The Motor Effect Lab

The Motor Effect Lab ⚡

Electricity and magnetism are two sides of the same coin. Whenever a **current flows through a wire**, it creates a **magnetic field** around that wire. Turn that idea around and you get the **motor effect** — the push that spins every electric motor, from a phone's vibrate to an electric car. Let's build it up.

A field around a wire 🧲

A current in a straight wire is wrapped by a magnetic field of **concentric circles** — rings centred on the wire. The field is strongest close to the wire and gets weaker further away. Reverse the direction of the current and the field circles reverse direction too.

What shape is the field?

What is the shape of the magnetic field around a long straight current-carrying wire?

• Concentric circles centred on the wire
• Straight lines parallel to the wire
• A bar-magnet pattern with two poles
• There is no field around a straight wire

The solenoid 🔩

Wind the wire into a coil — a **solenoid** — and the fields from every loop add together. The result looks just like a **bar magnet**: a strong, roughly **uniform field inside** the coil, with a north and south pole at the ends. This is the shape we can make useful.

Strengthen the solenoid

Select ALL THREE things that increase the strength of a solenoid's magnetic field.

• Adding more turns to the coil
• Increasing the current
• Placing an iron core inside the coil
• Spreading the same turns over a longer coil
• Putting a plastic rod inside the coil
• Painting the wire a darker colour

The electromagnet 🔌

A solenoid with an **iron core** is an **electromagnet**. Its greatest trick: it is only magnetic while the current flows, so it can be **switched on and off**. That is why electromagnets lift and drop cars in scrapyards, and why they are used in relays, bells and maglev trains.

Match each term to its meaning

The motor effect 🧩

Now the payoff. Put a **current-carrying wire inside a magnetic field** and the two fields interact, pushing the wire with a **force**. This is the **motor effect**. To find the direction of the force, use **Fleming's left-hand rule**: hold your left thumb and first two fingers at right angles — **thu**M**b = Motion (force)**, **First finger = Field**, se**C**ond finger = **Current**.

Fleming's left-hand rule

How big is the force? 📐

The size of the force on the wire is given by: **F = B I l** where **F** is the force in newtons (N), **B** is the **magnetic flux density** in tesla (T), **I** is the current in amps (A) and **l** is the length of wire in the field in metres (m). The wire must be at right angles to the field for the full force.

Work out the force

An interactive activity.

Another force

An interactive activity.

How a motor spins

An interactive activity.

Why the commutator?

What is the job of the split-ring commutator in a DC motor?

• It reverses the current in the coil every half turn
• It increases the current to make the motor faster
• It creates the magnetic field
• It stops the coil after one turn

Lab summary

A current in a wire makes a magnetic field; wound into a solenoid with an iron _____ it becomes a switchable electromagnet. A current-carrying wire in a field feels a force (the motor effect), whose direction is found with Fleming's _____-hand rule and whose size is F = B I _____. In a motor, the split-ring _____ reverses the current every half turn to keep the coil spinning.