An electric current is a flow of electric charge, carried by negatively charged electrons in atoms of metals. Electrons are free to move and flow in a direction opposite to the conventional current. Cells and batteries supply direct current (DC), which flows only in one direction. Generators produce alternating current (AC), in which the electrons change direction many times in a second. The current in a circuit is measured in amperes (A) using an ammeter, which must be connected in series to the circuit. The potential difference (pd) between two points in a circuit is the difference in voltage. This is measured in volts (V) using a voltmeter. A voltmeter is always connected in parallel.
A defibrillator delivers a controlled electric shock through a patient's chest to restart their heart. Two paddles are charged form a high voltage supply. They are placed firmly on the patient's chest to ensure good electrical contact. Electric charge is passed through the patient to make their heart contract. Once the heart is restarted, it is hoped it will continue normally. A typical shock from a defibrillator supplies around 400 J of energy in a few milliseconds (1 millisecond = 0.001 seconds). If we assume that a defibrillator is switched on for 5 milliseconds, we can calculate the power like this:
We will now answer some questions on electrical charge and power.