Physics 10 Homework Solutions

Chapter 22

(1) An quantity is conserved if, for a given isolated system, the net value of the quantity re-mains constant. To say, then, that charge is conserved is to say that the net charge of anyisolated system is constant. It does not mean that the total numbers of positive and negativecharges are individually constant. For instance, an electron and positron (the positivelycharged antiparticle of the electron) come together, they will annihilate each other into radia-tion, which carries no charge.

(2) Coulomb’s law and Newton’s law of gravitation are both inverse square laws; the electro-static force between two particles decreases as the inverse square of the distance between theparticles, which is also the case for the gravitational force. However, the gravitational forcebetween two particles is proportional to the product of their masses, while the electrostaticforce is proportional to the product of their charges.

(3) If one walks with a sliding motion across a carpet in dress shoes on dry day, one’s body willpick up a static charge. Touching a door knob or shaking someone’s hand will cause a dis-charge, which will produce a shock.

(4) The magnitude of the electric field at a point in space is the magnitude of the electric forceon a small positive test charge placed at the point divided by the value of the charge.

(5) The direction of the electric field at a point in space is the direction of the force on a smallpositive test charge placed at the given point.

(6) A dry cleaner’s garment bag is usually some sort of plastic such as polyethylene. As the suitis removed from the bag, the suit and bag rub against each other. The affinity of the wool forelectrons is greater than that of the plastic for electrons. Thus, electrons are rubbed of theplastic onto the wool, which gives the suit a negative charge.