Chapter 22 : Electric potentialastronomy.swin.edu.au/~cblake/Topic5_Electricity_Part2.pdfΒ Β· β’...
Transcript of Chapter 22 : Electric potentialastronomy.swin.edu.au/~cblake/Topic5_Electricity_Part2.pdfΒ Β· β’...
Chapter 22 : Electric potential
β’ What is electric potential?
β’ How does it relate to potential energy?
β’ How does it relate to electric field?
β’ Some simple applications
Electric potential
β’ What does it mean when it says β1.5 Voltsβ on the battery?
β’ The electric potential difference between the ends is 1.5 Volts
Electric potential
1.5 V230 V
100,000 V
So what is a volt?
Electric potentialβ’ The electric potential difference βππ in volts
between two points is the work in Joules needed to move 1 C of charge between those points
β’ βππ is measured in volts [V] : 1 V = 1 J/C
ππ = ππ Γ βππ
W = work done [in J]q = charge [in C]βV = potential difference [in V]
Electric potential
The 1.5 V battery does 1.5 J of work for every 1 C of charge flowing round the circuit
ππ = ππ Γ βππ
β’ The electric potential difference βππ in volts between two points is the work in Joules needed to move 1 C of charge between those points
Potential energyβ’ What is this thing called βpotentialβ?
β’ Potential energy crops up everywhere in physics
Potential energyβ’ Potential energy U is the energy stored in a system
(when work is done against a force)
β’ e.g. force of gravity β¦πΉπΉ = ππππ
β
ππ = πΉπΉ Γ β
Work = Force x Distance
= ππππβ
β ππ = ππππβ
πΉπΉ
Potential energyβ’ Potential energy may be released and converted
into other forms (such as kinetic energy)
Work is done, increasing the potential energy
Potential energyβ’ Potential energy difference is the only thing that
matters β not the reference (or zero) level
β’ For example, applying conservation of energy to a mechanics problem:
πΎπΎπΎπΎππππππππππ + πππΎπΎππππππππππ = πΎπΎπΎπΎππππππππππππππ + πππΎπΎππππππππππππππ
Final energy = Initial energy
πΎπΎπΎπΎππππππππππ = πΎπΎπΎπΎππππππππππππππ + (πππΎπΎππππππππππππππβπππΎπΎππππππππππ)
Difference in potential energy
Potential energyβ’ Potential energy difference doesnβt depend on the
path β only on the two points A and B
Potential energyβ’ Potential energy U is the energy stored in a system
β second example
β’ e.g. stretching a spring β¦ πΉπΉ = ππππ
ππ
ππ = οΏ½ππππ ππππ
Work = Force x Distance
= 12ππππ
2
β ππ = 12ππππ
2
πΉπΉForce is varying with distance!
Electric potentialβ’ e.g. moving a charge through an electric fieldβ¦
πΉπΉπΎπΎ
πΉπΉ = βπππΎπΎ
Work = Force x Distance = βπππΎπΎ βππππ = πΉπΉ βππ
β’ Potential difference βππ is work needed to move 1C of charge: ππ = ππ βππ
β’ Equate: ππ βππ = βπππΎπΎ βππ
ππ(minus sign because the force is opposite to E) βππ
πΎπΎ = ββππβππ
Electric potentialβ’ Electric field is the gradient of potential πΎπΎ = ββππ
βπ₯π₯
High V Low VπΎπΎ ππ
ππ
ππ
β’ Positive charges feel a force from high to low potential
β’ Negative charges feel a force from low to high potential
Two parallel plates have equal and opposite charge. Rank the indicated positions from highest to lowest electric potential.
