A hollow copper pipe is placed upright on an electronic balance, which shows a reading of 300 g. A 50 g magnet is suspended inside the pipe and subsequently released.

It was observed that the readings on the balance began to increase after the magnet began to fall, and that the reading reached a constant maximum of 350 g before the magnet reached the bottom of the tube.

Explain these observations.   (4 marks)

A DC motor connected to a 12 V supply is maintaining a constant rotational speed. An ammeter in the circuit reads 0.5 A .
 

Some material falls into the running motor, causing it to slow down.

What is the subsequent ammeter reading likely to be?

1. 0
2. Between 0 and 0.5 A
3. 0.5 A
4. Greater than 0.5 A

An electric motor is connected to a power supply of constant voltage. The motor runs at different speeds by adjusting a brake.

On the graph below, show the relationship between the current through the motor and its speed?  (2 marks)

How does back emf affect a DC motor?

1. It creates heat in the iron core.
2. It limits the speed of the motor.
3. It reverses the current in the coil.
4. It increases the torque of the motor.

A motor, battery and ammeter are connected in series. When the motor is turning at full speed, the ammeter has a reading of 0.1 A. While the motor is spinning, a person holds the shaft of the motor to stop it.

Which row of the table correctly identifies the change in the ammeter reading and an explanation for the change?
 

\begin{align*}
\begin{array}{l}
\rule{0pt}{1.5ex} \ \rule[-0.5ex]{0pt}{0pt}& \\
\rule{0pt}{2.5ex}\textbf{A.}\rule[-1ex]{0pt}{0pt}\\
\rule{0pt}{2.5ex}\textbf{B.}\rule[-1ex]{0pt}{0pt}\\
\rule{0pt}{2.5ex}\textbf{C.}\rule[-1ex]{0pt}{0pt}\\
\rule{0pt}{2.5ex}\textbf{D.}\rule[-1ex]{0pt}{0pt}\\
\end{array}
\begin{array}{|l|l|}
\hline
\rule{0pt}{1.5ex}\ \ \ \textit{Reading on ammeter}\rule[-0.5ex]{0pt}{0pt}& \quad \ \ \textit{Explanation} \\
\hline
\rule{0pt}{2.5ex}\text{Decreases}\rule[-1ex]{0pt}{0pt}&\text{Decrease in back emf}\\
\hline
\rule{0pt}{2.5ex}\text{Increases}\rule[-1ex]{0pt}{0pt}& \text{Increase in back emf}\\
\hline
\rule{0pt}{2.5ex}\text{Decreases}\rule[-1ex]{0pt}{0pt}& \text{Increase in back emf} \\
\hline
\rule{0pt}{2.5ex}\text{Increases}\rule[-1ex]{0pt}{0pt}& \text{Decrease in back emf} \\
\hline
\end{array}
\end{align*}

An AC generator is operated by turning a handle, which rotates a coil in a magnetic field.

The handle is turned at a constant speed and the AC voltage output of the generator causes a light globe connected to it to light up, as shown in Circuit 1.
 

A second identical light globe is then connected in series to the generator output, as shown in Circuit 2 . The handle is turned at the same constant speed.
 

Which statement describes and explains the effort required to turn the handle in Circuit 2, compared to Circuit 1 ?

1. The handle in Circuit 2 is easier to turn because the smaller current in Circuit 2 produces less opposing torque.
2. The handle in Circuit 2 is easier to turn because the voltage output is shared equally across the two identical light globes.
3. The handle in Circuit 2 is more difficult to turn because the larger current in Circuit 2 produces more opposing torque.
4. The handle in Circuit 2 is more difficult to turn because it takes more power to operate the two identical globes than it does to operate the single globe.

A student suspends an electric ceiling fan from a spring balance.

The fan is switched on, reaching a maximum rotational velocity after ten seconds.
 

1. Explain the changes that would be observed on the spring balance in the first 15 seconds after the fan is switched on.   (4 marks)

2. The student predicted that the current through the fan's motor would vary as shown on the graph.   
    
    
   

Assess the accuracy of the student's prediction.   (4 marks)

Which graph shows the magnitude of back emf induced in a DC motor rotating continuously at different angular velocities?