Magnetic Force on a Current-carrying Wire
AIM: To investigate the factors affecting the magnetic force on current-carrying wires by measuring the deflection of nearby compass needles caused by the reaction forces of the wires.
THEORY: According to the equation; F = nBIℓsinq, the magnetic force, F, on a current-carrying wire is directly proportional to;
n = the number of current carrying wires
B = the strength of the magnetic field about the wires
I = the current carried by the wires
ℓ = the length of wire within the magnetic field
sinq = the sine of the angle between the current and the magnetic field
According to Newton’s Third Law (FAB= –FBA), every action has an equal but opposite reaction. The force on a wire due to a nearby magnetic field should therefore be equal but opposite to the force of the wire on the object causing the magnetic field.
HYPOTHESIS: It is predicted that the compass needle will be deflected by different amounts depending on the (a) strength of the current, (b) the number of wires carrying the same current, (c) the angle between the current and the Earth’s magnetic field, (d) the total amount of magnetic field applied to the wire by one or more compass needles, and, (e) the distance across which two compass needles exert their magnetic force on the wire. Record your predictions in the Hypotheses Summary Table 4 marks
MATERIALS:
· 2× Power Pack (DC voltage source with switch, ammeter & variable resistor)
· 2× 30 cm (straightened) enamel coated copper wire (with exposed ends)
· 4× 30 cm connecting wires (crocodile clips on one end banana clips the other)
· 2× orienteering compasses (with rotating and graded mounts)
METHOD: Setting the test current (Control Condition):
Connect power pack to the test wire. Arrange the test wire so that the current running through it would be from north to south. Place a compass (N1) under the test wire so that the north facing needle is parallel to the wire. Switch on the Power Pack and turn the dial on the variable resistor until the deflection on the compass is equal to 45O from north: Record the magnitude of the current and the direction of the compass.
Current causing 45O deflection of compass needle: ______A
Direction of deflection 45O East or West of North (circle): EAST / WEST
2 marks
Test Conditions: Current, Wires, Angle, Field & Length (F = nBIℓsinq):
a) Current: Close the switch and turn the variable resistor until the current is twice the original test value. Measure the degree of deflection in the test compass (N1), then return the set-up to the original test conditions with the switch open.
Deflection of Compass Needle (N1): ______O EAST / WEST
b) Wires: Connect a second identically set-up power pack and test wire so that the second test wire is parallel to the first test wire, then close the switch on both circuits. Measure the degree of deflection in the test compass (N1), then return the set-up to the original test conditions with the switch open.
Deflection of Compass Needle (N1): ______O EAST / WEST
c) Angle: Arrange the test wire so that it makes an angle of 45O with the Earth’s magnetic field then close the switch. Measure the degree of deflection in the test compass (N1) and then open the switch.
Deflection of Compass Needle (N1): ______O EAST / WEST
Arrange the test wire so that it is perpendicular (90O) with the Earth’s magnetic field then close the switch. Measure the degree of deflection in the test compass (N1), then return the set-up to the original test conditions with the switch open.
Deflection of Compass Needle (N1): ______O EAST / WEST
d) Field: Place a second compass (Nd) 5.0 cm directly south of the first compass (N1) then close the switch. Measure the degree of deflection in both test compasses (N1 Nd), then return the set-up to the original test conditions with the switch open.
Deflection of Compass Needle (N1): ______O EAST / WEST
Deflection of Compass Needle (Nd): ______O EAST / WEST
e) Length: Place a second compass (Ne) 10.0 cm directly south of the first compass (N1) then close the switch. Measure the degree of deflection in both test compasses (N1 Ne), then return the set-up to the original test conditions with the switch open.
Deflection of Compass Needle (N1): ______O EAST / WEST
Deflection of Compass Needle (Ne): ______O EAST / WEST
Transfer all of your observed results to the Results Summary Table: 4 marks
Hypotheses Summary Table: complete this table before the experiment.
Condition / Factor / Deflection of Needle / Change in Deflection compared to the Control ConditionControl / Original / N1 45 O E / W / (circle correct answer)
a) Current / Doubled / N1 E / N / W / Decrease / Same / Increase
b) Wires / Doubled / N1 E / N / W / Decrease / Same / Increase
c) Angle / 45O / N1 E / N / W / Decrease / Same / Increase
90O / N1 E / N / W / Decrease / Same / Increase
d) Field / Doubled / N1 E / N / W / Decrease / Same / Increase
5.0 cm / Doubled / Nd E / N / W / Decrease / Same / Increase
e) Length / Doubled / N1 E / N / W / Decrease / Same / Increase
10.0 cm / Doubled / Ne E / N / W / Decrease / Same / Increase
Results Summary Table: complete the table based on your observations.
Condition / Factor / Deflection of Needle / Change in Deflection compared to the Control ConditionControl / Original / N1 45 O E / W / (circle correct answer)
a) Current / Doubled / N1 _____O E / W / Decrease / Same / Increase
b) Wires / Doubled / N1 _____O E / W / Decrease / Same / Increase
c) Angle / q (I-BE) 45O / N1 _____O E / W / Decrease / Same / Increase
q (I-BE) 90O / N1 _____O E / W / Decrease / Same / Increase
d) Field / Doubled / N1 _____O E / W / Decrease / Same / Increase
5.0 cm / Doubled / Nd _____O E / W / Decrease / Same / Increase
e) Length / Doubled / N1 _____O E / W / Decrease / Same / Increase
10.0 cm / Doubled / Ne _____O E / W / Decrease / Same / Increase
Results Description: Compare your predicted and observed results: 2 marks
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Discussion 10 marks
“Current”: Explain how and why the comparison of results for condition ‘a)’ and the control condition provides evidence for the affect of increasing the current in the wire on the magnetic force on a current-carrying wire. Be sure to refer to Newton’s 3rd Law of Motion in your explanation.
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“Wires”: Explain how and why the comparison of results for condition ‘b)’ and the control condition provides evidence for the affect of increasing the number of current carrying wires on the magnetic force on a current-carrying wire.
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“Angle”: Explain how and why the comparison of results for parts 1 and 2 of condition ‘c)’ and the control condition provides evidence for the affect of increasing the sine of the angle between the current and the magnetic field on the magnetic force on a current-carrying wire.
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“Field”: Explain how and why the comparison of results for parts 1 and 2 of condition ‘d)’ and the control condition provides evidence for the affect of increasing the strength of the magnetic field on the magnetic force on a current-carrying wire. Be sure to refer to the sum of forces in your answer!
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“Length”: Explain how and why the comparison of results for parts 1 and 2 of condition ‘d)’ and parts 1 and 2 of condition ‘e)’ provides evidence for the affect of increasing the strength of the magnetic field on the magnetic force on a current-carrying wire. Be sure to refer to the sum of forces in your answer!
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