Download Current Electricity * Learning Outcomes

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Current Electricity – Learning Outcomes
 Test electrical conduction in a variety of materials, and
classify each material as a conductor or insulator.
 Set up a simple electric circuit.
 Use appropriate instruments to measure current,
potential difference (voltage) and resistance, and
establish the relationship between them.
 Demonstrate simple series and parallel circuits
containing a switch and two bulbs.
 Perform simple calculations based on the relationship
between current, potential difference (voltage), and
resistance.
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Test Electrical Conduction
 Recall the definitions of conductors and insulators:
 A conductor allows charge to flow through it easily (e.g.
metals).
 An insulator does not allow charge to flow through it
easily (e.g. plastic, glass).
 To detect if a material is a conductor or an insulator, we
must know if charges can flow through it.
 An electric current is a flow of electric charge.
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Test Electrical Conduction
 Electrical diagrams use special symbols to mean certain
things:
Bulb
Power Supply
/ Battery
Switch
Resistor
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Test Electrical Conduction
1. Set up a circuit with power supply, bulb, switch, and
open connection in series.
2. Place an object in the open connection and check if
the bulb lights.
3. Replace the object with other objects of different
materials and check if the bulb lights.
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Set Up a Simple Electric Circuit
 To be a circuit, there must be at least one unbroken
connection between the two ends of the power supply.
 All circuits have a power supply, which provides a
certain amount of potential difference / voltage / e.m.f.
which gives energy to the circuit.
 The unit for potential difference is volt (V).
 A voltmeter is used to measure the potential difference
between / across two points in a circuit.
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Set up a Simple Electric Circuit
 An electric current is the flow of electric charges.
 The unit for current is the amp (A).
 An ammeter is used to measure current through a part
of a circuit.
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Set up a Simple Electric Circuit
 Recall that conductors allow current to flow and
insulators do not.
 In reality, insulators are just bad conductors – i.e. they
allow some current to flow, but not much.
 The amount of current that flows is determined by
resistance.
 Good conductors have low resistance and bad
conductors have high resistance.
 The unit of resistance is the ohm (Ω).
 Resistance can be measured with an ohmmeter.
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Set up a Simple Electric Circuit
 Some devices can change their resistance to become
good or bad conductors.
 These are called variable resistors or rheostats.
 They are used in e.g. dimmer switches, oven
temperature scales, etc.
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Measure Current
 Ammeters need to be wired in series with the object you
are measuring the current through.
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Measure Potential Difference
 Voltmeters need to be wired in parallel with the object
you are measuring the potential difference across.
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To Measure Resistance (Expt 30 part a)
1. Attach an ohmmeter to both ends of a coil of wire.
2. Note the resistance.
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To Measure Resistance (Expt 30 part b)
1. Set up a coil of wire in series with an ammeter, power
supply and variable resistor (or variable power supply).
2. Wire a voltmeter in parallel with the coil of wire.
3. Measure the potential difference across and current
through the coil.
4. Vary the resistance of the variable resistor to get
additional measurements for potential difference and
current.
5. For each measurement, divide potential difference by
the corresponding current and find an average.
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To Measure Resistance (Expt 30 part b)
6. Plot a graph of potential difference vs current for the
coil of wire and for the bulb.
7. Find the slope of the graph.
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Demonstrate Series Circuits
1. Wire a power supply, switch, and two bulbs in series.
2. Note what happens to the bulbs when the switch is
closed.
3. Disconnect one bulb and note what happens to other
bulb.
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Demonstrate Series Circuits
 Advantages:
 Uses less electricity than parallel.
 Disadvantages:
 One faulty component will prevent the whole circuit
from working.
 Used in e.g. Christmas lights.
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Demonstrate Parallel Circuits
1. Wire a pair of parallel bulbs in series with a power supply
and a switch.
2. Note what happens to the bulbs when the switch is
closed.
3. Disconnect one bulb and note what happens to the
other bulb.
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Demonstrate Parallel Circuits
 Advantages:
 If one component fails, the rest of the circuit still works.
 Disadvantages:
 Uses more power than series.
 Used e.g. in car headlights.
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Solve Problems About Resistance
 Fill in the blanks in the table below using the resistance
formula 𝑉 = 𝐼𝑅.
Current (I) Resistance (R)
Potential
Difference (V)
10
5
2
200
120
30
100
2
0.5
20
120
10