# 4.2.1 Electric Charge

### Positive & Negative Charges

• Objects can be given one of two types of electric charge:
• Positive
• Negative
• When two charged objects are brought close together, there will be a force between those objects

Like charges repel; opposite charges attract

• Remember:
• Opposite charges attract
• Like charges repel

#### Exam Tip

Whilst electrostatic forces share many similarities with magnetic forces, they are different phenomena – take care not to confuse the two!

Extended Only

### Measuring Charge

• Electric charge is measured in units called coulombs (C)

### Demonstrating Electric Charge

• In your exam you will be expected to be able to explain how to give insulating materials a charge by the process of friction and to describe how to demonstrate the charging of materials in this way

Method:

• Suspend one of the insulating materials using a cradle and a length of string so that the material can rotate freely
• Rub one end of the material using a cloth (in order to give it a charge)
• Now take a second piece of insulating material and charge that by rubbing with a cloth
• Hold the charged end of the second piece close to the charged end of the first piece:
• If the first piece rotates away (is repelled) from the second piece then the materials have the same charge
• If the first piece moved towards (is attracted to) the second piece then they have opposite charges

#### Exam Tip

Experimental demonstrations, such as the one above, are different from experiments in which you have to take measurements.

When describing a demonstration you should state a conclusion – in other words, explain what you expect to happen and what it means.

### Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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