Page 429 - Physics Coursebook 2015 (A level)

Chapter 26: Magnetic fields and electromagnetism
Comparing forces in magnetic, electric and gravitational fields
We have now considered three types of field: electric (Chapters 8 and 23), gravitational (Chapter 18) and magnetic (this chapter). What are the similarities and differences between these three types of field?
Modern physics sees magnetic fields and electric fields as two parts of a combined whole, an electromagnetic field. Gravitational fields, however, are different in nature to electromagnetic fields.
Gravitational and electric fields are defined in terms of placing a test mass or a test charge at a point to measure the field strength. Similarly, a test wire carrying a current can be placed at a point to measure the magnetic field strength. Therefore all fields are defined in terms of the force on a unit mass, charge or current.
Other features that all fields share include:
■■ Action at a distance, between masses, between charges or between wires carrying currents.
■■ Decreasing strength with distance from the source of the field.
■■ Representation by field lines, the direction of which show the direction of the force at points along the line; the density of field lines indicates the relative strength of the field.
How do the forces arising from these fields compare? The answer depends on the exact situation. Using ideas that you have studied earlier, you should be able to confirm each of the following values:
■■ The force between two 1 kg masses 1 m apart = 6.7 × 10–11 N ■■ The force between two charges of 1 C placed 1 m apart =
9.0×109N
■■ The force per metre on two wires carrying a current of 1 A
placed1mapart=2.0×10–7N
This might suggest that the electric force is strongest
and gravity is the weakest. Certainly if you consider an electron in a hydrogen atom moving in a circular orbit around a proton, the electrical force is 1039 times the gravitational force. So for an electron, or any other small charged object, electric forces are the most significant. However, over larger distances and with objects of
large mass, the gravitational field becomes the most significant. For example, the motions of planets in the Solar System are affected by the gravitational field but the electromagnetic field is comparatively insignificant.
Summary
■■ Moving charges produce a magnetic field; this is electromagnetism.
■■ A current-carrying conductor has concentric magnetic field lines. The magnetic field pattern for a solenoid or flat coil resembles that of a bar magnet.
■■ The separation between magnetic field lines is an indication of the field’s strength.
■■ Magnetic flux density B is defined by the following equation:
B=F IL
where F is the force experienced by a current-carrying conductor, I is the current in the conductor and L is the length of the conductor in the uniform magnetic field.
■■ The unit of magnetic flux density is the tesla (T). 1T=1NA−1m−1.
■■ The magnetic flux density is 1 T when a wire carrying a current of 1 A placed at right angles to the magnetic field experiences a force of 1 N per metre of its length.
■■ The magnetic force on a current-carrying conductor is given by F = BIL sin θ.
■■ The force on a current-carrying conductor can be used to measure the flux density of a magnetic field using a current balance.
■■ A force acts between current-carrying conductors due to the interaction of their magnetic fields.
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