Electric Charge

Section 1 of 10Positive and negative charge

Positive and negative charge

Two types

• Two types there are positive and negative charges.

In an atom

• In an atom electrons are negative, protons are positive and neutrons have no charge.

Neutral atom

• Neutral atom it has equal numbers of electrons and protons, so the charges cancel.

Unit

• Unit charge is measured in coulombs (C).

Section 2 of 10Forces between charges

Charge 1	Charge 2	Force
Positive	Positive	Repel
Negative	Negative	Repel
Positive	Negative	Attract

• Rule
  • like charges repel and opposite charges attract.

Section 3 of 10Charging by friction

1Method

• Method rubbing two different insulators together transfers electrons from one to the other.

2Charges stay

• Charges stay the charge remains on the insulators because it cannot flow away.

3Net charge

• Net charge the material that gains electrons becomes negative and the material that loses electrons becomes positive.

4Polythene rod and cloth

• Polythene rod and cloth electrons move from the cloth to the rod, so the rod is negative and the cloth is positive.

5Acetate rod and cloth

• Acetate rod and cloth electrons move from the rod to the cloth, so the rod is positive and the cloth is negative.

6Only electrons move

• Only electrons move protons stay fixed in the nuclei.

Section 4 of 10Investigating charging by friction

1

Rub both ends of a polythene rod with a cloth and hang it from a cradle on a nylon thread.

2

Rub a second rod with a cloth.

3

Bring the second rod close to one end of the suspended rod.

4

Record whether the suspended rod is attracted or repelled.

5

Repeat with rods of other materials.

Repulsion

• Repulsion the two rods have the same charge.

Attraction

• Attraction the two rods have opposite charges.

Good practice

• Good practice keep the room free from draughts, rub for the same time each time and do not touch the charged ends.

Section 5 of 10Gold-leaf electroscope

Gold-leaf electroscope

Structure

• Structure a metal plate, a metal stem and a thin gold leaf in an airtight case.

Charged rod brought near

• Charged rod brought near the leaf rises because charge in the stem and leaf is repelled or attracted, giving them the same charge so they repel.

Rod moved away

• Rod moved away the leaf falls because the charges redistribute.

Rod touches the plate

• Rod touches the plate the leaf stays up because charge is transferred to the stem and leaf, so the electroscope is charged.

Finger touches the plate

• Finger touches the plate the leaf falls because charge flows through the person to the earth, so the electroscope is discharged.

Section 6 of 10Electric fields

1Electric field

Electric field a region of space in which an electric charge experiences a force.

2Direction

• Direction the direction of the field is the direction of the force on a positive charge, so field lines go from positive to negative.

3Point charge

• Point charge the lines point radially outwards from a positive charge and radially inwards to a negative charge.

4Charged conducting sphere

• Charged conducting sphere the field is like that of a point charge and the lines meet the surface at right angles.

5Parallel plates

• Parallel plates the field between oppositely charged plates is uniform, with equally spaced parallel straight lines from the positive to the negative plate.

6Vector

• Vector an electric field has both magnitude and direction.

Section 7 of 10Conductors and insulators

Conductors and insulators

Conductor

Conductor a material that lets charge flow through it easily.

• Metals such as silver, copper, aluminium and steel are the best conductors because their outer electrons are delocalised and free to move.

Insulator

Insulator a material with no free charges that does not allow charge to flow easily, such as rubber, plastic, glass and wood.

Testing with an electroscope

• Testing with an electroscope charge the electroscope, touch the plate with the test material and watch the leaf.
• A good conductor makes the leaf fall quickly.
• A poor conductor makes it fall slowly.
• A good insulator leaves the leaf unmoved.

Section 8 of 10More from the specification

More from the specification

Electric charge

• Electric charge a property of matter, measured in coulombs, where electrons carry negative charge

Current

• Current rate of flow of charge, I = Q / t, and in a circuit energy is transferred by the cell to components

Potential difference and resistance

• Potential difference and resistance potential difference is energy per charge, resistance limits the current, R = V / I

Alternating current and power

• Alternating current and power current that reverses direction contrasts with direct current,
• and power equals voltage times current; a graph of I against V shows resistor behaviour

Electric charge

• Electric charge a property of matter, measured in coulombs, where electrons carry negative charge

Current

• Current rate of flow of charge, I = Q / t,
  • and in a circuit energy is transferred by the cell to components
    What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)

Potential difference and resistance

• Potential difference and resistance potential difference is energy per charge, resistance limits the current, R = V / I

Alternating current and power

• Alternating current and power current that reverses direction contrasts with direct current,
  • and power equals voltage times current; a graph of I against V shows resistor behaviour

Alternating current

• A current that continuously changes direction.

Electromotive force

• The electrical work done by a source in moving a unit charge around a complete circuit.

Current

• Flow of electrons in a circuit.

Resistance

• How difficult it is for current to flow through a material.

Terminal

• A connection point of a cell, marked + or -.

Voltage

• A measure of the electrical energy supplied or changed in a circuit.

Component

• A part of a circuit such as a lamp or buzzer.

Potential difference

• The work done by a unit charge passing through a component.

Section 9 of 10Key terms

Term	Meaning
Electric charge	A property of matter that causes electric forces, measured in coulombs.
Charging by friction	Transferring electrons between two insulators by rubbing them.
Electric field	A region where a charge experiences a force.
Conductor	A material through which charge flows easily.
Insulator	A material with no free charges that does not conduct.
Gold-leaf electroscope	A device that detects charge by the rising of a gold leaf.
Coulomb	The unit of electric charge.

Section 10 of 10Exam technique

1

Saying protons move when objects are charged by friction — only electrons are transferred.

2

Saying the object that gains electrons becomes positive — gaining electrons gives a negative charge.

3

Saying the electroscope leaf falls because the rod removes the charge — the charge redistributes when the rod moves away.

4

Saying field lines go from negative to positive — they go from positive to negative.

5

Saying insulators have no electrons — they have no free electrons.