how to get value of checked radio button ?

<input type="radio" value="1"/>Loan no. 1
<input type="radio" value="2"/>Loan no. 2
<select name="payment_category"  id="payment_category" class="form-control"  >	
						<option value="1">Installment</option>
						<option value="2">full payment</option>
 
					</select>
<input type="text" value="" name="amount" />

 $("#payment_category").change(function(){
      // want to check which radio selected based on that amount will be filled 
});

<input type="radio" class="radiobox" value="1"/>Loan no. 1
<input type="radio" class="radiobox" value="2"/>Loan no. 2

		var va=$(".radiobox:checked").val();