Transcript Resistance of a wire - The Thomas Cowley High School

Electric Current
11 April 2016
+
Remember: Copper atoms are much bigger than electrons
and are packed together in the wire.
Current
In an electric c____,
ircuit current is the flow of
e_______.
ore
lectrons The bigger the current, the m___
electrons are flowing around the circuit. The
w___
ires carry the electrons from one terminal
erminal
around the circuit to the other t______.
lamps
Ω
Omega
11 April 2016
5.2 Resistance
• Resistance is measured using an
ammeter and a voltmeter.
• Resistance = Voltage  Current
• This is called Ohm’s Law
• Resistance is measured in Ohm’s (Ω)
How many different words can you spell from the word
resistance?
Measuring Current
Current is
measured using an
ammeter
connected in
series.
What will the 3
ammeters read in
this circuit?
A
1
3
A
2
A
Ammeter 1 = 0.15A, Ammeter 2 = ____, Ammeter 3 = ____
Measuring Current
Current is
measured using an
ammeter
connected in
series.
What will the 3
ammeters read in
this circuit?
A
1
3
A
2
A
Ammeter 1 = 0.15A, Ammeter 2 = 0.15A, Ammeter 3 = 0.15A
In a series circuit the current is always the
same.
Current in parallel
In a parallel circuit the
current can split
depending on how
difficult each route is.
A1
The total current into
and out of the supply is
always the same.
What are the missing currents?
A1 = 0.6A
A2 = _____?
A3 = 0.2A
A4 = _____?
11 April 2016
A4
A2
A3
Current in parallel
In a parallel circuit the
current can split
depending on how
difficult each route is.
A1
The total current into
and out of the supply is
always the same.
What are the missing currents?
A1 = 0.6A
A2 = 0.4A (A1-A3 = 0.6A-0.2A)
A3 = 0.2A
A4 = 0.6A (A1 = A4)
A4
A2
A3
Measuring Voltage
Voltage measures how
energy changes around
the circuit. It
compares the energy in
one part with another
and shows the
difference.
Voltmeter
V1
V2
V3
Voltage
(V)
1.5
0.6
0.9
V1
NB Voltmeters are
connected in parallel
V2
V3
In series V1 = V2 +V3
Voltage in Parallel
Voltage only changes when
energy changes. In this
circuit energy changes
happen at the cell and the
bulbs.
V1 = V2 = V3
Voltmeter
V1
V2
V3
Voltage
(V)
1.5
1.5
1.5
V1
V2
V3
Measuring Resistance
Set up the circuit as shown below:
Resistance = Voltage
Voltage
(V)
A
V
Slowly move the variable resistor and for
each setting record the current in amps
and the voltage in volts.
Plot a graph of your results.
0
2
4
6
8
12
Current
Current Resistance
(A)
(Ω)
Correct type of graph
Graphs
Bar chart for categoric data
Line graph for continuous data
Current (Amps)
Voltage against current through a resistor
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0
0.5
1
1.5
2
2.5
3
3.5
4
Voltage (Volts)
Points plotted correctly
Suitable labels –
Quantity (Units)
Scale uniform
ISA Answers
1. I was trying to find out if the current flowing
through a resistor depends on the voltage across it.
2. Independent variable = Voltage
Dependent variable = Current
3. Voltmeter or Ammeter
4. The smallest scale division was 0.01V or 0.01A
5. Smaller scale division means more precise
measurements
6. Use the same resistor
Different resistance values change the current
8. As the Voltage increases the current increases
Section 2
10. As temperature increases the resistance falls
11. Range 0º C to 150ºC
No. 150ºC too high for room temperatures.
12. At these temperatures resistance only changes very
slightly.
13. 50ºC circled.
Not anomalous. More likely to be 25ºC
14. Thermistor tested might not be typical
between 50 and 200 (5 to 20%)
15. Results would be more reliable
16. Have smaller intervals between readings
17. Company was changing the results.
Avoiding bad publicity.
Would damage the companies reputation.
18. Improves the reliability
19. They have had a scale marked on them
A range either side of the expected value
that is considered acceptable
5%
Testing the effect of
current on a wire
Circuit
Results
Voltage Current Resistance
(V)
(A)
(Ohms)
2
A
Test Wire
4
6
8
12
Ω
Omega
11 April 2016
5.2 Resistance
• Resistance is measured using an
ammeter and a voltmeter.
• Resistance = Voltage  Current
• This is called Ohm’s Law
• Resistance is measured in Ohm’s (Ω)
How many different words can you spell from the word
resistance?
Resistance of a wire (ISA)
11 April 2016
• To investigate which factors change
the resistance of a wire.
Apparatus: Resistance wire, Voltmeter, Ammeter,
Power supply, Leads, Crocodile clips, metre ruler,
Variable resistor.
A
Test Wire
V