Capacitors and Electric Fields

Question 1

Define the electric field strength

Answer 1

The force per unit charge on a positive test charge placed at that point

Question 2

Define capacitance

Answer 2

charge per unit potential difference

Question 3

What is the Farad?

Answer 3

coulomb per volt

Question 4

Define the Electric Potential

Answer 4

Work done per unit charge to bring a positive charge from infinity to that point in the field

Question 5

Define the Electric Potential Energy

Answer 5

Work done to bring a small positive charge from infinity to that point in the field

Question 6

What is the time constant of a circuit containing a capacitorand a resistor?

Answer 6

the time taken for current (or charge or voltage) to fall to 1/e of its initial value

Question 7

How do you calculate the total capacitance for capacitors in parallel?

Answer 7

CT = C1 + C2 + C3 + ….

Question 8

How do you calculate the total capacitance for capacitors in series?

Answer 8

1/CT = 1/C1 + 1/C2 + 1/C3….

Question 9

If capcitors are connected in series what quantitiy is the same for all capacitors?

Answer 9

Charge stored

Question 10

If capcitors are connected in parallel what quantitiy is the same for all capacitors?

Answer 10

p.d.

Question 11

What is representd by the area under a p.d. - charge graphs?

Answer 11

Electrical energy transferred OR work done.

Question 12

What is represented by the gradient of a charge-p.d. graph for a capacitor?

Answer 12

Capacitance

Question 13

Out of Q, V and I which variable(s) follow the relationship x = x0 e-(t/RC) when discharging a capacitor?

Answer 13

Q, V and I

Question 14

Out of Q, V and I which variable(s) follow the relationship x = x0 e-(t/RC) when charging a capacitor?

Answer 14

I

Question 15

Out of Q, V and I which variable(s) follow the relationship x = x0 (1 - e-(t/RC) ) when charging a capacitor?

Answer 15

Q and V

Question 16

What is meant by the constant-ratio nature of exponential decay?

Answer 16

In equal time intervals the quantity decreases by equal factors.

Question 17

How much charge remains after 1 time constant when discharging a capacitor?

Answer 17

e⁻¹ OR 0.37 OR 37%

Question 18

How much charge remains after* n* time constants when discharging a capacitor?

Answer 18

e^(-n)

Question 19

What is represented by the gradient of charge-time graph when charging a capacitor?

Answer 19

Charging current

Question 20

State a two uses of capacitors.

Answer 20

Energy storage e.g. for camera flashes, spot welding…; timing circuits; voltage smoothing; audio filters.

Question 21

What does the direction of an electric field line show?

Answer 21

The direction of the force exerted on a positive test charge.

Question 22

What does the density of electric field lines show?

Answer 22

Field strength.

Question 23

When is the equation E = V/d used?

Answer 23

To find the uniform field strength between two parallel plates.

Question 24

State Coulomb’s law in words.

Answer 24

The force between two point charges is proportional to the product of the two charges and inversely proportional to the square of their separation distance.

Question 25

What is the relationship between the electrostatic force between two point charges and their seperation distance?

Answer 25

It is inversely proportional to the square of separation OR inverse square law

Question 26

What is the relationship between potential and distance from a point charge?

Answer 26

Potential α 1/distance

Question 27

What is a uniform electric field?

Answer 27

A region of space in which the electric field strength is the same at all points

Question 28

What is a radial field?

Answer 28

An electric field generated by a point charge OR a field that obeys the inverse square law.

Question 29

Is electric field strength a vector or a scalar quantity?

Answer 29

Vector

Question 30

Is electrical potential a vector or scalar quantity?

Answer 30

Scalar

Question 31

State a similarity between electric and gravitational fields.

Answer 31

Both obey inverse square law for field strength/an inverse proportionaility between potential (around a point mass/charge). Both have infinite range.

Question 32

What are the differences between electric and gravitational fields (at least 2)?

Answer 32

Gravitational fields act on masses, electric fields on charges: electric fields are much stronger that gravitational fields: electric fields can be attractive or repulsive, gravitational fields are always attractive: potential is always negative in a gravitational field.

Question 33

When is the equation V = Q / (4πε₀r) used?

Answer 33

To find the potential around a point charge/in a radial field.

Question 34

When is the equation C = 4πε₀R used?

Answer 34

To calculate the capacitance on an isolated sphere.

Question 35

What is represented by R in the equation C = 4πε₀R used?

Answer 35

he radius of an isolated sphere.

Question 36

What is the relationship between electric potential energy and electric potential?

Answer 36

EPE = V x q where q is the charge in the field.