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Electricity: Magnetic and Heating Effects - Lifting electromagnets

Grade 8CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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An electromagnet is a temporary magnet made by winding a coil of insulated wire around a soft iron core. It works on the principle of the magnetic effect of electric current discovered by Hans Christian Oersted.

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Lifting electromagnets are used in industries and junkyards to move heavy loads of iron, steel scrap, and machinery. These are attached to the arms of cranes.

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The magnetism of an electromagnet is temporary; it exists only as long as the electric current (II) flows through the coil. When the current is switched off, the magnetic field disappears, allowing the crane to release the heavy load.

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The strength of a lifting electromagnet depends on three main factors: the number of turns (nn) in the coil, the magnitude of the current (II) flowing through it, and the material of the core.

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Unlike permanent magnets, the strength of an electromagnet can be varied by changing the amount of current, and its polarity can be reversed by changing the direction of the current.

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The heating effect of current is often an unwanted side effect in electromagnets, governed by Joule's Law: H=I2RtH = I^2 R t, which can cause the coils to warm up during prolonged use.

📐Formulae

B∝n⋅IB \propto n \cdot I

V=I×RV = I \times R

H=I2⋅R⋅tH = I^2 \cdot R \cdot t

Power (P)=V×I\text{Power (P)} = V \times I

💡Examples

Problem 1:

An industrial electromagnet is used to lift two batches of iron scrap. In the first batch, it lifts 4500 kg4500 \text{ kg} and in the second batch, it lifts 3750 kg3750 \text{ kg}. Calculate the total weight lifted by the crane.

Solution:

4500+37508250\begin{array}{r} 4500 \\ + 3750 \\ \hline 8250 \end{array} The total weight lifted is 8250 kg8250 \text{ kg}.

Explanation:

To find the total weight, we perform vertical addition of the two individual loads lifted by the electromagnet.

Problem 2:

If the resistance (RR) of the coil in a lifting electromagnet is 10 Ω10 \, \Omega and the current (II) flowing through it is 12 A12 \text{ A}, calculate the potential difference (VV) required to operate it.

Solution:

Using Ohm's Law: V=I×RV = I \times R V=12 A×10 ΩV = 12 \text{ A} \times 10 \, \Omega V=120 VV = 120 \text{ V}

Explanation:

The voltage or potential difference is the product of the current and the resistance of the wire coil.

Problem 3:

Why is a soft iron core used in a lifting electromagnet instead of steel?

Solution:

Soft iron is used because it is a temporary magnetic material. It magnetizes quickly when current flows and demagnetizes almost completely when current stops. Steel, on the other hand, retains its magnetism even after the current is switched off, which would prevent the crane from dropping the iron load.

Explanation:

The efficiency of a lifting electromagnet relies on the ability to 'turn off' the magnetism instantly.