The oscilloscope trace below shows the output waveforms for the diode current (yellow) and the load voltage (blue) for a single phase full-wave uncontrolled bridge rectifier. The load attached to the rectifier was a resistive-capacitive load. I understand that there is a current spike every time that the capacitor charges up for every half cycle, however i do not understand why this spike alternates in polarity, isn't the spike always supposed to be positive?
The current should follow sign of the voltage. If you measure the current after the rectifier, it has already been rectified and so the current will only flow in one direction as the diodes will prevent it from flowing the 'wrong way'. However, if you measure the current before the rectifier, then it will follow the sign of the input signal. Since the input signal in this case is a sinewave that has both positive and negative peaks, you will see both positive and negative current flow.
It looks like you are measuring the current at the input of the bridge rectifier and the voltage at the output of the rectifier.
Look at the way the currents flow: -
1st half cycle current (red) is sourced from the top of the source through the top right diode to the load. Current from the load flows through the bottom left diode back to the bottom of the source.
For the 2nd half of the cycle current (colored blue) is sourced from the bottom of the source thru the lower right diode and feeds a positive voltage to the load (as previously). Current from the load flows back to the top of the source thru the top left diode and hence, for each half cycle the current (from the source) reverses.