Page 428 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
416
Base units
Derived units
because
The other way to explain the forces between the currents is to use the idea of the motor effect. Figure 26.24 again shows two like currents, I1 and I2, but this time we only consider the magnetic field of one of them, I1. The second current I2 is flowing across the magnetic field of I1; from the diagram, you can see that B is at right angles to I2. Hence there will be a force on I2 (the BIL force), and we can find its direction using Fleming’s left-hand rule. The arrow shows the direction of the force, which is towards I1. Similarly, there will be a BIL force on I1, directed towards I2.
These two forces are equal and opposite to one another. They are an example of an action and reaction pair, as described by Newton’s third law of motion.
I1 F F I2 B
Figure 26.24 Explaining the force between two currents. QUESTION
13 Two flat circular coils of wire are set up side
by side, as shown in Figure 26.25. They are connected in series so that the same current flows around each, and in the same direction. Will the coils attract or repel one another? Explain your answer, first by describing the coils as electromagnets, and secondly by considering the forces between parallel currents. What will happen if the current is reversed in both coils?
Relating SI units
In this chapter, we have seen how one SI unit, the tesla, is defined in terms of three others, the amp, the metre and the newton. It is an essential feature of the SI system that all units are carefully defined; in particular, derived units such as the newton and tesla must be defined in terms of a set of more fundamental units called base units.
We met the idea of base units in Chapter 3. The SI system of units has seven base units, of which you have met six. These are:
m kg s A K mol
(The seventh is the candela, cd, the unit of luminous intensity.) Each base unit is carefully defined; for example, the ampere can be defined in terms of the magnetic force between two parallel wires carrying a current. The exact definition is not required, but you should know that the ampere is itself a base unit. Other units are known as derived units, and can be deduced from the base units. For example, as shown in Chapter 3, the newton is given by:
1N = 1kgms−2
Similarly, in this chapter, you have learned about the tesla, the unit of magnetic flux density, given by:
1T = 1NA−1 m−1 or 1T = 1kgA−1 s−2
If you learn formulae relating physical quantities, you can replace the quantities by their units to see how the units are defined. For example:
force = mass×acceleration F = ma N =kgms−2
You should be able to picture how the different derived units form a logical sequence, as shown in Table 26.1.
m, kg, s
newton N = kg m s−2
joule J = kg m2 s−2
watt W = kg m2 s−3
coulomb C = A s
F = ma
W = Fd
P = Wt
Q = It
V = WQ
B= F IL
m, kg, s, A volt V = kg m2 A−1 s−3
tesla T = kg A−1 s−2
Figure 26.25 Two coils carrying the same current – see Question 13.
Table 26.1 How derived units relate to base units in the SI system.