How to update a custom field using trigger?

Here the trigger i tried,

trigger PrimeBaseRate on Prime_Rate__c (after update, after insert) {
Static Double x;
for(Prime_Rate__c  pr : trigger.new){
x= pr.Prime_Base_Rate__c;
}
System.debug('the vaulue of field is '+x);
//created a new list for bulkifying
list<opportunity> op = new list <opportunity>();
//running for loop for entire table. 
for(opportunity op1 : [select id, Base_Rate__c from opportunity where isclosed=false]){
//changing the value of the field with the static variable. 
op1.Base_Rate__c=x;
op.add(op1);
} 
update op;
}