1 2 3 4
0% 0%0%0%
+ + + + + + + + + + + + + + + +
- - - - - - - - - - - - -β’A
β’B
β’C
β’D
1. A=C, B=D2. A, B, C, D3. C, D=B, A4. A, B=D, C
Electric potentialβ’ Analogy with gravitational potential
ππ
ππ
Gravitationalpotential difference exerts force on mass
Electric potential difference exerts force on charge
ππ
Electric potential
High V Low VπΎπΎ
β’ The dashed lines are called equipotentials (lines of constant V)
β’ Electric field lines are perpendicular to equipotentials
β’ It takes no work to move a charge along an equipotential (work done = ππππ = οΏ½βοΏ½πΉ.ππππ =πππΎπΎ.ππππ = 0)
β’ Electric field is the gradient of potential πΎπΎ = ββππβπ₯π₯
β’ Summary for two plates at potential difference V
Electric potential
ππ
πΎπΎ β’ Electric field is the potential gradient
β’ Work W to move charge q from βve to +ve plate
πΎπΎ =ππππ
ππ = ππ ππ
β’ The electric potential difference βππ between two points is the work needed to move 1 C of charge between those points
β’ This work is also equal to the potential energy difference βππ between those points
β’ Potential V = potential energy per unit charge U/q
Link to potential energy
ππ = ππ Γ βππ
βππ = ππ Γ βππ
An electron is placed at βXβ on the negative plate of a pair of charged parallel plates. For the maximum work to be done on it, which point should it be moved to?
1 2 3 4 5 6
0% 0% 0%0%0%0%
+ + + + + + + + + + + + + + + +
- - - - - - - - - - - - -β’X
β’A β’C
β’B
β’D
1. A2. B3. C4. D5. A or C6. C or D
β’ What is the electric potential near a charge +Q?
+q
+Q
πΉπΉ =ππ ππ ππππ2
ππ
Work = Force x Distance
Force is varying with distance, need integral!
ππ = οΏ½β
πππΉπΉ ππππ = οΏ½
β
ππβππ ππ ππππ2 ππππ =
ππ ππ ππππ
Potential energy ππ = ππ ππ ππππ
Electric potential ππ = ππππ
= ππ ππππ
Electric potential
+q
+Qππ
Electric potential ππ = ππ ππππ
Electric potentialβ’ What is the electric potential near a charge +Q?
Electric potentialExercise: a potential difference of 200 V is applied across a pair of parallel plates 0.012 m apart. (a) calculate E and draw its direction between the plates.
The electric field is the gradient in potential
πΎπΎ =βππβππ
=200
0.012 = 1.7 Γ 104 ππ ππβ1 [ππππ ππ πΆπΆβ1]
πΎπΎππ = 200
ππ = 0
+ve plate
-ve plate
Electric potentialExercise: a potential difference of 200 V is applied across a pair of parallel plates 0.012 m apart. (b) an electron is placed between the plates, next to the negative plate. Calculate the force on the electron, the acceleration of the electron, and the time it takes to reach the other plate.
e = 1.6 x 10-19 C; me = 9.1 x 10-31 kg
πΎπΎ+ve plate
-ve plateβππ
Force πΉπΉ = πππΎπΎ = (β1.6 Γ 10β19) Γ (1.7 Γ 104) = β2.7 Γ 10β15 ππ
πΉπΉ = ππππ Acceleration ππ = πΉπΉππ
=2.7 Γ 10β15
9.1 Γ 10β31= 3.0 Γ 1015 ππ π π β2
ππ = 12πππ‘π‘
2 Time π‘π‘ = 2ππππ
π‘π‘ =2 Γ 0.0123.0 Γ 1015 = 2.8 Γ 10β9 π π
Electric potentialExercise: a potential difference of 200 V is applied across a pair of parallel plates 0.012 m apart. (c) calculate the work done on the electron as it travels between the plates.
e = 1.6 x 10-19 C; me = 9.1 x 10-31 kg
πΎπΎ+ve plate
-ve plateβππ
The potential difference is the work done on 1C charge
Work ππ = ππππ = 1.6 Γ 10β19 Γ 200 = 3.2 Γ 10β17 π½π½
Chapter 22 summary
β’ Electric potential difference V is the work done when moving unit charge: ππ = ππππ
β’ The electric potential energy is therefore also given by: ππ = ππππ
β’ The electric field is the gradient of the potential: πΎπΎ = ββππ/βππ
β’ Charges feel a force from high electric potential to low potential