Physics As Revision Need-To-Knows

Front 1

What is the equation for maximum thermal efficiency?

Back 1

Maximum thermal efficiency = (Th - Tc) / Th

Front 2

Current = what?

Back 2

I = Q / t

Q = charge

t = time

I = current (amps)

Front 3

potential difference = what?

Back 3

V = w / Q

w = work
Q = charge


or

V = I x R

I = current
R = resistance

Front 4

work = ?

Back 4

w = F x s

F = force
s = distance

Front 5

moment = what?

Back 5

moment = Force x perpendicular distance to the pivot

(in Nm)

Front 6

For an object at equilibrium (Principle of moments) ...?

Back 6

Principle of moments:
For an object in equilibrium the sum of the clockwise moments about any point must equal the sum of the anticlockwise moments.

Front 7

If a gas is compressed at a constant volume what happens?

Back 7

It’s temperature will rise.

Front 8

If a gas is heated at a constant pressure, what happens?

Back 8

It’s volume will rise.

Front 9

What are the six assumptions of the kinetic theory for an ideal or perfect gas?

Back 9

1. An ideal gas consists of a large number of identical molecules (so statistical treatment is valid) each of a mass “m”.
2. The molecules are in continual random motion (so many in one direction as any other).
3. The molecules never exert forces on each other. (Forces exerted are negligible)
4. The diameter of the molecules is negligible compared to the distance between them. (Also time taken for a collision is negligible compared with the time between collisions.)
5. All collisions between molecules and the container walls are perfectly elastic (i.e. there is no kinetic energy lost).
6. In between collisions, molecules move in straight lines at constant speed (obeying Newton’s laws of motion).

Front 10

What is the equation for the kinetic theory for an ideal gas (impact with the end wall of a container)?

Back 10

P = F / A

P = the pressure exerted by the molecules on the end of the wall

F = is the force exerted on the wall during collision

A = area (breadth x height)

Front 11

F x dt = (m x v) – (m x u) is the equation for the change in momentum on impact with the wall of a container. What is the equation for the change in momentum for an ideal gas?

Why is it different?

Back 11

F x dt = (m x v) – [m x (-v)]

F = force
dt = impact time
m = mass of one molecule
v = final velocity
(u = initial velocity)

it’s “–v” instead of “u” because the collision is perfectly elastic. (this is one of the five assumptions for an ideal gas).

Thus: F = (2 x m x v) / dt

Front 12

What is the equation relating time between impacts?

Back 12

t = distance / speed

time = (2 x l) / v

l = length
v = velocity

Front 13

What is the equation for avg force exerted on the wall over a long period?

Back 13

F = (m x v2) / l

F = avg force exerted on the wall
m = mass of one molecule
v = velocity
l = length

Front 14

What is the equation for avg force exerted on the end wall over a long period for more than one molecule?

Back 14

Assuming that 1/3 of the molecules are moving up and down, 1/3 sideways, and 1/3 lengthways:

F = 1/3 x N x [(m x v2) / (l) ]

F = avg force exerted on the wall
N = number of molecules
m = mass of one molecule
v = velocity
l = length

Front 15

Knowing that not all molecules are moving at one speed but at different speeds (some faster than others), but for simplicity’s sake assuming that they are all traveling at an avg speed, what is the equation for the total pressure exerted on the end wall from N molecules in the box?

Back 15

p = 1/3 x [ (N x m x 2) / (V) ]
or
p = 1/3 x ρ x 2

p = pressure exerted on the end wall
N = number of molecules
m = mass of one molecule
= average speed of molecules
V = volume = length x breadth x height
ρ = the density of the gas = mass / volume = (N x m) / V

Front 16

When considering the gas laws and the kinetic theory, what must you take note of?

Back 16

*The heat in a gas is approx. equal to the total kinetic energy of all the gas molecules.

*The Kelvin temperature of a gas is proportional to the kinetic energy of just one average molecule.

Front 17

What is the equation for the r.m.s. speed ċ ?

If p were increased what would happen to ċ ?

What is ċ dependent on?

Back 17

ċ^2 = (3p)/( ρ)

If p, pressure, were increased nothing would happen to ċ , ċ is constant. Instead the density ρ would just rise with p.

ċ is only dependent on the absolute temperature.

(Just like Ohm’s law R = V / I , V is constant.)

