5.4 Magnetic Effects of Electric Currents

The diagram shows a magnetic B field.

A circuit is built with a section of wire, between the points A and B, running perpendicular to a magnetic field.

When the force, the magnetic field and the current are all mutually perpendicular to each other, the directions of each can be interpreted using a technique known as Fleming's Left Hand Rule.

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A wire of length 15 cm has a mass of 30 g and current of 2.0 A flowing through it. When the wire is placed inside a uniform magnetic field it 'floats' in equilibrium in the magnetic field.

The equation used to calculate the force acting on a moving charged particle is

A beam of electrons is fired into a uniform magnetic field of flux density 0.5 T, as shown. An electron enters the magnetic field at point A.

The electron is travelling at a speed of 4.8 × 10⁷ m s⁻¹ in the magnetic field where magnetic flux density B = 0.5 T

When a moving charge enters a magnetic field the magnetic field produces a force on the charge, which can be calculated using

where q = charge, v = velocity, B = magnetic flux density, and θ = the angle between the velocity of the charge and the direction of the magnetic field.

The magnetic force provides the centripetal force which causes the charge to move in a circular orbit. The equation to calculate centripetal force acting on an object is

where m = mass of the object, v = speed of the object and r = radius of the circular orbit.

   

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An electron is travelling at right angles to a uniform magnetic field which has a magnetic flux density of 5.6 mT. The speed of the electron is 3.0 × 10⁶ m s^–1.

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