The emf acts in the anti-clockwise direction in Fig. 36.

The anti-clockwise current driven around the circuit by the motional
emf follows from Ohm's law:

Since the rod carries a current which flows perpendicular to a magnetic
field , the force per unit length acting on the rod is
(see Sect. 8.2). Thus, the total force acting on the rod is of magnitude

This force is directed parallel to the vector . It follows that the force is to the left in Fig. 36. In other words, the force

In order to keep the rod moving at a constant velocity, some external
agent must apply a force to the rod which is equal and opposite to
the magnetic force described above. Thus, the externally applied force acts to
the right.
The rate at which work is done
by this force is the product of the force and the velocity of the rod in the
direction of this force. Thus,

Every charge which circulates around the circuit in the anti-clockwise
direction acquires the energy . The amount of charge per unit
time which circulates around the circuit is, by definition, equal to the
current flowing around the circuit. Thus, the rate at which electric charges
acquire energy in the circuit is

Now, the rate at which electric charges acquire energy in the circuit is equal to the rate at which mechanical work is done on the rod by the external force, as must be the case if energy is to be conserved. Thus, we can think of this circuit as constituting a primitive generator which transforms mechanical into electrical energy.

The rate at which electrical energy is converted into heat energy in the
circuit is

Thus, all of the mechanical work done on the rod eventually ends up as heat dissipated in the circuit.