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A2 Physics > Chapter 16 - Charge & Field > Flashcards

Chapter 16 - Charge & Field Flashcards

1
Q

What are electric fields caused by?

A

Any object with charge has an electric field around it which interacts with other electrically charged particles

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2
Q

What is Coulomb’s Law?

A

A law used to calculate the force between two point charges

F = kQq/r^2

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3
Q

What is k?

A

k is Coulomb’s constant.

k = 8.99 x 10^9 Nm^2C^-2

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4
Q

What is epsilon?

A

permittivity of material between charges

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5
Q

What can be said about the forces on each point charge Q and q

A

Equal and opposite

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6
Q

What is electric field strength?

A

Force per unit positive charge

the force that a charge of +1 C would experience in an electric field

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7
Q

Electric field strength

A

E = F / q in NC^-1

E = V / d in Vm^-1

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8
Q

What is a point charge?

A

A body which behaves as if all its charge is concentrated at the centre

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9
Q

What is the force on a charge in a uniform electric field?

A

F = qE = qV/d

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10
Q

What can the electric field be described as?

A

Rate of change of electrical potential with respect to distance

E = dV/dx

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11
Q

For point charges what are their electric fields like?

A

Radial fields where E is a vector pointing in the direction a positive charge would move

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12
Q

Formula for electric field

A

E = F / q

Therefore

E = kQ/r^2

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13
Q

What is the relationship between the electric field and the electric potential (voltage)

A

Voltage is the integral of the electric field (i.e area under the graph)

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14
Q

What is the relationship between the force and the EPE?

A

EPE is the integral of the force

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15
Q

How can you tell the strength of an electric field in a diagram?

A

Field lines are more closely packed together when the field is stronger.

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16
Q

Formula for EPE

A

EPE = kQq/r

17
Q

What is the electric potential energy?

A

The work that would need to be done to move a small charge (q) from infinity to a distance (r) away from point charge (Q)

18
Q

Derivation of voltage formula from EPE

A

Since E = qV, V = E/q

Therefore V = kQ/r

19
Q

What is the electric potential?

A

Electric potential energy per unit positive charge

20
Q

What did Milikan’s experiment show?

A

Showed that charge was quantised and came in discrete “packets” of size 1.6 x 10^-19 C (Fundamental unit of charge - one elctron)

21
Q

How did Milikan discover this?

A

Watched oil drops for which F = qE = mg.

Found charge to be equal to n x e

22
Q

Within a spherical point charge how does the voltage and the electric field strength change?

A

Electric field strength = 0 inside of the sphere

Voltage remains at a constant value from the surface of the sphere to the center.

23
Q

Max angle of deflection in electric and magnetic fields

A

Electric field: 180 degrees due to a constant magnitude force (F = QE) in a constant direction

Magnetic fields: 360 degrees as there is a constant magnitude force BUT always perpendicular to the velocity

24
Q

Electron gun equation

A

v = root(2QV/m)

Can be derived from: E = QV = 1/2 mv^2 since a particles energy is converted to kinetic energy with acceleration.

25
Q

Why are charged particles in magnetic fields always deflected in circular paths?

A

Due to fleming’s left hand rule, the force on a moving charge (in a magnetic field) is always perpendicular to the velocity (i.e direction of travel).

26
Q

Why is the radius of curvature of the path of a deflected charged particle important?

A

Gives you information about the particle’s charge and mass.

Hence, different particles can be identified by studying their deflections in a magnetic field.

27
Q

What is the equation for the radius of curvature?

A

r = mv/qB

28
Q

Total energy of a free particle

A

Kinetic energy + rest energy

E total = ymc^2

29
Q

Calculation of the relativistic factor

A

E total / E rest = y mc^2 / mc^2 = y

30
Q

In linear accelerators, why does the distance between electrodes increase?

A

Because as the particles accelerate, their velocity increases and hence they travel a larger distance in the same time so they travel further per oscillation of the current. (alternating current which switch charge)

31
Q

Why do particles in magnetic fields spiral?

A

As they collide with other particles and lose energy and thus their velocity decreases.

Since velocity is proportional to radius ( r = mv/Bq ) their radius decreases with velocity.

32
Q

How can a particle travel in a straight line in a velocity selector?

A

Only if the electric force on it is equal and opposite to the magnetic force on it.

F = qE = qvB

A2 Physics (8 decks)

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