1. 19.1 Electric Current

2. can be positive or negative, but usually negative (electrons)
the flow of
charged particles;
can be positive or negative,
but usually negative (electrons)
move in a conducting metal. Conventional current is defined as flow of positive charge.

3. I = Q/t 1 Amp = 1 Coulomb per second = 1 C/s
Electric current is measured
in Amperes, in honor of
Andre Marie Ampere.
One Ampere is the flow of
one Coulomb of charge per second.
1 Amp = 1 Coulomb per second
= 1 C/s
I = Q/t

4. Sample Problem 16A
Serway, Raymond and Faughn, Jerry. Physics. Holt, Rinehart and Winston, 2002

5. electric cell - a device that converts one form of energy
to electrical energy
Chemical cells convert chemical
energy into electrical energy.
Chemical cells can be
“wet” or “dry”.

6. Solar cells convert light energy into electrical energy.
A generator converts mechanical
energy into electrical energy.
battery - two or more cells
connected in series or in parallel

7. Types of current Alternating Current Direct Current
Motion of charges continually changes direction
Direct Current
Charges move in one direction only

8. 19.2 Resistance

9. V R = I Resistance The SI unit of resistance is the Ohm, W, named
determines the amount of current flow
= the ratio of potential difference to current V
The SI unit of
resistance is the
Ohm, W, named
in honor of Georg Simon Ohm. R = I
One Ohm of resistance is the resistance
such that one Volt of potential difference
is needed to obtain a current of one Amp.

10. Ohm’s Law If R = V/I is a constant value
The ratio of potential difference to current
is constant.
If R = V/I is a constant value
for a given resistor, then that
resistor is said to obey Ohm’s Law.
Click here and here to link to pages describing resistor
color codes.

11. Sample Problem 19B
Serway, Raymond and Faughn, Jerry. Physics. Holt, Rinehart and Winston, 2002

12. Resistance depends on 
Serway, Raymond and Faughn, Jerry. Physics. Holt, Rinehart and Winston, 2002

13. 1. the length of the conductor
The resistance of a circuit element depends on:
1. the length of the conductor
as length increases, resistance increases proportionally
2. the cross-sectional area of the conductor
as area increases, resistance decreases proportionally
3. the resistivity of the conductor
as resistivity increases, resistance increases proportionally

14. Resistors Control current in a circuit
For a constant potential difference (voltage) increasing the resistance….
Decreases the current
Human body resistance is approximately 500,000.
If the skin gets soaked with salt water, it could drop to 100.
Currents above 0.15A through the chest cavity can be fatal!!
Serway, Raymond and Faughn, Jerry. Physics. Holt, Rinehart and Winston, 2002

15. Many circuit elements
do not obey Ohm’s Law. Resistors that get hot,
like light bulbs
and heating elements,
do not keep a constant
resistance.
Resistance generally increases as
objects become hotter.

16. electric potential difference
Ammeter
a device that measures current
Voltmeter
a device that measures
electric potential difference

17. 19.3 Electrical Power

18. P (Watts) = V (Volts).I (Amps)
power = work/time
= (work/charge).(charge/time)
= electric potential difference . current
P (Watts) = V (Volts).I (Amps)
= I2.R = V2/R

19. Click for answer

20. Practice
How much does it cost to operate a W light bulb for 24 hours if electrical energy costs $0.080 per kW·h?

21. Solution
How much does it cost to operate a W light bulb for 24 hours if electrical energy costs $0.080 per kW·h? Energy = Power * time = kW * 24 =2.4 kW·h Cost = 2.4 kW·h * $0.080 per kW·h

22. Water circuit analogy

23. Resistor Circuits Series RT = R1 + R2 + R3 + ...
1. total resistance is the sum of the
separate resistors
RT = R1 + R2 + R
2. current is the same through each resistor
IT = I1 = I2 = I3 = ...
3. total potential difference is the sum of each
VT = V1 + V2 + V
In other words, in a series circuit,
resistance and voltage add,
but current stays the same.

24. Click here for answer!

25. 8.0 2.0 5.0 RT = VT = IT = PT = E = 12 V R1 R1 R3 R2 R3 R2 R, W V, VA P, W
E = 12 V R1
8.0 R1 R3 R2
2.0 R3
5.0
RT = R2
VT =
IT =
PT =

26. 8.0 2.0 5.0 RT = 15 Ω VT = 12 V IT = 0.80 A PT = 9.6 W E = 12 V R1 6.4
5.1 R1 R3 R2
2.0
1.6
0.80
1.3 R3
5.0
4.0
0.80
3.2
RT = 15 Ω R2
VT = 12 V
IT = A
PT = 9.6 W

27. Click for answer!

28. Parallel 1/RT = 1/R1 + 1/R2 +1/R3 + ... IT = I1 + I2 + I3 + ...
1. reciprocal of the total resistance is the
sum of the reciprocals of the separate
resistors
1/RT = 1/R1 + 1/R2 +1/R
2. total current is the sum of the current
through each resistor
IT = I1 + I2 + I
3. potential difference is the same across
each resistor
VT = V1 = V2 = V3 = ...
In other words, in a parallel circuit,
resistance adds as reciprocals,
voltage stays the same, and current splits.

29. 12 8.0 12 RT = VT = IT = PT = E = 12 V R1 R2 R1 R3 R2 R3 R, W V, V I,A P, W
E = 12 V R1 12 R2
8.0 R1 R3 12 R2
RT = R3
VT =
IT =
PT =

30. R, W V, V I, A P, W
E = 12 V R1 12 12
1.0 12
8.0 12
1.5 R2 18 R1 12 12
1.0 12 R3 R2
RT = 3.42 Ω R3
VT = 12 V
IT = A
PT = 42 W

31. Click for answer!