Magnetic Field AS & A Level

Magnetic Field

by Azizah Mutiarani

​Today's Learning Goals

​1. Understand that a magnetic field is an example of a field of force produced either by current-carrying conductors

2. Represent a magnetic field by field lines

3. Appreciate that a force might act on a current-carrying conductor placed in a magnetic field

4. Recall and solve problems using the equation F = BIL sin θ, with directions as interpreted by Fleming’s left-hand rule

5. Define magnetic flux density and the tesla

6. Understand how the force on a current-carrying conductor can be used to measure the flux density of a magnetic field using a current balance

​A magnetic field is a field of force that is created either by:

• Permanent magnets

• Moving electric charge

​A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials.

​Representing Magnetic Fields

​Magnetic field lines are directed from the north pole to the south pole

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​The magnetic field into or out of the page is represented by circles with dots or crosses

​Force on a Current-Carrying Conductor

• A current-carrying conductor produces its own magnetic field

  When interacting with an external magnetic field, it will experience a force

A current-carrying conductor will only experience a force if the current through it is perpendicular to the direction of the magnetic field lines

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​Magnetic Force

​A simple situation would be a copper rod placed within a uniform magnetic field

When current is passed through the copper rod, it experiences a force which makes it move

​A wire carrying a current creates a magnetic field. This can interact with another magnetic field, causing a force that pushes the wire at right angles. This is called the motor effect.

The force F on a conductor carrying current I at right angles to a magnetic field with flux density B is defined by the equation :

F = BIL sinθ

• Where:

  • F = force on a current-carrying conductor in a B field (N)

  • B = magnetic flux density of external B field (T)

  • I = current in the conductor (A)

  • L = length of the conductor (m)

  • θ = angle between the conductor and external B field (degrees)

​The direction of the force can be found using Fleming's left-hand rule.

​A current of 0.87 A flows in a wire of length 1.4 m placed at 30^o to a magnetic field of flux density 80 mT.

Calculate the force on the wire!

​Magnitude of the force on a current carrying conductor depends on the angle of the conductor to the external B field

​Magnetic Flux Density Definition

​Rearranging the equation for magnetic force on a wire, the magnetic flux density is defined by the equation:

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• Note: this equation is only relevant when the B field is perpendicular to the current

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A straight conductor carrying a current of 1A normal to a magnetic field of flux density of 1 T with force per unit length of the conductor of 1 N m^-1

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​Measuring B from a Current Balance

When current I flows through the wire, an amperemeter reads its value

According to Newton’s third law, there is an equal and opposite force on the magnets

The magnets are therefore pushed downwards and a reading appears on the scale of the balance

This force is given by F = mg

​A 15 cm length of wire is placed vertically and at right angles to a magnetic field. When a current of 3.0 A flows in the wire vertically upwards, a force of 0.04 N acts on it to the left.

Determine the flux density of the field and its direction.

​a current-carrying wire placed in auniform magnetic field

​a. when does the wire experience the maximum force due tu the magnetic field?

​b. when does the wire experience no force due to the magnetic field?

​The magnetic flux density in the space between the poles of the magnet is uniform and is zero outside this region.

The length of the metal wire normal to the magnetic field is 4.5 cm. When a current in the wire is switched on, the reading on the balance increases by 1.2 g. The current in the wire is 8.7 A.

Calculate the magnitude of the magnetic flux density between the poles of the magnet.