# What is the difference between curly and derivative (d) sign

1. Mar 19, 2015

### masyousaf1

Dear All,

Please see the image below in attachment where Energy is function of K. I want to understand how is it possible to understand the last expression ( dE = ? ). Additionally, what is the difference between curly and derivative (d) sign ?

Many thanks to the mentors on this forum
Best wishes

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2. Mar 19, 2015

### RaulTheUCSCSlug

the "curly" derivative (d) sign is called a partial derivative, and is when you only take derivative of a function that has multiple variables. It works very similar to a regular derivative.

3. Mar 19, 2015

### RaulTheUCSCSlug

The last expression is just taking the partial derivative of E with respect to each k, and summing them up since the derivative of the sum is the sum of derivatives!

4. Mar 19, 2015

### Khashishi

This subject is covered in multivariable calculus

5. Mar 20, 2015

### masyousaf1

Thanks RaulTheSCSlug, kindly mention what is the physical meaning of the quantity ∂E/∂k1 * dk1 .

Best wishes