Hello Guys, i have created an Opportunity page. Problme is for a new record if i change the stageName the probability is not changing. But its changing when i Edit the record. Do i need to write an Apex class or assign field dependency, Can anyone help me out please Below is the VF Page

<apex:page standardController="Opportunity">
<apex:sectionHeader title="Edit Opportunity" subtitle="{!opportunity.name}"/>

<apex:form >
<apex:pageBlock title="Opportunity Edit" helpTitle="Opportunity" helpUrl="Opportunity">


<apex:pageBlockButtons >
<apex:commandButton action="{!save}" value="Save" />
<apex:commandButton action="{!QuickSave}" value="QuickSave"/>
<apex:commandButton action="{!Cancel}" value="Cancel"/>

</apex:pageBlockButtons>


<apex:pageBlockSection title="Opportunity Information" collapsible="false" >

<apex:outputText value="{!Opportunity.Owner}" 
label="Opportunity Owner"/>

<apex:inputField value="{!Opportunity.Amount}"
label="Amount"/>

<apex:inputCheckbox value="{!Opportunity.IsPrivate}"
label="Private"/>

<apex:inputField value="{!Opportunity.CloseDate}" required="true"
label="CloseDate"/>

<apex:inputField value="{!Opportunity.Name}" required="true"
label="Opportunity Name"/>

<apex:inputText value="{!Opportunity.NextStep}"
label="NextStep"/>

<apex:inputText value="{!Opportunity.Account}" 
label="Account Name"/>

<apex:inputField value="{!Opportunity.StageName}"/>


<apex:inputField value="{!Opportunity.Type}"
label="Type"/> 

<apex:inputField value="{!Opportunity.Probability}"
label="Probability(%)"/>

<apex:inputField value="{!Opportunity.LeadSource}"
label="LeadSource"/> 

<apex:inputText value="{!Opportunity.Campaign}"
label="Primary Campaign Source"/>

<apex:inputText value="{!Opportunity.Rating__c}"
label="Rating"/>

</apex:pageBlockSection>

<apex:pageBlockSection title="Additional Information" collapsible="false">

<apex:inputfield value="{!Opportunity.OrderNumber__c}"
label="Order Number"/>

<apex:inputfield value="{!Opportunity.MainCompetitors__c}"
label="Main Competitor(s)"/>

<apex:inputField value="{!Opportunity.CurrentGenerators__c}"
label="CurrentGenerator(s)"/>

<apex:inputfield value="{!Opportunity.DeliveryInstallationStatus__c}"
label="Delivery Installation Status"/> 

<apex:inputField value="{!Opportunity.TrackingNumber__c}"
label="Tracking Number"/>

</apex:pageBlockSection>


<apex:pageBlockSection title="Description Information" collapsible="false">

<apex:inputField value="{!Opportunity.Description}" 
label="Description" />


</apex:pageBlockSection>

</apex:pageBlock>

</apex:form>
</Apex:page>