Electric and Magnetic Fields Flashcards
Define an Electric Field
- A region where a charged particle experiences a force
- E = F/Q
What is the equation for Electric force in a radial field (Coloumbs law)?
- F = kQ1Q2 / r2
Derive the equation E = kQ / r2
- F = EQ and F = kQ1Q2 / r2
- EQ = kQ1Q2 / r2
- E = kQ / r2
Define Electric potential
- The energy required to move a postively charged particle from infinity to a point
What are the equations for a uniform electric field?
- E = F / Q
- E = V / d
- Hence F / Q = V / d
What law does every particle obey in radial electric fields?
- Inverse square law
- F ∝ 1 / r2
What is the equation for electric potential in a radial field?
- V = kQ / r
What is the equation for electric potential in a uniform field?
- V = E / Q
- JC-1
State the similarities between G fields and E fields
- Both obey the inverse square law F ∝ 1 / r2
- Both act over an infinite range
- Both fields are radial for point objects
State the differences between G fields and E fields
- 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
What is the definition of capacitance?
- C = Q / V
What is the equation for energy stored in a capacitor and how do you derive it?
- W = 1/2QV
- You can derive this by using the area under a graph of potential difference against charge stored
How do you derive W = 1/2CV2 and W = 1/2 Q2 / C
- Use W = 1/2QV and C = Q/V
- W = 1/2CVV which is W = 1/2CV2
- W = 1/2QQ / C which is W = 1/2Q2 / C
How do you draw equipotential lines?
- They are always perpendicular to the field lines
- For radial fields the circles get further apart as field strength weakens
Describe the process of a capacitor discharging?
- When discharging, charges move (electrons) from negative plate to positive plate until the p.d is 0.
How do you know when a capacitor is fully charged?
- The p.d across the capacitor will equal the emf of the cell
Describe the process of a capacitor charging?
- Charge moves as the electrons in wire are attracted to the positive terminal of the battery
- They then move to one plate
What does time constant equal?
- Time constant = RC
Define time constant?
- The time taken to fall to 37% of initial value
State the discharging equations for capacitors
- Q = Q0 e-t/RC
- V = V0 e-t/RC
- I = I0 e-t/RC
What are the log equations for discharge and what does this allow you to plot on a graph?
- lnQ = lnQ0 - t/RC
- Allows you to plot a straight line of lnQ against t with gradient -1/RC
What does half time mean in capacitors?
- Q / Q0 = 0.5