through one resistor flows through one resistor flows through the other)
Sum of potential differences across the resistors is equal to total potential difference across the combination
Slide 13-16 Nov 28

Question 7

What are parallel circuits?

Answer 7

Have same potential difference across them (voltage drop is same)
Slide 20 nov 28
Slide 8 Nov 30

Question 8

What’s the difference between a fuse and circuit breaker?

Answer 8

Fuses burn out and need to be replaced
Circuits automatically shut off and can be switched back on
Slide 10 nov 30

Question 9

What does the degree of damage on the body from an electrical current depend on? (3 things)
Is the body a good conductor?

Answer 9

Magnitude of the current
Length of time it is present
Part of the body through which it passes

Body is a very good conductor cause of the large number of ions

Example of slide 13 nov 30

Question 10

What do three prong plugs do?

Answer 10

Third wire is called a case ground which prevents shocks
Add a short circuit in an appliance
Slides 18-19 Nov 30

Question 11

What is a ground fault circuit interrupter (GFCI)?

Answer 11

Special outlet in high risk locations 
Senses currents (of about 5mA or greater) leaking to the ground and shits off current when above that level