Front 18

What is the equation that best describes what the r.m.s. speed is dependent on?

Back 18

ċ^2 = (3 x R x T) / Mm


Where:
ċ = the r.m.s. speed (also written as <ċ^2>)
R = the universal molar gas constant (the measure of the heat energy or work that would have to be applied to one mol of gas in order to make its temperature rise by 1K.)
T = temperature
Mm = the molar mass of a gas

Making ċ proportional to T

Front 19

What does S.T.P mean?

Back 19

S.T.P. means Standard Temperature and Pressure.
Which is 0 degrees C and 1.0 x 10^5 Pa.

Front 20

What does “r.m.s.” mean and how do you calculate a r.m.s. “average”?

Back 20

r.m.s. stands for “root mean square”.

To work out an r.m.s. average:

take all the numbers you are finding the average for, square them, take the mean of the squares and the take the root of the mean.

Example:
1 + 5 + 7
12 + 52 + 72
(12 + 52 + 72) / 3 = 25

root of 25 = 5

Front 21

What is the Boltzmann constant?

Back 21

The Boltzmann constant “k” is the gas constant per molecule (instead of per mol).

k = (R) / NA

R= the universal gas constant per mol (in J K-1 mol-1)
NA = Avogadro’s number (6.02 x 10^23 mol-1)


k = 1.38 x 10^-23 J/K

Front 22

The kinetic energy of just one “average” molecule of gas is directly related to the temperature of that gas through what two equations?

Back 22

For one mol N = NA

½ x m x = (3 x R x T) / ( 2 x NA)

or

½ x m x = 3/2 x k x T

Where “½ x m x ” is the kinetic energy of one avg gas molecule.

Front 23

Are the kinetic theory equations in agreement with the gas laws?

Back 23

p x V = 2/3 x N x (½ x m x 2)

½ x m x 2 is proportional to T (temperature in Kelvin)

thus:

p x V T

which is in exact agreement with the gas laws.

Front 24

What is an example of a key experiment that proves that gas molecules move as the kinetic theory suggests.

Back 24

A key experiment is Brownian motion shown in a smoke cell.

Light is shone onto smoke in a smoke cell and the smoke is observed through a low power microscope.

The smoke shows up as specks of light moving randomly. Larger specks are seen to move slower than smaller specks. We can see smoke particles but not air molecules, thus we believe that air molecules are much smaller molecules than smoke molecules and the air molecules must thus move very fast to have enough momentum in order to move the smoke. Both smoke and air molecules move randomly.

Front 25

What is internal energy?

What is mean kinetic energy proportional to?

Back 25

Internal energy is the random distribution of kinetic and potential energy among molecules.
For an ideal gas, this is entirely kinetic since there are no intermolecular forces being considered.

Mean kinetic energy is proportional to the absolute temperature.

Front 26

When a current passes through a conductor what happens?

Back 26

It sets up a magnetic field around the conductor.

Front 27

What is the symbol used for the magnetic field strength, and what is it measured in?

What are other names for magnetic field strength?

Back 27

“B” is the symbol used for magnetic field strength. Magnetic field strength is measured in teslas (T).

“magnetic induction” or “the induction”.

Front 28

What causes B (the magnetic field strength) to increase?

Back 28

• An increase in current
• A decrease in distance from the conductor
• An increase in the number of turns (coil) on the conductor (if it’s wound into a coil).

Front 29

How can you work out the direction of the magnetic field?

Back 29

Using the Right-hand thumb rule.

-Imagine gripping the wire with your right thumb in the direction of the current – your fingers curl around in the direction of B (the magnetic field strength).

Imaging a screw: Screwing it tighter (aka: to the right) means the current would go down. Loosing it, (aka to the left) current goes up.

Front 30

Which direction do the lines of magnetic force go?

Back 30

From North to South.

Front 31

What is a solenoid? And how can you work out the poles for a solenoid?

Back 31

A solenoid is a long thin coil of wire with current flowing through it.
The extended wire idea still works, however, it is usual to work out the poles using the “clock rules”: a clockwise current seen from the end of the solenoid gives a south pole, and an anti-clockwise current gives a north pole.

Front 32

Magnetic fields are really useful when a current carrying conductor is positioned how?

Back 32

At right angles to the field.

Front 33

What is an important equation when dealing with magnetism?

