Download Voltage, Current, and Resistance Ohm`s Law

Electricity
Part 1
Electric Current
• The rate of the flow of electrons (charge)
– Charge Movement / Time
• Symbol = I
• SI unit = Ampere, A (or Coulomb/ second, C/s)
– 1 ampere = 1coulomb/second
Electric Current Equation
• Electric current = Charge Movement / Time
I = Dq /Dt
• Ex. A charge of 1.67 C passes through a wire in 2
seconds. What is the current of the wire?
I = Dq /Dt
I = 1.67C / 2.00s
I = 0.835 A
Electric Resistance
• Slows down electron flow in the circuit
– Everything has some resistance.
– Like friction for electric charges
• Symbol: R
• Units: W (Ohms)
Resistance Factors
• Electric Resistance depends on:
– Length of wire
• longer wires have more resistance than
short wires.
– Thickness of wire
• thicker wires have less resistance than thin
wires.
– Resistivity of wire
• Materials with less resistivity allow
electricity to flow easier
Resistivity of various materials
Material
Resistivity (10-8 Wm)
Material
Resistivity (10-8 Wm)
Silver
Copper
Gold
Aluminum
1.61
1.70
2.20
2.65
Bismuth
Plutonium
Graphite
Germanium
106.8
141.4
1375
4.6x107
Pure
Silicon
Calcium
3.5
Diamond
2.7x109
3.91
1.8x1013
Sodium
Tungsten
Brass
Uranium
Mercury
4.75
5.3
7.0
30.0
98.4
Deionized
water
Iodine
Phosphorus
Quartz
Alumina
Sulfur
1.3x1015
1x1017
1x1021
1x1022
2x1023
Voltage
• Amount of energy given to each unit of
electric charge
• Symbol: V
• Units: V (volts)
Ohm’s Law
• Equation:
V=IR
• Voltage (V) = Current (I) * Resistance (R)
• So …
– If current increases, voltage increases
• Directly proportional
– If current increases, resistance decreases
• Inversely proportional
Ohm’s Law
Example Problem
• A potential difference of 10 V applied
across a wire produces a 0.2 A current.
What is the resistance?
V=IR
10 V = 0.2 A * R
50 W. = R
Circuits
• A circuit is a closed or complete path
through which electrons can flow.
• An Open circuit has a break in it, electricity
cannot flow.
• A short circuit is just wires from the one
battery terminal to the other.
– Because flow is too quick, electrons back up and
energy is lost as heat
• Switch – can open or close circuits.
– Can act as a gap or a wire
Batteries
• The current (electrons) flows from the
negative terminal to the positive terminal in
batteries.
– Uses chemical energy to push electrons through
Insulators vs Conductors
• Conductors – conduct electricity well
– Usually metals
• Insulators – do not conduct electricity well
– Need high energy to conduct
Series Circuit
• When electrons have to flow through one
part to get to the next part
– More components = more resistance
– Increase resistance = decrease current (flow)
– Less current = less bright bulbs
– As voltage increases, current increases
Circuit Diagram Symbols
Wire
Battery
Light bulb
Resistor
Switch