Circuit - a continuous path for current to flow
- must go from and back to the source of
voltage
- this is why plugs on electrical devices
have two prongs
- "closed" circuit - current can flow, usually
referring to a switch being "on"
- "open" circuit - circuit is not complete,
no current, switch is "off"
Switch
Electrochemical Cells
(Battery)
+
Light Bulb
(Load)
electron flow
Around 1827, Georg Ohm investigated the affect of changing the potential difference (voltage) in a simple circuit on the corresponding current.
The circuit consisted of a source of potential difference (battery) and a set length of wire which has a low resistance. He found that as the voltage increased, the current increased proportionally.
If voltage is divided by current, the answer is a constant. He called this constant "resistance".
V/I = R or V = IR Ohm's Law
Potential Difference or Voltage (V)
- measured in volts (V)
- provides the "push" or "pull" to make emove through a circuit
- electrons travel from neg. terminal to pos.
- voltage is like water pressure making
water travel through a hose
Electric Current (I)
- measured in amperes or "amps" (A),
- the rate at which electrons flow
through a circuit
- 1 ampere = 1 Coulomb per second or
6.25 x 1018 electrons per second
I=q/t
current = charge / time
- current is like the amount of water
flowing out of a hose
Resistance (R)
- measured in ohms (Ω)
- a measure of how difficult it is for electrons
to flow through a substance
- electrons do not flow easily through
materials with high resistances
- is like the diameter of a hose or a kink in the
hose, a thinner hose "resists" the flow of
water more than a thicker one
EMF - "electromotive force" - total voltage available
from the battery - "open" circuit voltage
T.V. - terminal voltage - voltage available from battery
to the external circuit - "closed" circuit voltage
EMF is always larger than T.V. The difference between
them is the voltage lost in the battery due to
internal resistance.
EMF = T.V. + Voltage Loss in Battery
EMF = T.V. + I (r)
EMF - I(r) = T-V.
Answers to "Sources of Current" Reading:
1) Anode (-), cathode (+)
2) Generators
3) Microphones, headphone speakers, gas grill lighters,
stylus (needle) on a record player
4) Storage (secondary) cells
5) Primary cell
6) When different temperatures are applied to the
junction between wires of two different metals
7) They last longer
8) Light (electromagnetic energy)
9) A device in a circuit that uses the electrical energy
10) False, he received it for explaining the photoelectric
effect
Electromagnetic Induction
- generators - produces alternating current (AC)
- converts mechanical energy to electricity
Electrochemical Cells - "batteries"
- a battery is really a combination of 2 or more cells
- converts chemical energy to electricity
Photoelectric Effect - photovoltaic (solar) cells
- converts electromagnetic energy (light) to electricity
Thermoelectric Effect - thermocouple
- more commonly used as temperature sensors
- converts heat energy into electricity
Piezoelectric Effect - pressure on crystals
- used in microphones, phonograph stylus, lighters
- converts mechanical energy to electricity
(Ammeter)
Primary Cells
­ fixed amount of chemicals, once used up they are "dead"
­ Duracell or Energizer batteries, watch batteries
Secondary Cells
­ rechargeable batteries, sometimes called "storage" cells
­ reaction can be reversed when electricity is applied ­ cell phone battery, laptop battery, Ni­Cd, Li­ion
Fuel Cells
­ additional reactants can be added or replenished as they run out
­ hydrogen fuel cells for cars,