Electrical homework

1. A sports car company boasts that its car can accelerate from 0 to 60 mph in 5.1 seconds.
Let’s see how much current an electric car battery would need to deliver to do that.

Compute the current drawn from a 12 Volt DC battery bank needed to accelerate a 1250kg car from a stop to 60 miles per hour in that same 5.1 seconds. Assume the DC motor in the car is 77% efficient in converting electrical power to mechanical power. Also. assume constant current flow and constant acceleration. Please neglect aerodynamic drag. etc. (Note : The result of this problem illustrates why electric car makers don’t use 12 Volt DC motors

2. Speaking of DC motors. the part of the motor that makes electrical contact between the external wiring and the rotating part of the motor is called a brush. (So named because in old motors. it literally was a brush with wire bristles that made contact with the rotor as it spun.)
Consider a brush made from graphite that is rectangular in shape, 5 mm long with a square cross section 1.5 mm on a side.

(a). Compute the resistance of this brush.

(b). Compute the resistance if the brush were made of copper.

3. In class last week we computed the drift velocity of electrons in a cube of copper. Let’s dig a little deeper. Consider a wire that is 1 mm in diameter made from silver. If it carries 1 Amp of current

(a). Compute the electron drift velocity in the wire. The atomic mass of silver is 107.9 gram/mol and its mass density is 10.49 gram/cm3.

(b). Compute the kinetic energy of a free electron in the wire in electron volts (i.e. take the kinetic energy in Joules and divide through by the electron charge. 1.6X10’19 Coulomb.) Assume an electron rest mass of 9.1x 10’31 kg.

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