What does it mean?

Back 33

F = B x I x l

F= force acting on the conductor
B= magnetic field strength
I= current or current balance
l= length of conductor inside the magnetic field or length of wire inside the magnetic field at 90 degrees to the field.

A current-carrying conductor of length l at right angles to a uniform magnetic field B will experience a force F. (Where I is the current.)

Front 34

The direction of the force is positioned where (in relation to the magnetic field strength and the current)?

Back 34

The direction of the force is at right angles to B and I.

Front 35

What is an “amp”?

Back 35

Ampere (A), the SI unit of electrical current

The amp is defined in terms of its magnetic field. 1 A is the current which, when maintained in two parallel wires placed 1 meter apart, causes each meter of wire to experience a force of 2.0 x 10^-7 N.

Front 36

Draw the fields around two wires with anti-parallel currents on that have a combined field. What is the effect?

Back 36

Effect is of a strong field pushing the conductor towards the weak field.

Front 37

What are the base units for magnetic field strength (B), force acting on the conductor (F), current (I), and length (L)?

Back 37

B = T (telsa)
F = N (Newtons) = kg x m x s-2
I = A (ampere)
l = m (meters)

Front 38

What are two ways of calculating the current?

Back 38

F = B x I x l
I = (B x l)/F

Or

I = Q/t
Q is the amount of charge that passed through the surface in the time t.

Front 39

F = B x I x L

You don’t know the current. But you do know the speed. How do you solve?

Back 39

F = B x I x l
I = Q/t

F = B x (Q/t) x l

F = (B xQ x l)t

l/t = velocity!! (thus v)

F = B x Q x v <- solve.

Front 40

F can = negative quantity of N

Correct?

Back 40

Yes correct. The negative indicates the direction of the Force!

Front 41

What is a Moment, what is it measured in, and what is another word for it?

Back 41

The rotational driving force represented by the product of the radius and the force.
* Moment = force x perpendicular distance to pivot. *

(perpendicular means: two lines that form 90 degree angles)

Measured in Nm (or Ncm).

Another word for a Moment is “torque”.

Front 42

What rule can be applied, regarding moments, for an object that is in equilibrium?

Back 42

For an object that is in equilibrium the sum of the clockwise moments about any point must equal the sum of the anti-clockwise moments.

Front 43

When talking about moments, what is a couple?

Back 43

A couple is two equal and opposite moments.

Front 44

What is the formula, regarding moments, for an object in equilibrium?

Back 44

Sum of clockwise moments = sum of anticlockwise moments

m1y1 + m2y2 = M1x1 + M2x2

Front 45

What must you always remember regarding pivots?
and
What is another word for pivot?

Back 45

At the pivot, reaction acts from the pivot onto the beam but it has no moment since distance = 0.

Fulcrum

Front 46

When objects are in equilibrium then:

Back 46

-There is no resultant force or moment acting on them. (The sum of all forces in one direction on the body must equal the sum of all forces in the opposite direction on the body –all around. & The sum of all the cockwise moments must equal that of the sum of all the anti-clockwise moments about that point.)
-They remain stationary or move with a constant velocity.
-Adding the forces produces a closed triangle or polygon.

Front 47

What is a free-body diagram?

Back 47

A free body diagram is a pictorial representation often used by physicists and engineers to analyze the forces acting on a free body. It shows all contact and non-contact forces acting on the body.

Front 48

How can you (physically) determine the CG of a flat shape?

Back 48

To determine the Center of Mass / Center of Gravity one must dangle weights from different points of the lamina –making sure that the shape is ‘loose’ and free to rotate – and where the lines cross is the center of mass (M).

Front 49

What is the displacement of a point called?

And what is a way to provide a turning effect with out this problem? (What is the term used for this?)

Back 49

A translation



Provide two equal moments with the forces opposite each other. Called a couple

Front 50

A beam is 1.0 m long and is pivoted at its center. A force of 200N is applied to one end. How far from the other end must the 300N force be hung on in order to stop the beam from rotating clockwise?

Back 50

200 x 0.5 = 300 x (0.5 x l )

l = (approx.) 0.17m

Front 51

What is density? What is the equation for density? What is the symbol for density?

And what are three typical (approx.) values for 3 substances (air, water and silver)?

Back 51

Density is the measure of how tightly the matter (molecules) within it is packed together.

