Compare the phase difference between points at different distances along a stationary wave with the phase difference between points along a progressive wave. C. 5 Calculate the number of electrons in a beam hitting the screen of a TV tube each second when the beam current is 1. Electric field around a point charge You have already studied general ideas about electric fields (Chapter 8). is anticlockwise (the ‘direction’ of E2) so, for a resistor where the current is clockwise, the ‘IR’ term will be negative. This exercise provides practice drawing electric fields and understanding the rules that should be followed.

c When the frequency of the vibrator is doubled, the number of loops on the stationary wave changes from three to six. c Calculate the value of the contact force that acts on the box at the point where the box touches the ground.

d If a negative charge of −20 C moves from C to D, deduce the change in its electrical potential energy.

Now picture an explosion in which a stationary object splits into two parts that move away from each other in opposite directions.Getting a feel for momentum changes You can use the idea of conservation of momentum to picture what happens when two objects collide head-on, or when two objects explode apart.

Exam-style questions 1 A bullet of mass 25 g is travelling at 450 m s −1 when it strikes the armour plating of a tank.

a Show that this leads to the equation for the gravitational field strength at a distance r from a point mass M: g = GM2 . c Use your answers to parts a and b to calculate the time taken for the spacecraft to complete half an orbit, and hence find its orbital period.