Download Currents

Currents, Power, &
Electrical Energy
Currents
• Electric Current (I)
– Flow of electric charge, by
movement of electrons
– Electrons spontaneously flow from areas of
high negative charge to areas of high positive
charge.
– Flow stops when potential difference is equal
• In other words, there is no more charge
– To keep a current flowing, there must be a
charge pump
Charge Pumps
• Battery
– Chemical energy to electric energy
• Photovoltaic Cells
– Light energy into electric energy
• Generator
– Kinetic energy to electric energy
• Ex: water, steam, wind, etc.
Basic Battery
Current Flow
• Current is the result of the flow of electrons.
• What direction to electrons move?
• (-)  (+)
• Conventional Current flows from the
positive terminal to the negative terminal.
• In other words, opposite the direction of
electron flow
• The symbol for current is “I”
• The unit for current is the Ampere
Current Flow
Current Flow
Positive Terminal
Current Flow
Battery
Resistor
Negative Terminal
Resistor
Current Flow
Measuring Currents
• Ampere (A)
– Flow of one Coulomb per second
– (C/s) = A
– Measured by an Ammeter
• (Voltmeters measure voltage)
Calculating Current
q
I
t
q = Charge (C)
I = current (A; C/s)
t = time (s)
Sample Problem #1
• If a lightning strike has a charge of 10C
and has current of 10,000 Amps, how long
does it take to strike?
Ohm’s Law
• Resistance
– Hindrance to the flow of charge
– Unit: Ohms (Ω)
V
I
R
V = Voltage (V)
I = Current (A)
R= Resistance (ohms; Ω)
Sample Problem #2
• What is the resistance of a circuit with a
9V battery and a 3A current?
Electric Power [Unit: Watts (W)]
• Rate at which work is done by electrical energy
– i.e. the rate at which energy is changed from one form to another
• Remember: P = W/t, and W = ΔE
– So P = ΔE/t
• Light bulb: Electrical energy  heat energy &light energy
• Home appliances:
– Coffee pot, hair dryer, computer, TV, etc
• Power of an electric circuit depends on 3 things:
1. Voltage
2. Current
3. Resistance
Electric Power
2
V
P  IV  I R 
R
2
P = Power (watts (W) or J/s)
I = current (amps (A) or C/s)
V = voltage (volts (V) or J/C)
R = resistance (ohms(Ω))
Electric Bills
Electric companies charge by
the kilowatt-hour (kWh). This is
a unit of ENERGY.
1 kWh = 3,600,000 (3.6 x 106) J
Electric Energy (Joules)
2
V t
E  Pt  VIt  I Rt 
R
2
E  qV
These are not new
equations, just
substitutions derived from
previous knowledge!
Units: Energy MUST be in joules (J)!!
Sample Problems
• How long will it take a 500 Watt coffee pot
to warm pot of water if such a task requires
500,000J of energy?
• How much current will run through the
coffee pot if it is plugged into the wall which
has a potential difference of 120V?
Sample Problems
• Your hairdryer uses 120 V while pulling
10.0 amps of current. If it takes five
minutes to dry your hair, how much total
energy is used?
Testing Electric Current
• We test electric circuits with a
MULTIMETER which measures
resistance, current, and power.
• An AMMETER tests current. The leads
must be across an OPEN switch to
prevent a blown fuse.
• A VOLTMETER tests voltage. The leads
must be across a CLOSED switch to get
an accurate reading.
• A battery (AAA, AA, C, or D) runs on
1.5V each.