Video Transcript
The diagram shows a moving-coil galvanometer. The terminals of the galvanometer are connected to a direct-current source. Which of the terminals 𝐴 or 𝐵 connects to the positive output of the source?
To begin, we’ll need to remember that conventional electric current is the flow of positive charge. For that reason, current in the system will be pointing away from the positive terminal toward the negative terminal. So as long as we can determine which way current is pointing in the galvanometer’s coils, we can tell which terminal 𝐴 or 𝐵 is positive.
Looking at the diagram, we can see that the galvanometer has rotated clockwise. This rotation corresponds to a downward force on the right side of the coils and an upward force on the left side of the coils. For simplicity though, let’s just focus on this section of the coils on the left-hand side. Here, the current-carrying wire that makes up the coils is pointing more or less into and out of the screen. So let’s determine whether the current itself is pointing into or out of the screen.
To do this, we can recall Fleming’s left-hand rule. Using our left hand, we stick out our thumb, index, and middle fingers like this so that each one is perpendicular to the others. The left-hand rule states that if our index finger is pointing in the direction of the magnetic field and our middle finger is pointing in the direction of conventional current, then our thumb will point in the direction of the resulting force on the current-carrying wire.
Here in this section of the coil, we’ve already established that the force points upward. And although it’s not explicitly shown in the diagram, we do know which way the magnetic field points because it always points from north to south and also radially about the iron core of the galvanometer. Since we’re focusing on the left side of the coils, the magnetic field over here is pointing to the left.
Now let’s rotate the left hand so that our index finger representing the magnetic field points to the left and the thumb representing the force points up. This causes the middle finger, which represents current, to point away from us into the screen. Because the current points into the screen on the left side of the coils, it’s moving clockwise as viewed from above in the diagram. This means the current is moving toward terminal 𝐵. And so terminal 𝐵 must be negative. The current is pointing away from terminal 𝐴. And therefore, we know that terminal 𝐴 is connected to the positive output of the source.