Density = mass / volume
ρ = m / v

Air = 1 kg/m3

Water = 1000 kg/m3

Silver = 10500 kg/m3

Front 52

True or false: if a body moves at constant speed, or remains at rest, there are no forces acting on it.

Explain.

Back 52

False. There may be forces, but there are not net forces. (A net force is the resultant force, the resultant force thus = 0.)

Front 53

True or false:
a.) Sitting in a chair, if your action force is your weight (acting down), than the Newton III reaction force is the upward push of your chair.
b.) In projectile motion, the horizontal velocity is always the same as the vertical velocity component.
c.) A moment produces both a translation and a turning effect.
d.) Density is a ratio of the weight to volume of a body.

Back 53

a.) false
b.) false
c.) false
d.) false

Front 54

Fill in:
The center of mass (or center of ________) of a body is that point at which all the _________ of the body can be assumed to act. If a body is suspended by its center of mass its said to be in ___________.

Back 54

Gravity

Mass

Equilibrium

Front 55

What is a force you should label but isn’t always in the question info?

At what angle does it work in a free-body diagram?

What is this force in the example of a car?

Back 55

The “Normal reaction”.

At right angles to the surface.

Aerodynamic lift.

Front 56

What would most likely happen to a ball bearing falling through a liquid?

Back 56

A ball bearing falling through a liquid has upthrust and viscous friction opposing the weight. The viscous friction increases with speed and eventually (probably quite quickly) the forces are equal and the bearing reaches terminal velocity.

Front 57

A tennis ball is suspended in the air and then released. What happens force wise for it to be able to fall?

Back 57

The ball’s weight and the force of gravity are greater than the force of air resistance, thus the ball falls.

Front 58

What are the three Newton laws? And what do they describe?

Back 58

Newton I: Describes what happens when there s no net force present : If there is no net force on a body it will either: *remain at rest or *continue moving with constant velocity (thus constant speed AND direction).

Newton II: Describes what happens if there is a net force: The net force on a body is equal to the rate of change of momentum of that body. F =m x a ; a = (vf-vi)/t so F = [(m x vf)-(m x vi)] / t
F = rate of change of momentum.

Newton III: That every action force has a reaction force and these actions and reactions are all *equal in magnitude ; *opposite in direction and *act on separate bodies.

Front 59

Never forget to label the reaction force.
There is a descending parachutist. If the action force is the combined weight of the man and the parachute, what is the reaction force?

Back 59

The upward gravitational pull of man on earth.

NOT: the air resistance (though that also is a force that must be labeled, it is not ‘the reaction force’).

Front 60

Why does a Geiger counter always register radiation? And what is it from?

Back 60

A Geiger counter always registers radiation due to the fact that there is always some Background Radiation.

This is due to: radon gas (bout 50%), cosmic rays, rocks (such as granite), medical uses, the nuclear industry, and even food.

Background radiation is about 20 counts (per min).
Thus 1/3 Bq.

Front 61

What is β – emission and what is it’s decay equation expression?

Back 61

β – emission consists of fast moving electrons. Β – emission occurs when a neutron decays into a proton and an electron. The electron is then ejected.

Decay expression:
β + = 0-1 β + 00-v or β- = 0+1e + 00v
Proton number: rises
Nucleon number: remains unchanged. (proton “replaces” the decayed neutron. # - 1 + 1 = #)

Front 62

For α, β, and γ radiation:
What is the air range, and what is needed to absorb them?

Back 62

- α: range through air: a few cm. Absorbed by: thin paper or skin.
-
- β: range through air: several cm. Absorbed by: A few mm of Al. (and partially by skin)
-
- γ: range through air: many meters (falls off w/ inverse square relationship I 1/d2 ) Absorbed by several cm or Pb or several meters of concrete.

Front 63

What is α – emission and what is it’s decay equation expression?

Back 63

α –emission occurs when an unstable nucleus ejects two protons and two neutrons. (A helium nucleus)

42 He

Proton # falls by 2.
Nucleon # falls by 4. (2 protons and 2 neutrons = 4.)

Front 64

What is γ – emission and what is it’s decay equation expression?

Back 64

γ – emission is of high frequency electromagnetic waves that accompany α and β emissions.

No decay equation expression (due to there being no change in composition of the nucleus).

Front 65

How can you tell it’s α – emission?

