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Investigates how moving electric charges create magnetic fields.
How can a simple piece of copper wire, which doesn't stick to a fridge, suddenly gain the power to lift a two-ton car in a scrapyard?
In 1820, Hans Christian Ørsted noticed a compass needle move near a live wire. This revealed a fundamental truth: moving charges create magnetic fields. For a straight wire carrying a current , the magnetic field forms invisible concentric circles around the wire. The strength of this field is directly proportional to the current and inversely proportional to the distance from the wire. To find the direction of these circles, we use the Right-Hand Rule (RHR). Point your right thumb in the direction of the conventional current (positive to negative); your curling fingers show the direction of the magnetic field lines.
Imagine a vertical wire where the current is flowing straight up toward the ceiling.
1. Point your right thumb toward the ceiling. 2. Observe your fingers curling around the wire. 3. From a 'top-down' view, the magnetic field lines are moving in a counter-clockwise direction.
Quick Check
If the current in a horizontal wire is flowing from left to right, what is the direction of the magnetic field directly above the wire?
Answer
The magnetic field points out of the page (toward you).
A student has a solenoid with 100 loops and a current of . They want to quadruple the magnetic field strength.
1. They could increase the current to (). 2. Alternatively, they could keep the current at but increase the number of loops to 400. 3. The most efficient way is often adding an iron core, which increases significantly without changing the electrical input.
Quick Check
Why does adding an iron core make a solenoid stronger?
Answer
The iron core has high magnetic permeability, meaning its internal magnetic domains align with the solenoid's field, greatly amplifying the total magnetic flux.
When current flows through a solenoid, it behaves exactly like a permanent bar magnet. It has a distinct North and South pole. You can find the North pole by using a variation of the RHR: curl your fingers in the direction of the current loops, and your thumb points toward the North pole. Unlike bar magnets, solenoids are temporary; they lose their magnetism the moment the current stops. Furthermore, you can flip the poles of a solenoid simply by reversing the direction of the current, a feat impossible for a standard bar magnet.
An industrial electromagnet must lift a 1500 kg car. The current is fixed at .
1. To increase lifting force, the engineer increases the 'turn density' by winding the wire tighter. 2. They select a 'soft' iron core because it loses magnetism quickly when the power is cut, allowing the car to be dropped safely. 3. If the car won't detach due to residual magnetism, the engineer briefly reverses the current direction to neutralize the field.
If you look down the axis of a solenoid and the current is moving clockwise, which pole are you looking at?
Which change would NOT increase the magnetic field of a solenoid?
The magnetic field inside a long solenoid is considered uniform (constant in strength and direction).
Review Tomorrow
Tomorrow morning, try to visualize a wire carrying current toward you and use your right hand to 'see' the circular field lines. Can you remember the three factors that make an electromagnet stronger?
Practice Activity
Find a bolt, some copper wire, and a battery. Wrap the wire around the bolt and see how many paperclips you can lift. How does the number of wraps change the result?