electric current and series and parallel circuits
TRANSCRIPT
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ELECTRIC CURRENT
Chapter 22
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What is an electric current?An electric current is a flow of microscopic particles called electrons flowing through wires and components.
+ -
In which direction does the current flow? from the Negative terminal to the Positive terminal of a cell.
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For current to flow you must have a complete
circuit
A power source is needed to create a
POTENTIAL DIFERENCE
When these conditions are met free electrons move about inside the metal.
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Potential Difference• Charges will continue to move as long as
the potential difference (voltage) is maintained.
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Chargetime
Measuring Current
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A Charge pump
How do we maintain a charge?
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The CELLThe CELLThe cell stores chemical energy and transfers it to electrical energy when a circuit is connected.
When two or more cells are connected together we call this a Battery.
The cells chemical energy is used up pushing a current round a circuit.
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Animation on page 599
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simple circuitssimple circuitsHere is a simple electric circuit. It has a cell, a lamp and a switch.
To make the circuit, these components are connected together with metal connecting wires.
cell
lamp
switch
wires
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simple circuitssimple circuitsWhen the switch is closed, the lamp lights up. This is because there is a continuous path of metal for the electric current to flow around.
If there were any breaks in the circuit, the current could not flow.
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POWERPower is the rate at which energy is transfer or transformed. It is measured in Watts (W).Example: Most of our appliances at home are rated in watts.
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Measuring Power
If the current through the motor is 3.0and the potential difference is 120V. What is the
motor’s power?
(3.0 C/s) (120 j/C) = 360 J/s (W)
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Electrical Resistance• Electrical resistance – resistance to the flow
of electrical charges
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Ohm’s Law
• For many conductors, current depends on:• Voltage - more voltage, more current– Current is proportional to voltage
• Resistance - more resistance, less current– Current is inversely proportional to resistance
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Adding resistances in series
• Each resistance in a series circuit adds to the total resistance of the circuit.
Rtotal = R1 + R2 + R3...Total
resistance(ohms)
Individual resistances ()
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Total resistance in a series circuit• Light bulbs, resistors, motors, and heaters usually have
much greater resistance than wires and batteries.
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Voltage in a series circuit• Each separate resistance creates
a voltage drop as the current passes through.
• As current flows along a series circuit, each type of resistor transforms some of the electrical energy into another form of energy
• Ohm’s law is used to calculate the voltage drop across each resistor.
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Measuring resistance / Ohm’s Law
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Simple problem solving exercises1. A 30 V attery is connected to a 10 Ω resistor.
What is the current of the circuit? 2. A lamp draw a current of 0.50 A when it is
connected to a 120 V source. a. What is the resistance of the lamp? b. What is the power consumption of the
lamp?3. A 75 W lamp is connected to 125 V. a. What is the current through the lamp? b. What is the resistance through the lamp?
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Simple problem solving exercises
4. The current in a certain wire is 0.35A. Calculate the charge passing a point in the wire a)In 10 s b) in 10 minute5. Calculate the average current in a wire through which a charge of 15C passes in a) In 10 s b) in 10 minute
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SERIES AND PARALLEL CIRCUITS
Chapter 23
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Series Circuits• In series circuits, current can only take one path.• The amount of current is the same at all points in a
series circuit.
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Parallel Circuits• the current can take more than one path.• Because there are multiple branches, the current is not the same at all points
in a parallel circuit.• If one bulb ‘blows’ there is still be a complete circuit to the other bulb so it
stays alight.
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circuit diagramscircuit diagrams
cell battery
switch
lamp
motorammeter
voltmeter
buzzer
resistor
variable resistor
Diode
LED
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Measuring currentElectric current is measured in amps (A) using an ammeter connected in series in the circuit.
A
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Diagraming a circuit
A A
This is how we draw an ammeter in a circuit.
SERIES CIRCUIT PARALLEL CIRCUIT
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Measuring currentSERIES CIRCUIT
PARALLEL CIRCUIT
• current is the same at all points in the circuit.
2A 2A
2A
• current is shared between the components
2A2A
1A
1A
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measuring voltageThe ‘electrical push’ which the cell gives to the current is called the voltage. It is measured in volts (V) on a voltmeter
V
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Different cells produce different voltages. The bigger the voltage supplied by the cell, the bigger the current.
measuring voltage
Unlike an ammeter a voltmeter is connected across the components
Scientist usually use the term Potential Difference (pd) when they talk about voltage.
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measuring voltage
V
This is how we draw a voltmeter in a circuit.
SERIES CIRCUIT PARALLEL CIRCUITV
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V
measuring voltage
VV
V
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series circuit
1.5V
• voltage is shared between the components
1.5V
3V
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• voltage is the same in all parts of the circuit.
3V
parallel circuit
3V
3V
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Three circuit laws
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Direct Current VS Alternating current
Direct current (DC) - If the voltage is maintained between two points in a circuit, charge will flow in one direction - from high to low potential. (Ex. Battery-powered circuits, electrified railway lines)
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Direct Current VS Alternating currentAlternating current (AC) - If the high & low voltage terminals switch locations periodically, the current will flow “back and forth” in the circuit. (Ex. Circuits powered by electrical outlets, domestic electricity )
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Converting AC to DC• AC is converted to DC using devices called
diodes, which allow charges to move in only 1 direction.
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Exercise on Drawing electric currents
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Try Drawing the following circuits:1. Series circuit: 1 (3 cell) battery, 1 switch, 3 light bulbs, 1
ammeter and connecting wires
2. Series circuit: 1 (4 cell) battery, 1 resistor, 1 light bulb, 1 switch and connecting wires
3. Paralle circuit: 1 battery, 1 switch, 2 light bulbs connected in parallel and connecting wires.
4. Parallel Circuit: 1 battery, light bulbs connected in parallel, a voltmeter to measure the potential difference of the batteryand connected wires.
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complete the missing current and voltage readings.
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answers
3V 3V
6V
4A 4A6V
6V
6V4A 4A
2A
2A
4A
a)a) b)b)
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Helpful simulation sites
• http://www.docircuits.com/#home• http://phet.colorado.edu/en/simulations/cate
gory/physics/electricity-magnets-and-circuits• https://www.youtube.com/watch?v=x2EuYqj_
0Uk• http://www.sengpielaudio.com/calculator-oh
m.htm