Electric and Magnetic Fields

Question 1

Define an Electric Field

Answer 1

• A region where a charged particle experiences a force
• E = F/Q

Question 2

What is the equation for Electric force in a radial field (Coloumbs law)?

Answer 2

• F = kQ₁Q₂ / r²

Question 3

Derive the equation E = kQ / r²

Answer 3

1. F = EQ and F = kQ₁Q₂ / r²
2. EQ = kQ₁Q₂ / r²
3. E = kQ / r²

Question 4

Define Electric potential

Answer 4

• The energy required to move a postively charged particle from infinity to a point

Question 5

What are the equations for a uniform electric field?

Answer 5

• E = F / Q
• E = V / d
• Hence F / Q = V / d

Question 6

What law does every particle obey in radial electric fields?

Answer 6

• Inverse square law
• F ∝ 1 / r²

Question 7

What is the equation for electric potential in a radial field?

Answer 7

• V = kQ / r

Question 8

What is the equation for electric potential in a uniform field?

Answer 8

• V = E / Q
• JC^-1

Question 9

State the similarities between G fields and E fields

Answer 9

• Both obey the inverse square law F ∝ 1 / r²
• Both act over an infinite range
• Both fields are radial for point objects

Question 10

State the differences between G fields and E fields

Answer 10

• G fields are always attractive whilst E fields can be attractive or repulsive
• E fields are stronger than G fields
• E fields only act on charged particles whilst g fields act on all particles with mass
• E fields can be shielded (with a Faraday cage) whilst g fields cannot

Question 11

What is the definition of capacitance?

Answer 11

• C = Q / V

Question 12

What is the equation for energy stored in a capacitor and how do you derive it?

Answer 12

• W = 1/2QV
• You can derive this by using the area under a graph of potential difference against charge stored

Question 13

How do you derive W = 1/2CV² and W = 1/2 Q² / C

Answer 13

1. Use W = 1/2QV and C = Q/V
2. W = 1/2CVV which is W = 1/2CV²
3. W = 1/2QQ / C which is W = 1/2Q² / C

Question 14

How do you draw equipotential lines?

Answer 14

• They are always perpendicular to the field lines
• For radial fields the circles get further apart as field strength weakens

Question 15

Describe the process of a capacitor discharging?

Answer 15

• When discharging, charges move (electrons) from negative plate to positive plate until the p.d is 0.

Question 16

How do you know when a capacitor is fully charged?

Answer 16

• The p.d across the capacitor will equal the emf of the cell

Question 17

Describe the process of a capacitor charging?

Answer 17

• Charge moves as the electrons in wire are attracted to the positive terminal of the battery
• They then move to one plate

Question 18

What does time constant equal?

Answer 18

• Time constant = RC

Question 19

Define time constant?

Answer 19

• The time taken to fall to 37% of initial value

Question 20

State the discharging equations for capacitors

Answer 20

• Q = Q₀ e^-t/RC
• V = V₀ e^-t/RC
• I = I₀ e^-t/RC

Question 21

What are the log equations for discharge and what does this allow you to plot on a graph?

Answer 21

• lnQ = lnQ₀ - t/RC
• Allows you to plot a straight line of lnQ against t with gradient -1/RC

Question 22

What does half time mean in capacitors?

Answer 22

• Q / Q₀ = 0.5