4.2 Energy, power and resistance Flashcards
What is the definition of electromotive force?
The energy gained per unit charge by passing through the supply, when a form of energy is transferred to electrical energy carried by the charges.
What is the definition of potential difference?
The energy transferred per unit charge by the charges passing through the component.
What are e.m.f and p.d both measured in?
Volts (v) or joules per coulomb (JC^-1).
Where is e.m.f measured?
At a supply.
Where is p.d measured?
Across a component.
What is the equation for e.m.f?
e.m.f = energy transferred / charge
What is the equation for p.d?
p.d = work done / charge
What are e.m.f and p.d measured by?
A voltmeter connected in parallel with a supply or component.
What happens when an electron is accelerated by a voltage?
The electron gains energy equal to eV, this energy is also equal to the kinetic energy of an electron, Ek = 1/2mv^2.
What is the resistance of an ammeter assumed to be?
Very small so that it doesn’t affect the circuit.
What is the resistance of a voltmeter assumed to be?
Very high so that it doesn’t affect the circuit.
What is Kirchhoff’s second law?
Around any series circuit, the sum of the e.m.fs is equal to the sum of the p.ds.
How do current and voltage behave in a series circuit?
Current is the same at any point, voltage splits up over each component.
How do current and voltage behave in a parallel circuit?
Current splits up down each branch, voltage is the same across each branch.
What is the equation for resistance?
Resistance = voltage / current
What is resistance measured in?
Ohms (Ω).
What is the relationship between voltage and current across a fixed resistor?
Voltage is directly proportional to current.
What does Ohm’s Law state?
The voltage across a conductor is directly proportional to the current flowing through it, provided all physical conditions and temperature, remain constant.
What factors affect electrical resistance?
The material the wire is made from, the length of the wire, the cross-sectional area of the wire, the temperature of the wire.
What are features of LEDs?
Switch on instantly, very robust, very versatile, operate with low potentials, long working life, more efficient than incandescent light bulbs.
What does a thermistor do?
Changes its resistance based on the surrounding temperature (higher temperature = lower resistance).
What does an LDR do?
Changes its resistance based on the surrounding light intensity (higher light intensity = lower resistance).
What is the explanation for the VI graph for a filament lamp?
As more pd is applied across the filament, it heats up more, increasing its resistance and reducing the rate of increase of current flow with increasing pd.
What is the explanation for a graph of temperature against resistance?
As temperature increases, ions vibrate more so there are more delocalised electrons meaning more current can be carried so resistance decreases.
