3.11 Electric Charges and Fields

Electric Charges and Fields

Electric Fields

• Electric fields are created by electric charges, which can be either positive or negative. Positive charges create an electric field that points away from the charge, while negative charges create an electric field that points towards the charge.
• The strength of an electric field is determined by the amount of charge producing the field and the distance from the charge. The electric field is stronger the closer you are to the charge and weaker the further away you are.
• Electric fields can be represented graphically using lines of force, which are lines that show the direction that a positive test charge would be pushed if placed in the field. The density of the lines indicates the strength of the field.
• Electric fields can be used to understand and analyze how electric forces and charges interact with each other.
• Electric fields are an important concept in electromagnetism and are used in a variety of applications, including electricity generation, motors and generators, and electronic devices.

• Field lines are vectors and must be drawn with arrows.

• Lines go away from a positive charge and towards a negative charge.

• The strength of the field can be visually represented by the density of the field lines. Therefore field, lines must never touch or cross. This would represent an infinitely strong field.

Simple Fields

• Point Charges

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• Two Point Charges

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• Two Parallel Plates
• Notice how the field is the same everywhere between the plates.

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Electric Field Strength

• Electric field strength is represented by the symbol "E" and is measured in units of volts per meter (V/m).
• The electric field strength at a given point is determined by the amount of charge producing the field and the distance from the charge. The electric field is stronger the closer you are to the charge and weaker the further away you are.
• Electric field strength can be calculated using the formula E = F/q, where E is the electric field strength, F is the electric force experienced by the charged particle, and q is the charge of the particle.
• Electric field strength is an important concept in electromagnetism and is used to understand and analyze how electric forces and charges interact with each other.
• The electric field strength at a point can be determined by measuring the force experienced by a charged particle placed at that point. The electric field strength can also be calculated from the distribution of charges producing the field.

Practice Questions

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Answer:

• The force at each location is the same. The field is uniform so E is constant everywhere and q is the same for each case. Fe​=Eq, so the force must be the same.

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Answer:

• Point A must have an electric field strength of 0. The point must be closer to the smaller charge (Q) than the larger charge (-4Q), so it can't be D or E. It must also be where the force vectors between the test charge point in the opposite direction so that the net force is 0. Therefore, it can't be point C either. Since the negative charge (-4Q) is 4x greater than the positive charge, the point must be 2x as far from the -4Q charge as it is from the Q charge.

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Answer:

• Graph A is correct. At x = 2 and x = 4, the distance from the charges is 0, so the field strength must trend towards infinity. At x = 3, the repulsion from the 2 charges cancels out so the field must be 0 there.