Back 65

• α particles are 800 X that of β; and have 2X the charge. –they are thus more energetic, and interact strongly w/ gas molecules. Removing electrons.
• excellent ioniser, thus thick trails.
• Clear emission paths (cloud chamber) – not deflected by collision w/ air molecules. (can knock air molecules aside) Thus straight trails.
• Weakly deflected by a magnetic field: and in one direction (Fleming’s Left-hand rule).
• Slowest moving (5% speed of light).
• Positively charged
• 4 x the size of a proton
• weakest penetrating power. Thus short trails.

Front 66

How can you tell it’s β – emission?

Back 66

• β particles are much weaker ironisers than α particles. Thus thin trails.
• Less clear emission path (cloud chamber)
• Strongly deflected by air molecules, in opposite direction to α particles, by a magnetic field. Thus erratic tracks
• β- is negatively charged (electrons), β+ positive electron (positrons).
• β particles travel at 90% speed of light
• ionize air molecules but not as well as α
• can penetrate farther than α but not as far as γ.
• Longer trails.

Front 67

How can you tell it’s γ – emission?

Back 67

• Very weak ionising ability.
• Cloud chamber poorly reveals the paths of γ – emissions (only via emission secondary electrons) undeflected by air molecules.
• Is undeflected by a magnetic field.
• Can penetrate the farthest
• Fastest moving (speed of light)
• Not particles but waves (photons of electromagnetic radiation)
• Most penetrating
• No mass, no charge

Front 68

What are the 3 main types of radioactive emission?

Back 68

- α (alpha), β (beta), and γ (gamma).

(β can be “subcategorized“ into: β+ and β-)

Front 69

What does a cloud chamber do?

Back 69

- Reveals track by ionisation (Alcohol vapour condenses on ion trail left by ionising particle.)

- Can show the sign of charge when a magnetic field is applied.

Front 70

What is the difference between β+ and β- emission?

Back 70

β- - emission is the emission of an electron and an anti-neutrino, and β+ - emission is the emission of a positron and a neutrino.

β- is what most refer to as beta emission. It’s when a neutron decays into a proton and an electron and the electron is ejected.

β+ is when a proton is converted to a neutron, a positron, (also known as the "beta plus particle", the antimatter counterpart of an electron), and a neutrino.

Front 71

What is a parent nuclide and what is a daughter nuclide?

Back 71

A daughter nuclide is the decay product. A daughter nuclide is the result of the radioactive decay of a parent nuclide. The parent nuclide is the “origin”.

Front 72

Why are neutrinos/anti-neutrinos emitted w/ β decay?

Back 72

Neutrinos/anti-neutrinos are emitted w/ β decay in order to conserve momentum, energy and lepton number.

Front 73

A nucleus tends to decay if… what?

Back 73

A nucleus tends to decay if:

- The ratio of protons to neutrons does not fit the stability range
- The nucleus has 84 protons or more

Or

- If the nucleus has too much energy to be stable.

For lighter elements isotopes with approx. equal # protons:neutrons are stable.
More massive elements lie (between the stability lines on a graph) nearer to the β- decay range.

Front 74

What kind of process is radioactive decay?
And how predictable is it?

Back 74

Radioactive decay is a random process. With a large enough sample of material it decays to a predictable pattern. The more radioactive nuclei there are the greater the chance that one will decay.

Front 75

What is the decay equation and what does it represent?

What is the Activity?

Back 75

A = λ x N

Where:
A = amount of nuclei decaying per second (Activity)
(A = ΔN/Δt N)
λ = decay constant (probability of decay) (in s-1)
N = number of nuclei in sample (at this time).

Activity is the total number of emissions in all directions per second. The Activity is measured in Becquerels (Bq).

N = N0e^(- λ x t)

Front 76

What is the half-life equation and what do you have to remember about it?

Back 76

T½ = 0.693 / λ

(T½ is the half-life and λ is the decay constant)
The half-life (T½ ) is the time that is taken for half of the nuclei in the sample to decay.

*λ and T½ can be in s-1 or min-1 or day-1 or year-1. But if λ is in day-1 then so is T½ . So don’t forget to convert if necessary.

*S-1 used the most.

*When calculating T½ from a graph, don’t forget to calculate two T½ values and then use the average.

