7.3.2

# Electric Field Strength

Test yourself

## Electric field strength

Electric field strength is a way of displaying how the force from an object will act at a point far away, without any apparent connection.

### Electric field strength

• The electric field strength is the force per unit charge felt by an object.
• $E=\frac{F}{Q}$
• Using Coulomb's law we can find $E$
• $F=\frac{1}{4{\pi}{{\epsilon}_0}}{\times}\frac{{Q_1}{Q_2}}{r^2}$
• $E=\frac{1}{4{\pi}{{\epsilon}_0}}{\times}\frac{Q}{r^2}$

### Field concepts

• The field is the force experienced if we placed a positive point charge at a point.
• The field will act in the same direction as the force, so if we are looking at the field from a positive charge the force and field will point away.
• If we are looking at the field from a negative charge the field and the force will point towards the charge.

## Radial Field Strength

The field from a charge is the force per unit charge. Field lines show the direction of motion of a positively charged particle.

### Direction of the field

• From a positive charge, the field lines go radially outwards
• From a negative charge, the field lines go radially inwards (towards the charge)

### Position of the field

• Radially outward means in straight lines from the centre, like spokes on a wheel.
• The field lines from a charge are radial lines from the centre.