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Figure 1   Current flowing in a loop of wire interacts with the B field to produce a force, F. The net force on the wire is upward. An equal but opposite downward force is exerted on the magnet.

Figure 2   Forces generated by currents flowing in the magnetic field are measured by the 0.01 gram balance scale.

Figure 3   The experimental setup is shown. A quadruple-beam balance measures the weight of the magnet assembly. A power supply provides current to the wire in the magnetic field.

Figure 4   The apparatus shown allows the direction of the current relative to the magnetic field to be varied.

Phys 223 University Physics III
Lab 5     Magnetic Forces

In this lab, we investigate magnetic forces on a current carrying wire. Magnetic fields exert forces on charges, but the charge must be moving as they are in currents. The force exerted on a current carrying wire by a magnetic field is,
where I is the current, L is the length of the wire, and B is the magnetic field in Teslas.

This assumes the wire is straight and the magnetic field is uniform.

Recall the cross product. The force vector in equation 1, is perpendicular to both the current and the magnetic field, and is in the direction a right hand screw will advance when the current vector is turned into the magnetic field vector.

For the current and magnetic field shown in Figure 1, the force on the wire will be up. An equal but opposite force is exerted on the magnet. The force on the magnet will be down. The magnet rests on the scale. The force exerted on the magnet effects the weight of the magnet seen by the scale. The setup is shown in Figure 2.

The magnitude of the force on a current carrying wire in a magnetic field is,

Since the current is perpendicular to the magnetic field, the magnitude of the force is,
where I is the current, L is the length of the wire segment, and B is the magnetic field.

In the course of performing this experiment, we shall reinforce

  1. proper use of electrical meters,

  2. notions of direct and indirect measurement,

  3. use of a quadruple-beam balance to measure force,

  4. use of experiment to corroborate or falsify theory, and

  5. error analysis.

Force versus Current

Force as a function of the Length of Wire

Force versus Angle


Reflect upon your observations.

Q: What is the value of the B field?

Q: How do measured force values compare with the formula, ?