Energy, Power & Resistance Flashcards
What is the difference between p.d and E.M.F?
p.d - The energy transferred from electrical to other forms per unit charge
E.M.F - The energy transferred from other forms to electrical per unit charge
How to calculate EMF?
Energy (W) / Charge (Q)
How to calculate energy transferred in a circuit?
Power (W) = EMF x charge (Q)
Base units for power (w)
kg m² s⁻²
Base Units for the volt (V)
kg A⁻¹ m² s⁻³
Define Resistance
The p.d across a component divided by the current going through it
Equation for resistance
R = V/I
Base units of the ohm
kg A⁻² m² s⁻³
How do you find the base units of the ohm
Volt base units divided by A because R=V/I
Ohm’s law
In a ohmic conductor the current is proportional to the p.d at a constant temperature - V=IR
What is the gradient of a I against V graph equal to? And what should it look like?
Resistance - A straight line through the origin
IV characteristics of filament lamp
- Current is not proportional to the p.d
- The lamp does not obey ohms law
- The p.d is increasing out a faster rate than the current
- The resistance is not constant it is increasing
- As the temperature increases there are more collisions between the conducting electrons and the positive ions which increases the current which increases the resistance
IV characteristics of a thermistor
- The current is not proportional to the p.d
- The thermistor does not obey ohms law
- The current is increasing at a faster rate than the p.d
-As the current and temperature increase resistance decreases as the number density increases as more conducting electrons are freed
IV characteristics of a diode
- p.d is not proportional to current
- The diode does not obey ohms law
- The diode’s behaviour depends on the polarity (i.e doesn’t conduct with reverse p.d)
- At the beginning the resistance is high with no current then at the threshold p.d It starts conducting and the resistance decreases as the current rises at a faster rate than the p.d. Towards the end the diode has a small resistance.
Equation for resistivity
R is directly proportional to pL/A
R = p L/A
Units of resistivity
(Rearrange equation for resistivity)
Ohm/m
Equation for power
P = VI or P=I²R or P= V²/R or
P = energy/time or P = VIT
Unit for power
Watt (W)
Kilowatt hour
The amount of energy used by a 1Kw device in one hour
- unit of energy
- w(kwh) = p x t(hours)
Define charge
A property of matter that can be positive or negative
Experiment determining the resistivity of the wire
- Set up a circuit with a power supply, ammeter, potentiometer and voltmeter in parallel to a length of wire
- Use a micrometer to determine the cross-sectional area of the wire
A = πd² / 4 - Adjust the potentiometer to where the ammeter reads a value (This needs to be kept constant through throughout the experiment)
- Record the value on the voltmeter at different points across the wire (using a ruler and crocodile clips) 20cm - 100cm
- If the amps changes, just the potentiometer
- Calculate the resistivity at each length using the equation p = RA/ L
- Plot a graph of R against L - the gradient will = p/A
What is resistivity?
R = P L/A
P = resistivity