Front 77

What is an equation, used in questions regarding electricity, that can be applied to radioactivity questions?

Back 77

Q = I x t

Q = charge
I = current
t = time

Front 78

What is a count rate?
and
When using a Geiger counter what must you always remember to do?

Back 78

A count rate is the detected radiation.
Subtract the background count from the count rate.

Also: (when considering other calculations):
Count rate is not in seconds! But per minute!

Front 79

What is rate of decay not affected by?

Back 79

Temperature and pressure.

Front 80

Once you work out the T½ time, can you work out how long it takes per decay?

Back 80

No. (At least not easily.) The amount it takes for one decay to take place can vary largely from the next amount of time needed for the next decay. That’s why half-life is used: as an indication of approximate (total ->½)decay time.

Front 81

α, β and γ radiation are three types of what kind of radiation?

Back 81

All come from the nucleus, and thus they are called nuclear radiation.

Front 82

What are the three ways of detecting nuclear radiation?

Back 82

• photographic paper
• Geiger-Muller tube (with thin mica window)
• Cloud or bubble chamber

Front 83

What is a bubble chamber? How does it work?

Back 83

A bubble chamber is like a cloud chamber, but instead of a trail of vapor caused by droplets of liquid forming on newly ionised molecules, bubbles of gas form on the ionised liquid molecules leaving a trail of bubbles left in the wake of the radiation. (Liquid used is probably liquid hydrogen or freon.)

Front 84

What’s better, a bubble chamber or a cloud chamber?

Back 84

The cloud chamber is the weaker of the two. Far weaker ionising rays will produce a much clearer trail in the bubble chamber.

Front 85

What would happen to α, β and γ rays in an electric field?

Back 85

• Gamma rays would go straight, remain undeflected.

• Alpha rays would be somewhat deflected to the positively charged alpha rays would be attracted to the negative side, and the trajectory would be parabolic.

• Negatively charged beta rays would be attracted to the positive side of the field, and would have a steeper curve than alpha particles due beta particles being less massive. Beta ray trajectories would also be parabolic.

Front 86

What would happen to α, β and γ rays in a magnetic field?

Back 86

• Gamma rays would remain straight and undeflected.
• Negatively charged beta rays would rotate anticlockwise for the field direction shown.
• Positively charged alpha rays would rotate clockwise.
• Since beta particles are smaller than alpha particles they would have a smaller radius of curvature.
• Both alpha and beta ray trajectories would be circular.

Front 87

What are the symbols of the nucleons?

Back 87

• Protons = Z (proton number –or- the atomic number –or- charge number)

• Neutrons = N (neutron number)

• # of nucleons = A (mass number)

Front 88

What do you have to change in an atom in order to get a different (new) atom?

Back 88

The number of protons.

Front 89

How do you create an isotope?
What does this do?

Back 89

By adding neutrons.

*this makes the atom heavier
*makes no difference chemically << its still the same atom
*might make the atom unstable and therefore radioactive

Front 90

What isotope is used in carbon dating?

Back 90

Isotope carbon - 14

Front 91

What are the advantages and disadvantages of nuclear fission over fusion?

Back 91

Fusion is better because:
• very little radioactive waste
• far larger quantities of energy
• A large supply of fuel (deuterium/tritium) readily available from the sea.

Disadvantages of using fusion instead of fission:
• Need temperatures of approx. 100,000,000 K to give nuclei enough kinetic energy to overcome the repulsive forces due
to positive charges in the nucleus. (Which is not easily accomplished)

Front 92

State Hooke's law.

Back 92

the extension of a spring is in direct proportion with the load added to it as long as this load does not exceed the elastic limit.

Front 93

define acceleration

Back 93

acceleration is the rate of change of velocity.

Front 94

What does the crumple zones of the car do for car safety?

Back 94

The crumpling of the car makes the impact forces smaller because it increases the time of impact (thus smaller deceleration).

Front 95

Give the equation for final velocity.

Back 95

V^2 = U^2 + (2 x s x s)

V = final velocity
U = initial velocity
a = acceleration
s = distance

Front 96

What is the equation for kinetic energy of an object?

Back 96

Ek = (1/2)(m)(v^2)

Ek = kinetic energy
m = mass
v = velocity

Front 97

power = ?

Back 97

power = v x I

v = p.d.
i = current


and


P = E/ change of t

E = energy
t = time