Electricity

Question 1

What is Ohm’s Law?

Answer 1

The current and voltage of a component are directly proportional if the temperature is constant

Question 2

Define resistance

Answer 2

Voltage across a component divided by the current through a component

Question 3

Define current

Answer 3

The rate of charge flowing through a component

Question 4

Define voltage

Answer 4

The energy transferred per unit of charge

Question 5

Sketch the circuit you would use to produce a current voltage characteristic graph

Answer 5

(cell, ammeter, bulb, variable resistor, voltmeter in parallel)

Question 6

Explain the current voltage characteristic for a diode

Answer 6

Resistance is very high at low voltage (until 0.6V), diodes only let current flow in one direction

Question 7

Explain the shape of the filament bulb graph

Answer 7

• Initially, I is proportional to V,
• As the current flows, electrons collide with the metal particles
• As this happens the temperature of the metal increases
• this happens as particles vibrate more rapidly - so more resistance.
• As a result, the gradient of the graph decreases - a greater change in the potential difference is required to cause a change in the current

Question 8

What experiment do you do to determine the resistivity of a wire

Answer 8

-Measure the thickness of the wire with a micrometre, 5 measurements, get an average
-Set up circuit
-Tape wire to ruler to ensure it is straight and taught
-Attach crocodile clips so wire length is 10cm
-Record current and voltage
-Turn off power supply, increase length by 10cm
-Record current and voltage
-Repeat

Question 9

What are superconductors

Answer 9

Superconductors are a group of certain metals that have 0 resistance when they are below a certain temperature.
They can be used to produce strong magnetic fields and there is no energy loss

Question 10

Current in a series circuit…

Answer 10

Is the same at any point

Question 11

Current flowing into a junction in a parallel circuit…

Answer 11

Is equal to the current flowing out of a junction

Question 12

Potential difference across the cell of a series circuit…

Answer 12

Is the sum of the potential difference across all of the components

Question 13

Potential difference in a parallel circuit…

Answer 13

Is the same across each branch

Question 14

Potential difference in a closed loop

Answer 14

Is the same as in a series circuit

Question 15

What are the 3 equations for power?

Answer 15

P=IV P=I2R P=V2/R

Question 16

Define electromotive force, emf

Answer 16

the energy provided to a unit charge passing through a battery (energy in)

Question 17

Which way does convectional current flow?

Answer 17

from the positive terminal to the negative terminal (clockwise) - opposite to electron flow

Question 18

Why is the potential difference across the load resistance always less than the EMF of the cell?

Answer 18

There is always a pd across the internal resistance, which reduces the pd across the output
terminals of the cell.

Question 19

As temperature decreases, thermistor’s resistance ……….. which affects the ………..

Answer 19

decreases / current

Question 20

Ideal voltmeters have …..

Answer 20

infinite resistance

Question 21

Ideal ammeters have…..

Answer 21

zero resistance

Question 22

When EMFs are in parallel

Answer 22

do not add them

Question 23

How can cells be added to a circuit to make the bulbs light at normal brightness for a longer time?

Answer 23

-cells added in parralell
-so less internal resistance
-so less power wasted

Question 24

Why is a resistor needed in the circuit with a variable resistor?

Answer 24

-to limit current when variable resistor is set to 0
-so components aren’t damaged

Question 25

What is E.M.F?

Answer 25

Emf is defined as the amount of energy supplied to each unit of charge (from a battery or other energy source)

Question 26

What does a diode do?

Answer 26

Only lets current flow in one direction. The positioning of the ‘ | ‘ on the symbol tells you which was it cannot go
(arrow in direction I can flow)

Question 27

What is the difference between the P.d. across the load resistance and the e.m.f.?

Answer 27

The p.d. across the load (R) is the ‘terminal P.d.’ in other words the potential difference between the two terminals of the battery.
this DOES NOT count for the ‘lost volts’ due to the internal resistance of the battery.

Question 28

What is meant by ‘terminal potential difference’?

Answer 28

It is the e.m.f take away the energy per coulomb wasted in internal resistance.

the actual p.d. available to the circuit.

Question 29

What is a potential divider?

Answer 29

A circuit that splits P.D.
You have a voltage supply and two resistors in series, the ‘potential’ is then divided between the two resistors as per kirchhoffs laws.

Question 30

Why are potential dividers useful?

Answer 30

if you need varying supplies of voltage. E.G. LDR that decreases in resistance as light levels increase.

That would mean in the dark far more p.d. can be taken to a lamp for example as V/R = I, the R in the lamp would be fairly constant the voltage grows so the current of the light grows making it more bright in the dark.