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General Physics - Currents and Magnetic Forces - Lab 8 | PHYS 112, Lab Reports of Physics

Material Type: Lab; Class: General Physics-GTSC1; Subject: Physics; University: Mesa State College; Term: Spring 2008;

Typology: Lab Reports

Pre 2010

Uploaded on 08/19/2009

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Phys 112L
Spring 2008
Laboratory 8: Currents and Magnetic Forces
1 Forces between Two Currents
The force exerted by one current carrying wire on another parallel current carrying
wire of the same length is
F=µ0I1I2
2πd L(1)
where I1and I2are the currents in the two wires, dis the distance between the wires
and Lthe length of the wires.
This relationship can be investigated using a current balance, whose essential working
parts are two rigid parallel wires through which the same current is passed. The current
balance is designed so that one of the upper wire is free to pivot and the lower wire is
fixed. When currents pass through the wires in opposite directions the lower wire exerts
force on the upper wire which causes it to rotate upwards. This can be counteracted
by adding masses to the small balance pan on the upper wire, eventually returning
the upper wire to its equilibrium position. When the balance is in equilibrium, the
gravitational force due to the additional mass equals the force exerted by the lower wire
on the upper wire.
a) Predict the current required to balance the current balance arm when there is
10 mg masspiece in the balance pan. Repeat this for the case where there is 20mg
masspiece in the balance pan.
b) Set up the current balance with no additional mass in the pan. Record the equilib-
rium point by marking the position of the reflected laser beam on the wall. Measure
the distance between the two wires and the length of the upper wire.
c) Place a 10 mg masspiece in the balance pan. Adjust the current until the arm
balances and measure the current. Reverse the direction of the current through the
apparatus (this should partially cancel the effects of the earth’s magnetic field) and
again balance the arm and measure the current. Determine the average current
and calculate the difference between this and the predicted current.
d) Repeat the previous part using a 20 mg masspiece in the balance pan.
2 Force Exerted by Current Carrying Coil on a Magnet: Determining the
Earth’s Magnetic Field
This experiment uses a circular current-carrying coil with Ncomplete loops of wire.
The magnetic field at the center of the loop points along the axis of the loop in a direction
pf2

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Phys 112L Spring 2008

Laboratory 8: Currents and Magnetic Forces

1 Forces between Two Currents The force exerted by one current carrying wire on another parallel current carrying wire of the same length is

F =

μ 0 I 1 I 2 2 πd

L (1)

where I 1 and I 2 are the currents in the two wires, d is the distance between the wires and L the length of the wires.

This relationship can be investigated using a current balance, whose essential working parts are two rigid parallel wires through which the same current is passed. The current balance is designed so that one of the upper wire is free to pivot and the lower wire is fixed. When currents pass through the wires in opposite directions the lower wire exerts force on the upper wire which causes it to rotate upwards. This can be counteracted by adding masses to the small balance pan on the upper wire, eventually returning the upper wire to its equilibrium position. When the balance is in equilibrium, the gravitational force due to the additional mass equals the force exerted by the lower wire on the upper wire.

a) Predict the current required to balance the current balance arm when there is 10 mg masspiece in the balance pan. Repeat this for the case where there is 20 mg masspiece in the balance pan. b) Set up the current balance with no additional mass in the pan. Record the equilib- rium point by marking the position of the reflected laser beam on the wall. Measure the distance between the two wires and the length of the upper wire. c) Place a 10 mg masspiece in the balance pan. Adjust the current until the arm balances and measure the current. Reverse the direction of the current through the apparatus (this should partially cancel the effects of the earth’s magnetic field) and again balance the arm and measure the current. Determine the average current and calculate the difference between this and the predicted current. d) Repeat the previous part using a 20 mg masspiece in the balance pan.

2 Force Exerted by Current Carrying Coil on a Magnet: Determining the Earth’s Magnetic Field

This experiment uses a circular current-carrying coil with N complete loops of wire. The magnetic field at the center of the loop points along the axis of the loop in a direction

given by the right hand rule. The magnitude of the magnetic field at the center of the loop is given by

B = N

μ 0 I 2 R

where R is the radius of the loop and I the current through the loop. This can be used in the following way to determine the earth’s magnetic field.

a) Place the compass at the center of the loop and orient the loop so that the plane of the loop lies along the north-south direction. b) Suppose that current passes through the coil. Indicate the two possible directions of the magnetic field produced by the coil. In each case, sketch qualitatively the net magnetic field vector at the center of the loop, indicating the contributions from the earth’s magnetic field vector and the magnetic field vector produced by the coils. c) Adjust the current through the coil so that the compass needle deflects by about 20 ◦. Measure the current through the coil, determine the magnitude of magnetic field produced by the coil and use this to determine the magnitude of the earth’s magnetic field. d) Repeat the previous part for compass needle deflections of 30◦, 40 ◦, 50 ◦, and 60◦. e) Determine an average value for the earth’s magnetic field based on your measure- ments.

3 Magnetic Field Produced by a Helmholtz Coil Standard electromagnetic theory predicts that the field at the center of a pair of Helmholtz coils is

B =

μ 08 N I 5

5 R

where N is the number of coils and R is the radius of the coil. The aim of this experiment is to check this relationship.

a) Align the Helmholtz coils so that they are parallel to the north-south direction. Adjust the current in the coils so that the compass deflection is about 45◦. Measure deflection of the compass needle and the current in the coils. b) Based on the measurement of the earth’s magnetic field and the compass deflection, calculate the magnetic field produced by the Helmholtz coils. c) Based on the measured value for I and Eq. (3), calculate the magnetic field produced by the Helmholtz coils. d) Determine the percentage difference between the two values for the magnetic field produced by the Helmholtz coils.