B_B0034
1
MEASURING MAGNETIC FIELD STRENGTH USING CURRENT BALANCE: A
STUDENT PROJECT
Apinya Ketkong, Kueakul Khowamnuychok, Kheamrutai Thamaphat,* Pichet Limsuwan
Physics Learning Innovation Group, Department of Physics, Faculty of Science, King
Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand
*e-mail: s903607@yahoo.com
Abstract: In this work, we proposed a method to measuring the magnitude of a uniform
magnetic field in space using current balance. The apparatus can be constructed easily using
low-cost materials and its operation is simplicity. This constructed current balance consists
of copper wire with a diameter of 1.175 mm, weight pan, acrylic cord, bearing, and wood
frame. A copper wire was bent in U-shaped and mounted at the end of acrylic balance arm.
A weight pan was hanged in the opposite side of the balance arm with high sensitivity to a
small torque. The horizontal segment of the U-shaped copper wire, which the length was l,
was located inside the influence of a magnetic field. When a current is supplied to the copper
wire, the magnetic force acting on a horizontal segment of length l carrying a current of
r
r r
r
r
intensity i in a magnetic field B is given by FB = il × B . The direction of l and i is
coincidence. The magnetic force on a straight wire of length l is measured by hanging small
weights on the pan until reaching equilibrium between opposing gravitational and magnetic
torques. In our experiment, the uniform magnetic field was produced by two parallel bar
magnets with opposite poles facing each other. The gap of magnets was set as 1.0 cm. The
horizontal segment of the U-shaped copper wire with the length l of 4.125 cm was placed at
the center of this gap. The magnetic field direction in the gap is perpendicular to the
horizontal segment. The U-shaped copper wire was connected to a DC power supply (15 V).
The current intensity was varied from 0 – 2.8 A. At each value of current intensity, the added
masses to the pan until the balance arm moved to the equilibrium position were recorded.
The results showed that the magnetic force increased linearly with increasing the current
intensity. By plotting a linear graph of magnetic force versus current intensity, the magnetic
slope
field B inside a central part of the gap can be calculated from B =
. The calculated and
l
actual values of B were 92.0 and 96.1 mT, respectively. This constructed current balance is
an excellent tool for high school and undergraduate fundamental physics courses. Students
will be excited when they see the balance arm rising or going down due to magnitude and
direction of current flowing in a conductor wire.
References:
1. Erlend HG. Physics Teacher 2004;42:500-501.
2. Ganci S. Physics Education 2011;46:14-16.
Keywords: current balance, magnetic field, magnetic force, current-carrying wire
© 38th Congress on Science and Technology of Thailand (Abstract)