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22.1 Objectives
Students define an electric current and the ampere.
Students describe conditions that create current in an electric circuit.
Students draw circuits and recognize they are closed loops.
Students define power in electric circuits.
Students define resistance and describe Ohm’s law.
Electric Current Electric current is simply the flow of charge
Electrons flowing in a wire constitute a current Measured in Coulombs per second, or Amperes
Colloquially, Amp (A) refers to amount of charge crossing through cross-
sectional area per unit time Electrons have a charge of –1.610-19 Coulombs
so (negative) one Coulomb is 61018 electrons one amp is 61018 electrons per second subtle gotcha: electrons flow in direction opposite to
current, since current is implicitly positive charge flow, but electrons are negative
Circuit in Diagram Form_ +battery
bulb
In a closed circuit, currentflows around the loop
Current flowing through thefilament makes it glow.
No Loop No Current No Light
current
electrons flow opposite theindicated current direction!
(repelled by negative terminal)
Current is the Flowrate
It sometimes helps to think of current as flow of water, which is more familiar to
us. High current means lots of water flow per unit time. Low current is more
like a trickle.
In electronics, it is the flow of charge, not water, that is described by the
word current. And it’s always electrons doing the flowing (thus
electronics)
Voltage is the Pressure
It sometimes helps to think of voltage as water pressure. High voltage means lots
of water pressure. If the water faucet valve is closed, no water flows no matte how high a pressure there is. No current
is present if no electrons flow.
In electronics, it is the flow of charge, not water, that is described by the
word current. And it’s always electrons doing the flowing (thus
electronics)
Resistance is the Valve
It helps to think of resistance as the valve. High resistance means the valve is closed and little water flows through. Low resistance means the valve is open
and a lot of water flows through.
Currents Divide and Merge at Junctions
How much would the current through the battery change if I unscrewed one of the 2 bulbs?
How would the brightness of “A” change if I unscrewed “B”?A B
+
Answer
The battery is supplying an equal amount of current to each of the two bulbs. If one of the bulbs is disconnected, the current through the battery will be halved.
Unscrewing “B” would not affect the current through “A” so it will stay the same brightness.
Why wouldn’t more current flow through A?The battery does not supply constant current
(is there current even when the battery is disconnected? NO. But there is voltage!)
Series Circuits
A connection or arrangement of devices that provides only one path for a current.
The same current flows through each device (resistance).
Parallel Circuits
A connection or arrangement of two or more devices that provides more than one path for a current.
The current is divided through each path and merges again after each device.
Each device gets that same voltage.
Meters in a Circuit – Ammeter
The ammeter measures current (I) in amperes or amps or Coulombs/second
It must be connected in series. Why? The voltmeter measures potential difference (ΔV) in volts or
Joules/Coulomb It must be connected in parallel. So what is I(ΔV)?
Circuit DiagramCircuit Diagram
This is useful for measuring resistance, flow of charge is impeded by fixed atoms in conductors
R = ΔV/I measured as volts/amp or ohms (Ω)
An iron has a 6.9 A current when connected to a 120 V outlet, what is its resistance? How is this a circuit?
Resistance
Increasing ΔV is like increasing the slope of a water run, it increases current (I), they are directly related
The constant of proportionality is the resistance of the conductor
Obstructions such as rocks act as the resistance, decreasing the current
Resistance in a circuit is due to collisions between the electrons carrying the current with the fixed atoms inside the conductor
30
Georg Simon Ohm
1787 – 1854 Formulated the concept of
resistance Discovered the proportionality
between current and voltages For many materials, including
most metals, resistance remains constant over a wide range of applied voltages or currents
This statement has become known as Ohm’s Law, ΔV = IR
Ohmic Devices
Ohm’s Law is an empirical relationship that is valid only for certain materials
Materials that obey Ohm’s Law are said to be ohmic
The resistance is constant over a wide range of voltages
The relationship between current and voltage is linear
How do light dimmers work?
Non-ohmic Materials
The resistance increases as the voltage increases
Notice that current is small and resistance is high when reversed
It therefore acts as a one way valve for current
A diode is a common example of a non-ohmic device
22.2 Objectives
Students explain how electric energy is converted into thermal energy.
Students determine why high-voltage transmission lines are used to carry electric energy over long distances.
Students define kilowatt-hour in electric energy delivery.
Power
The POWER dissipated in a resistor (or resisting appliance) is proportional to the square of the current that passes through it and to the resistance.
€
P = IV = I(IR) = I2R
Resistance Becomes Heat
Electric energy becomes thermal energy when the friction of electrons moving in a wire dissipates heat. Resistors are coils of wire that can dissipate heat.
€
P = IV = I(IR) = I2R
Alternating Current Current comes to your house as a sine wave that
alternates between positive and negative current. The is called alternating current (AC).
This oscillating current has to be converted to direct current (DC = one direction current like a battery).
Wonder how we get direct current?
Let’s see if Mr. Mosher can Draw!
Mr. Mosher will now explain how diodes can be used to separated the up-humps and down-humps of a sine wave.
A capacitor is used to flip (rectify) the down-humps to up-humps.
The combination is DC! P.S. - Please be patient with Mr. Mosher!
AC/DC Converter A diode is used to capture current in ONE DIRECTION
ONLY. Use another diode to get the OTHER DIRECTION.
Put them together and what have you got? Direct Current!
Now go and play!!!!!!!!
Power Plants and You
Electric energy becomes thermal energy due to the friction of electrons. By keeping the current very low and wire resistance minimized.
Some long distance lines use 500,000 volts with low amperes. Current can be increase in your home with transformers.