Electricity and Magnetism E

1
Current, voltage and resistance
2
Consider a flashlight ..
3
Wiring Diagram or a Schematic
4
Predict the relative brightness
5
Lets start with a pre-test to get a sense of
where we are and where we are going ...

• Consider some electrical circuits rank the
  circuits with the brightest and dimmest light
  bulbs. Explain your reasoning.

6
CURRENT

• Measure of the rate of flow of charge.
• I Dq/Dt
• Measured in coulombs/second, which we define as
  the Ampere (A).

7
Special Note

• The current is actually carried by electrons (-)
  but we usually work as if ve charge is moving,
  which we call conventional current.

8
electron flow
9
electron flow
10
electron flow
11
electron flow
12
electron flow
13
electron flow
14
electron flow
15
electron flow
16
electron flow
17
electron flow
18
electron flow
19
electron flow
20
How do we measure current?

• use an ammeter
• How do we insert it?

What drives the current?
21
Electromotive Force (EMF)

• The battery acts like a (current) pump.
• EMF is the energy that one coulomb of charge
  gains in passing through a battery.
• Measured in J/C or volts.
• Well just call this the battery voltage
• Also given the symbol ebatt.

22
EMF and Voltage
ebatt 12 volts
Loses 12 J/C
DVAB
12 J/C
DVCD
-12 J/C
23
e 12 volts
Loses 12 J/C
24
e 12 volts
Loses 12 J/C
25
e 12 volts
Loses 12 J/C
26
e 12 volts
Loses 12 J/C
27
e 12 volts
Loses 12 J/C
28
e 12 volts
Loses 12 J/C
29
e 12 volts
Loses 12 J/C
30
e 12 volts
Loses 12 J/C
31
e 12 volts
Loses 12 J/C
32
e 12 volts
Loses 12 J/C
33
-1
e 12 volts
Loses 12 J/C
34
e 12 volts
Loses 12 J/C
What goes around, comes around!
35
How do we measure voltage?
DVA,B

• use a voltmeter
• How do we insert it?

36
How do we measure voltage?

• use a voltmeter
• How do we insert it?

In 1 second
DPE -0.012 J
DV -0.012J/.001C -12 J/C -12 volts
37
Consider a simple circuit

• What if we increase the push?
• more push means more flow
• What if we change the bulb?
• not all identical pushes produce identical flows

38
Resistance...
is futile
39
Resistance
V I R

• Represents the ability of a circuit element to
  impede the flow of current.
• R DVelement/Ielement
• volt/ampere ohm W
• Georg S. Ohm (1789-1854)

• Constant R ? ohmic

40
Which has more resistance?
steel
wood
41
Which has more resistance?
steel
steel
42
Which has more resistance?
steel
steel
43
Is the earth a good conductor or a good
insulator?
44
What is constant for a battery, DV or I?
DV I R