Exploring Forces · Lesson 3 of 7
Non-Contact Forces: Magnetism and Static Charge
“Investigate magnetic and electrostatic forces that can attract or repel without direct physical contact.”
• Define non-contact force and distinguish it from contact force. • Explain magnetic attraction and repulsion as examples of force at a distance. • Describe how rubbing can produce static charges on materials. • Infer that like charges repel and unlike charges attract. • Define electrostatic force and connect static charge with electric current.
Forces That Act from a Distance
Some forces act even when the interacting objects do not touch. Their effects can be detected across a gap. Magnetic force and electrostatic force are two important examples in this chapter.
A force that can act between objects without physical contact is called a non-contact force.
Magnetic Force
Magnets can attract magnetic materials such as iron. Two magnets can also attract or repel one another. Unlike magnetic poles attract, while like magnetic poles repel. Since these effects can occur without the magnets touching, magnetic force is a non-contact force.
The force exerted by a magnet on another magnet or on a magnetic material is called magnetic force.
Problem
Two ring magnets are placed on a vertical wooden stick with like poles facing. The upper magnet stays above the lower one. What does this show?
- 1.The magnets are separated, so they are not touching.
- 2.Like poles repel each other.
- 3.The upward magnetic effect on the upper magnet demonstrates a force acting without physical contact.
Charging Objects by Rubbing
A plastic scale or straw rubbed with polythene can attract tiny pieces of paper without touching them first. Rubbing certain materials causes electrical charges to build up on their surfaces. Because these charges remain in place rather than flowing through a circuit, they are called static charges.
Electrical charge that builds up on the surface of an object and does not move by itself is called static charge.
An object that has acquired static charge is called a charged object.
A charged object can exert force on nearby objects. For example, a rubbed plastic scale can attract small uncharged paper pieces. This force acts across a small distance, so it is another non-contact force.
Like Charges Repel, Unlike Charges Attract
If two balloons are rubbed in the same way with wool, they acquire similar charges and move apart when brought close. This shows that like charges repel. The rubbed balloon and the woollen cloth acquire opposite kinds of charge, and they attract each other. The two kinds of charge are called positive and negative.
The force exerted by a charged body on another charged body or on an uncharged body is called electrostatic force. It is a non-contact force.
Static charges remain on the surface of an object. When electric charges move through a circuit, their motion constitutes electric current—the same current that can produce light, heating, and magnetic effects.
An electroscope is a device used to detect whether an object is electrically charged. A simple classroom electroscope can be made using a jar, metal wire, and thin aluminium foil.
Quiz
Which statement correctly describes magnetic force?
What happens when like magnetic poles face each other?
What is produced when suitable materials are rubbed together?
What happens between two objects carrying like static charges?
Which is an example of electrostatic force?
Practice Problems
- Explain why magnetic force is classified as a non-contact force.
- Describe what happens when two like magnetic poles face each other.
- Explain how rubbing a plastic scale with polythene can make it attract paper pieces.
- Two balloons are rubbed in the same way and then brought close. Predict what happens and explain why.
- Explain the difference between static charge and electric current.
- Describe how a simple electroscope can help test whether an object is charged.
Key Takeaways
• Non-contact forces can act across a distance without physical touching. • Magnetic force can attract or repel depending on the magnetic poles involved. • Rubbing certain materials can produce static charges. • Like charges repel while unlike charges attract. • Electrostatic force acts between charged bodies and can also act on some uncharged objects.