Back to Exercise: Motors, Generators, and Transformers

Exercises: Motors, Generators, and Transformers

Work through each section in order. Show your work where indicated.

Grade 11·21 problems·~30 min·OpenStax Physics (High School)·section·sec-20-3

Printable layout

Recall / Warm-Up

An electric motor converts electrical energy to mechanical energy. What is the fundamental physical principle that causes the coil in a motor to rotate?

Electromagnetic induction — a changing flux induces a current in the coil.

Magnetic force on a current-carrying loop creates a torque that rotates the coil.

The battery pushes current through the coil, and the current itself generates rotation.

Electrostatic attraction between the coil and the magnets causes rotation.

How does a generator differ from a motor in terms of energy conversion and operating principle?

A generator uses a different physical law than a motor — they share no common principle.

A generator converts mechanical rotation to electrical energy via induction; a motor converts electrical energy to mechanical rotation via magnetic force — they are the same device operated in opposite directions.

A generator only produces DC; a motor only runs on AC.

A generator requires permanent magnets; a motor requires electromagnets.

A transformer has $N_p$ turns in the primary coil and $N_s$ turns in the secondary coil, with voltages $V_p$ and $V_s$ . Which equation correctly states the turns ratio?

$\frac{V_s}{V_p} = \frac{N_s}{N_p}$

$V_s \cdot N_s = V_p \cdot N_p$

$\frac{V_s}{V_p} = \frac{N_p}{N_s}$

$V_s = V_p + (N_s - N_p)$

Fluency Practice

In a DC motor, a commutator reverses the current direction every half turn. Why is this necessary?

To increase the current through the coil and produce more torque.

To prevent the coil from overheating by alternating the current direction.

To ensure the torque always acts in the same rotational direction, maintaining continuous rotation.

To convert AC from the power supply to DC for the coil.

A generator coil rotates in a uniform magnetic field. How does the induced EMF vary with time as the coil rotates?

The EMF is constant because the field is uniform.

The EMF varies sinusoidally because the rate of flux change is sinusoidal.

The EMF increases linearly as the coil rotates faster.

The EMF is zero whenever the coil is parallel to the field.

Transformer diagram showing primary coil with N_p = 50 turns and input voltage V_p connected through an iron core to secondary coil with N_s = 500 turns and unknown output voltage V_s

A step-up transformer has $N_p = 50$ turns in the primary coil and $N_s = 500$ turns in the secondary coil. The primary voltage is $V_p = 120\ \text{V}$ . Calculate the secondary voltage $V_s$ in volts.

A transformer has a primary voltage of $V_p = 240\ \text{V}$ and a secondary voltage of $V_s = 12\ \text{V}$ . If the primary coil has $N_p = 1000$ turns, how many turns does the secondary coil have?

An ideal transformer has primary voltage $V_p = 120\ \text{V}$ and secondary voltage $V_s = 1200\ \text{V}$ . The primary current is $I_p = 10\ \text{A}$ . Using power conservation ( $V_p I_p = V_s I_s$ ), calculate the secondary current $I_s$ in amperes.

You're viewing 2 of 6 sections.

Create a free account to continue the full exercise set and save your progress.

Create free account